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Pressure Converter

Convert between SI, atmospheric, and gravitational pressure units (pascal, kilopascal, megapascal, bar, millibar, atmosphere, torr, mmHg, psi, kgf/cm², inH2O, cmH2O) with NIST-grade exact factors and conventional liquid-column values.

psi

Result

0.0689476 bar

1 psi= 0.068948 bar

UnitValue
Pascal (Pa)6,894.76
Kilopascal (kPa)6.89476
Megapascal (MPa)0.00689476
Bar (bar)0.0689476
Millibar (mbar)68.9476
Standard atmosphere (atm)0.068046
Torr (Torr)51.7149
Millimeter of mercury (mmHg) (mmHg)51.7149
Pound per square inch (psi) (psi)1
Kilogram-force per square centimeter (kgf/cm²)0.070307
Centimeter of water (cmH₂O) (cmH₂O)70.307
Inch of water (inH₂O) (inH₂O)27.6799

Pressure converter. SI, atmospheric, imperial and gravitational pressure units with NIST-exact factors.

A pressure converter changes a value from one pressure unit to another using NIST-exact factors anchored to the SI pascal. Eight of the twelve units (Pa, kPa, MPa, bar, mbar, atm, psi, kgf/cm²) are definitionally exact; the four liquid-column units (torr, mmHg, cmH₂O, inH₂O) are conventional values that depend on the density of mercury or water and are marked inexact.

What Is a Pressure Converter?

A pressure converter is a tool that takes a measurement in one unit of pressure and returns the equivalent value in any other supported unit. Every conversion in this calculator routes through a single base unit, the pascal (Pa), using each unit's defined factor to pascals. Pressure is force per area — formally 1 Pa = 1 N/m² — so anchoring on the pascal lets the same engine cross-convert SI engineering units (Pa, kPa, MPa), atmospheric units (bar, millibar, standard atmosphere), an imperial industrial unit (psi), a gravitational technical unit (kgf/cm²), and the liquid-column units used in medicine and HVAC (torr, mmHg, cmH₂O, inH₂O).
The single most-important fact about a serious pressure converter is honesty about which factors are exact and which are not. Eight of the twelve units in this set have exact rational factors to the pascal: pascal (1, SI base), kilopascal (1 000), megapascal (1 000 000), bar (100 000, BIPM-accepted), millibar (100, identical to the hectopascal), standard atmosphere (101 325 Pa exact, fixed by the 10th CGPM in 1954 and embedded in ISO 2533:1975), pound-force per square inch (6 894.757293168361 Pa, the IEEE-754 product of NIST-exact upstream rationals 4.4482216152605 lbf/N and 0.0254 m/in), and kilogram-force per square centimetre (98 066.5 Pa, the so-called technical atmosphere, fixed by CGPM 1901 standard gravity × 1 kg/cm² with no rounding). The four remaining units — torr, mmHg, cmH₂O and inH₂O — are conventional values and the converter tags them as inexact. Most online converters call every unit exact; we do not, and the difference matters in metrology and clinical contexts where the gap between two near-identical units is the entire point.
The distinctions inside the inexact group are subtle but real. Torr is defined as exactly 1/760 of a standard atmosphere — a clean rational definition (101 325 ÷ 760) — but the resulting quotient 133.32236842105263… Pa is irrational in IEEE-754 double-precision binary, so it cannot be stored without truncation. Millimetre of mercury (mmHg) is a separate empirical convention from ISO 31-3 / NIST SP 811 fixed at exactly 133.322387415 Pa, derived from a mercury density of 13.5951 g/cm³ at 0 °C under standard gravity. Torr and mmHg therefore differ by about 5 × 10⁻⁵ Pa — less than 0.000015 % — close enough to be interchangeable in clinical blood-pressure work but distinct in vacuum metrology. Centimetre of water (cmH₂O = 98.0665 Pa) and inch of water (inH₂O = 249.0889 Pa) both assume a water density of exactly 1 000 kg/m³ at 4 °C; real water density at room temperature is 999.972 kg/m³, so the underlying physical assumption drifts by ~28 ppm versus the conventional value.
This converter supports the entire industrial and medical pressure landscape. Mechanical and pneumatic engineers move between MPa, bar and psi; weather services publish in hPa (numerically identical to millibar — the WMO migrated officially in 1986 but kept the same scale, so 1013 hPa and 1013 mbar are the same atmospheric reading); automotive technicians read tyres in psi, kPa or bar (a typical passenger-car spec of 32 psi = 220.6 kPa = 2.21 bar); medical clinicians read systolic blood pressure in mmHg (a healthy adult value of 120 mmHg = 16.0 kPa per the 2017 ACC/AHA guideline); CPAP and ventilator technicians read airway pressure in cmH₂O (a typical CPAP spec of 5–20 cmH₂O = 0.49–1.96 kPa); HVAC designers read duct static pressure in inches of water column (a residential system runs at 0.3–0.6 inH₂O = 75–150 Pa); divers read absolute depth pressure in bar (~1 bar per 10 m of seawater; the Mariana Trench bottom is roughly 1 086 bar ≈ 15 750 psi); ASME boiler engineers split low-pressure (≤ 15 psi = 103.4 kPa, Section IV) from high-pressure (> 15 psi, Section I); and physics students do the routine Pa↔atm↔bar conversions that show up in every gas-law problem.
What distinguishes a serious pressure converter from a marketing widget is calling out the conflations every other tool ignores. Torr is not mmHg. hPa is mbar (numerically) but the official meteorological unit is the hPa. Standard atmosphere (101 325 Pa) is not technical atmosphere (98 066.5 Pa = 1 kgf/cm²) — they differ by 3.21 % and the technical atmosphere survived in older European industrial drawings long after BIPM deprecated it. The pound per square inch is exact only because it inherits from two exact upstream definitions (the IYP-1959 inch at 0.0254 m and the CGPM-1901 pound-force at 4.4482216152605 N); strip either upstream definition and psi loses its exact-rational status. Every result in this calculator displays an "exact" badge when both endpoints are exact, and is silent when either endpoint depends on a conventional liquid-column value — full transparency, not marketing.

How to Convert Between Pressure Units

Every pressure conversion is one multiplication and one division through the pascal. The general formula is:
y=x⋅afromatoy = x \cdot \frac{a_{\text{from}}}{a_{\text{to}}}
where $x$ is your input value, $a\text{from}$ is the source unit's factor to pascals, and $a\text{to}$ is the target unit's factor to pascals. To do it by hand:
1. Look up the source-to-pascal factor. For psi, $a\text{from} = 6894.757293168361$.
2. Multiply the input by that factor to get pascals. 32 psi × 6894.757293168361 = 220 632.2333813876 Pa.
3. Look up the target-to-pascal factor. For bar, $a\text{to} = 100000$.
4. Divide the pascal value by the target factor. 220 632.2333813876 ÷ 100 000 = 2.20632 bar.
The same procedure works for every supported unit. Going from atmospheres to pascals: 1 atm × 101 325 Pa/atm = 101 325 Pa exact (the canonical ISO 2533 standard-atmosphere definition). Going from bars to psi: 1 bar × 100 000 ÷ 6 894.757293168361 = 14.5037738… psi (the irrational SI-to-imperial pressure quotient). Going from torr to mmHg: 1 Torr × 133.32236842105263 ÷ 133.322387415 = 0.99999985… mmHg — a useful sanity check that confirms the two units are not bit-identical despite being interchangeable in clinical work.
To use this calculator, pick the source unit from the "From" dropdown, type a value, then pick the target unit from the "To" dropdown. The result updates on every keystroke. Click the result card to copy it to the clipboard. Use the precision selector to switch between auto (6 significant figures), or a fixed 0, 2, 4, 6, 10 or 15 decimals. Auto-precision switches to scientific notation when the result is larger than 10¹² (one trillion) or smaller than 10⁻³, so converting a Mariana Trench-bottom pressure (~1 086 bar ≈ 108 600 000 Pa) all the way down to inches of water (≈ 436 000 inH₂O) still renders readably at every step. The "exact" badge appears when both endpoints have exact rational factors and is absent when either endpoint is a conventional liquid-column value (torr, mmHg, cmH₂O, inH₂O).

Pressure Conversion Formula

y=x⋅afromatoy = x \cdot \frac{a_{\text{from}}}{a_{\text{to}}}
  • yy = The converted value, expressed in the target unit of pressure.
  • xx = The input value, expressed in the source unit of pressure.
  • afroma_{\text{from}} = Factor that converts the source unit to pascals (e.g. 100000 for bar, 6894.757293168361 for psi, 101325 for atm, 98066.5 for kgf/cm²).
  • atoa_{\text{to}} = Factor that converts the target unit to pascals (e.g. 1000 for kPa, 100 for mbar/hPa, 133.322387415 for mmHg, 98.0665 for cmH₂O).
The formula is a two-step pivot through the pascal base unit. The factor table this calculator uses is sourced from NIST SP 811 Appendix B.8 (pressure section), the BIPM SI Brochure (accepted non-SI units), ISO 2533:1975 (standard atmosphere) and ISO 31-3 (mmHg conventional value), and is anchored to four exact upstream definitions — the SI pascal, the 1959 International Yard and Pound Agreement (1 in = 0.0254 m, 1 lb = 0.45359237 kg, both exact), the 3rd CGPM (1901) declaration that gn = 9.80665 m/s² exact, and the 10th CGPM (1954) declaration that 1 atm = 101 325 Pa exact:
  • Pascal (Pa): 1 Pa (exact, SI derived unit; 1 Pa ≡ 1 N/m² ≡ 1 kg/(m·s²))
  • Kilopascal (kPa): 1 000 Pa (exact, SI)
  • Megapascal (MPa): 1 000 000 Pa (exact, SI)
  • Bar (bar): 100 000 Pa (exact, BIPM-accepted non-SI)
  • Millibar (mbar): 100 Pa (exact, = hectopascal hPa numerically; WMO-standard meteorological unit)
  • Standard atmosphere (atm): 101 325 Pa (exact, 10th CGPM 1954; ISO 2533:1975)
  • Torr: 101 325 / 760 = 133.32236842105263… Pa (definitionally exact rational, representationally inexact in IEEE-754 binary)
  • Millimetre of mercury (mmHg): 133.322387415 Pa (NIST SP 811 conventional value, ISO 31-3; mercury density 13.5951 g/cm³ at 0 °C × gn)
  • Pound-force per square inch (psi): 6 894.757293168361 Pa (exact, = 4.4482216152605 N / 0.0254² m² = IEEE-754 product of NIST-exact upstream rationals)
  • Kilogram-force per square centimetre (kgf/cm², technical atmosphere): 98 066.5 Pa (exact, = 9.80665 N × 10⁴ cm²/m²)
  • Centimetre of water (cmH₂O): 98.0665 Pa (conventional, water density 1 000 kg/m³ at 4 °C × gn; real water density at 25 °C ~999.972 kg/m³ drifts ~28 ppm)
  • Inch of water (inH₂O): 249.0889 Pa (conventional, = 2.54 × cmH₂O)
For the pascal → psi direction the calculator displays the irrational quotient 1 / 6 894.757293168361 = 1.45037737730209… × 10⁻⁴ psi/Pa. For the bar → atm direction: 100 000 / 101 325 = 0.98692326671601… atm/bar (the slightly-less-than-one ratio because the standard atmosphere is a touch above 1 bar). Both endpoints (Pa, psi) are exact rationals, so the exact badge fires; the printed inverse is mathematically irrational but the underlying definition is exact.

Worked Pressure Conversion Examples

32 psi to bar (the daily tyre-pressure question)

Set From = Pound per square inch (psi), To = Bar, Value = 32. The formula gives 32 × 6 894.757293168361 ÷ 100 000 = 2.2063 bar at auto-precision, more precisely 2.206322333813876 bar at 15 decimals. A typical passenger-car spec calls for about 32 psi (2.21 bar = 220.6 kPa) cold, and most owner's-manual stickers list all three units side by side so the same wheel works at a US station, a European pump, and an Asian compressor. Both endpoints (psi, bar) are exact rationals, so the "exact" badge fires — there is no rounding error, only the irrational quotient printed in decimal form. The casual rounding 1 psi ≈ 6 894.76 Pa vs the engine's exact 6 894.757293168361 Pa introduces about 0.003 Pa per psi — so a 32-psi tyre reading drifts by ~0.087 Pa total (≈ 8.7 × 10⁻⁷ bar) — invisible on any gauge, but it accumulates over millions of cycles in heavy-industrial pneumatics. The IYP-1959 inch (0.0254 m exact) and the CGPM-1901 pound-force (4.4482216152605 N exact) are what make this conversion definitionally exact rather than empirical.

1 atm to pascals, bar, psi, mmHg, torr (the standard atmosphere that anchors everything)

Set From = Standard atmosphere, To = each of the others, Value = 1. By ISO 2533:1975 definition: 1 atm = 101 325 Pa = 1 013.25 mbar = 1 013.25 hPa = 1.01325 bar = 14.6959 psi = 760 Torr = 759.9999 mmHg. The atm-to-Torr ratio is exactly 760 by construction (Torr was defined as 1/760 atm precisely to make this conversion clean), but mmHg uses a slightly larger empirical value (133.322 387 415 Pa per ISO 31-3 vs 133.322 368 421… Pa per Torr) and lands at 759.9999 mmHg instead of exactly 760 mmHg — slightly less than 760 because each mmHg is fractionally bigger than each Torr, so it takes fewer of them to span one atmosphere. This is the famous Torr vs mmHg gap of ~5 × 10⁻⁵ Pa per unit, inverted: 760 mmHg corresponds to 101 325.014 Pa (about 1.00000014 atm), not 1 atm exactly. The 14.696 psi value is the canonical sea-level atmospheric pressure printed on every US barometer and aircraft altimeter. The 1 013.25 hPa figure is the WMO sea-level reference that every weather forecast on Earth normalises against. This is the single most-important set of equalities in the whole pressure landscape, and it is the anchor for every other conversion in this table.

120 mmHg to kilopascals (the blood-pressure conversion)

Set From = Millimeter of mercury (mmHg), To = Kilopascal, Value = 120. The formula gives 120 × 133.322387415 ÷ 1 000 = 15.999 kPa at auto-precision (≈ 16.0 kPa). A systolic blood pressure of 120 mmHg is the upper bound of "normal" per the 2017 ACC/AHA hypertension guideline (Normal: SBP < 120 mmHg AND DBP < 80 mmHg); higher values cascade through Elevated (120–129 SBP), Stage 1 (130–139), Stage 2 (≥ 140). European clinical-physics teaching often quotes the same threshold as 16 kPa for SI-trained students. The "exact" badge does NOT fire here because mmHg is a conventional value (ISO 31-3, 133.322 387 415 Pa); even though kilopascals are exact, the conversion inherits the conventional liquid-column status of the source unit. For routine clinical work the gap is negligible — far below the 1–2 mmHg precision of any oscillometric cuff.

10 cmH₂O to pascals (the CPAP ventilator setting)

Set From = Centimeter of water (cmH₂O), To = Pascal, Value = 10. The formula gives 10 × 98.0665 = 980.665 Pa ≈ 0.98 kPa ≈ 0.0097 atm. A CPAP machine treating obstructive sleep apnoea typically delivers 4–20 cmH₂O of continuous positive airway pressure; the most common starting setting in clinical practice is around 8–10 cmH₂O. Mechanical-ventilation pressure-support modes for adults with normal lung compliance start at 12–15 cmH₂O. The cmH₂O unit survives in clinical and biomedical engineering precisely because it matches the natural scale of airway and pulmonary pressures, and the relationship 1 cmH₂O ≈ 1 mbar / 1.02 keeps it numerically close to the meteorologist's millibar without being identical. Like mmHg, this conversion is tagged inexact — the 98.0665 Pa per cmH₂O assumes water at 1 000 kg/m³, which drifts to ~999.972 kg/m³ at room temperature.

1 086 bar Mariana Trench bottom to psi and atmospheres (the deepest pressure on Earth)

Set From = Bar, To = Pound per square inch (psi), Value = 1086. The formula gives 1 086 × 100 000 ÷ 6 894.757293168361 = 15 751.10 psi at auto-precision. To atmospheres: 1 086 × 100 000 ÷ 101 325 = 1 071.80 atm. To megapascals: 1 086 × 0.1 = 108.6 MPa. This is the pressure at Challenger Deep (~10 935 m below sea level), the deepest point in any ocean, measured by the bathyscaphe Trieste's 1960 descent and re-verified by multiple expeditions since — a closed historical anchor that does not decay with time. At this depth, seawater is roughly 5 % denser than at the surface, and any unprotected human or conventional steel pressure vessel collapses instantly. The bar unit dominates oceanographic specifications because the 1-bar-per-10-m rule of thumb makes mental conversion trivial; for US-trained engineers the equivalent ~16 000 psi is the canonical number quoted in popular accounts.

Comparative table: 1 unit of pressure in pascals and bar

UnitValue in PaValue in barValue in psiValue in mmHgExact?
Pascal (Pa)11e-51.4504e-47.5006e-3yes
Centimetre of water (cmH₂O)98.06659.80665e-41.4223e-20.73556no (conventional)
Millibar (mbar) / Hectopascal (hPa)1001e-31.4504e-20.75006yes
Torr133.32236841.3332e-31.9337e-20.999 999…no (irrational quotient)
Millimetre of mercury (mmHg)133.3223874151.3332e-31.9337e-21no (conventional)
Inch of water (inH₂O)249.08892.4909e-33.6127e-21.8683no (conventional)
Kilopascal (kPa)1 0000.010.145047.50062yes
Bar (bar)100 000114.50377750.0617yes
Standard atmosphere (atm)101 3251.0132514.6959759.9999yes
Kilogram-force per cm² (kgf/cm²)98 066.50.98066514.22334735.5592yes
Pound per square inch (psi)6 894.7572936.8948e-2151.71493yes
Megapascal (MPa)1 000 00010145.03777 500.617yes
Five definitional shortcuts worth memorising: 1 hPa = 1 mbar = 100 Pa exact (the WMO meteorological identity); 1 atm = 101 325 Pa = 1.01325 bar = 760 Torr exact (the ISO 2533 standard atmosphere); 1 bar = 100 kPa = 10⁵ Pa exact (the cleanest atmospheric/SI bridge); 1 psi ≈ 6.895 kPa with the full 1 psi = 6 894.757293168361 Pa exact (the IEEE-754 product of IYP-1959 and CGPM-1901 rationals); 1 kgf/cm² = 1 technical atmosphere = 98 066.5 Pa exact (≠ standard atmosphere by 3.21 %). All twelve units cross-convert through the pascal base, and the "exact" column tells you when the underlying conversion is definitionally exact vs conventional.

Pressure Conversion Tips

  • Torr is not mmHg. They are interchangeable in clinical blood-pressure work but differ by about 5 × 10⁻⁵ Pa per unit (less than 0.000015 %). Torr is defined as exactly 1/760 of a standard atmosphere = 133.32236842105263… Pa (a clean rational definition that produces an irrational IEEE-754 binary representation). mmHg is defined as exactly 133.322387415 Pa by NIST SP 811 / ISO 31-3, from a mercury column of density 13.5951 g/cm³ at 0 °C under standard gravity. The two units differ in vacuum metrology and high-precision gas-law work; they are indistinguishable on any clinical sphygmomanometer.
  • 1 hPa = 1 mbar exactly. Meteorology officially uses the hectopascal (since the WMO unit-system harmonisation in the 1960s and 1980s), but every weather hobbyist tool still labels readings in millibars because 1 hPa equals exactly 100 Pa equals exactly 1 mbar. "1 013 hPa" and "1 013 mbar" are the same atmospheric pressure with two labels. Use whichever the audience expects — this calculator lists mbar; convert to hPa by reading the same number.
  • Standard atmosphere (atm) is not technical atmosphere (at = kgf/cm²). 1 atm = 101 325 Pa exact, by 10th CGPM 1954 / ISO 2533. 1 at = 1 kgf/cm² = 98 066.5 Pa exact, by CGPM 1901 standard gravity × 10⁴ cm²/m². They differ by 3 258.5 Pa, about 3.21 %. The technical atmosphere survives in older European industrial drawings (German Druckluft specs, Russian and Czech hydraulic catalogues from before the 1990s); modern engineering uses bar or pascal. If a spec just says "atm" with no qualifier, it almost always means the standard atmosphere, but the difference matters on calibration certificates.
  • Memorise the six exact pivots through the pascal: bar (100 000), kilopascal (1 000), megapascal (1 000 000), millibar/hectopascal (100), standard atmosphere (101 325) and psi (6 894.757293168361). Every conversion in this tool is implemented as "input × from-factor ÷ to-factor", so memorising these six gives you the entire exact cross-table in your head. Add the technical atmosphere (kgf/cm² = 98 066.5) for legacy European drawings, and the four liquid-column units (torr, mmHg, cmH₂O, inH₂O) for clinical and HVAC work.
  • Use the precision selector to match your audience. Tyre-pressure work rounds to whole psi or kPa (32 psi, not 32.0046 psi). Engineering specs quote 2–4 decimals. Vacuum metrology and high-altitude aerospace cabin-pressure work need 6–10 decimals because rounding error stacks in differential pressure chains. Metrologists calibrating against a primary pressure standard (dead-weight tester, gas piston gauge) want 15 decimals — the full IEEE-754 double mantissa exposed by this calculator. Switch precision at the final reporting step to keep rounding error out of intermediate math.
  • Tyre pressure is gauge, not absolute. A car-tyre spec of 32 psi means 32 psi above ambient atmospheric pressure — so the absolute pressure inside the tyre is about 32 psi + 14.7 psi (1 atm) ≈ 46.7 psi absolute = 322 kPa absolute. The distinction matters for compressed-air calculations, hyperbaric medicine, and any time you cross-check a gauge reading against a thermodynamic property table that uses absolute pressure. Industrial sensors are usually labelled psig (gauge) or psia (absolute); this calculator does pure unit math and does not add or subtract atmospheric — the caller decides whether the input is gauge or absolute.
  • HVAC duct static pressure lives in the inches-of-water range, not psi. A typical US residential air handler runs at 0.3–0.6 inH₂O (75–150 Pa) total external static pressure, and anything above 0.9 inH₂O (224 Pa) signals an airflow restriction. Commercial low-velocity systems run below 2 inH₂O (498 Pa); high-velocity VAV systems run above. The cmH₂O and inH₂O units survive in HVAC and respiratory medicine precisely because the natural scale of those applications is sub-kilopascal, and quoting 100 Pa as "0.4 inH₂O" reads more cleanly to a duct designer than "0.001 bar" or "1 mbar".
  • When the result is larger than 10¹² (one trillion) or smaller than 10⁻³ at auto-precision, the display switches to scientific notation. This is on purpose: writing the Mariana Trench bottom pressure (~1.086 × 10⁸ Pa) as 108 600 000 Pa is fine but its conversion to millibar (1 086 000 mbar) or inches of water (~436 000 inH₂O) is more readable as 1.086e6 mbar or 4.36e5 inH₂O at a glance.
  • Every SI / industrial / atmospheric unit in this set is exact. Pa, kPa, MPa, bar, mbar, atm, psi and kgf/cm² all have rational factors that fit cleanly in IEEE-754 double precision. The four liquid-column units (torr, mmHg, cmH₂O, inH₂O) are conventional values and marked inexact — not because the math is sloppy, but because the underlying physical assumption (mercury density at 0 °C, water density at 4 °C) is an idealisation that drifts with real temperature. The "exact" badge fires when both endpoints are in the exact set; otherwise it stays silent.
  • A psi result above ~14 500 psi (≈ 100 MPa, 1 000 bar) lives in the world of deep-sea exploration, high-pressure forging, and water-jet cutting. Below 0.01 psi (≈ 69 Pa, 0.69 mbar) you are in HVAC duct static-pressure land or low-vacuum work. Below 0.001 psi (≈ 6.9 Pa) you are in true vacuum metrology, where torr and mmHg are still standard despite their conventional status.

Pressure Converter — Frequently Asked Questions

Is this pressure converter free?

Yes. The calculator is free, requires no account, runs entirely in your browser, and is ad-free. The embeddable iframe version at /en-US/widget/pressure-converter is also free and ad-free, so you can drop it into mechanical-engineering portals, HVAC blogs, dive-school sites, weather-hobby pages or medical-physics teaching material without exposing readers to third-party trackers.

How accurate are the pressure conversion factors?

Eight of the twelve units are definitionally exact and tagged accordingly. Pa, kPa, MPa, bar, mbar are exact by SI / BIPM definition. Standard atmosphere (101 325 Pa) is exact by 10th CGPM 1954 and ISO 2533:1975. psi (6 894.757293168361 Pa) is exact as the IEEE-754 product of NIST-exact upstream rationals (0.0254 m/in and 4.4482216152605 N/lbf). kgf/cm² (98 066.5 Pa) is exact by CGPM 1901 standard gravity. The four liquid-column units (torr, mmHg, cmH₂O, inH₂O) are conventional values from NIST SP 811 / ISO 31-3 and tagged inexact, because they depend on idealised mercury or water density.

What is the difference between Torr and mmHg?

Torr is defined as exactly 1/760 of a standard atmosphere = 101 325/760 = 133.32236842105263… Pa (an exact rational definition that yields an irrational IEEE-754 binary representation). mmHg is defined as exactly 133.322387415 Pa by NIST SP 811 / ISO 31-3, derived from a mercury column at 13.5951 g/cm³ density and standard gravity. The two differ by about 5 × 10⁻⁵ Pa per unit — less than 0.000015 %. They are interchangeable in clinical blood-pressure work and routine vacuum gauges; they are distinct in primary vacuum metrology and high-precision gas-law work.

Is 1 hPa the same as 1 mbar?

Yes, exactly. 1 hectopascal = 100 Pa = 1 millibar by definition. Meteorology officially adopted the hectopascal in 1986 (WMO), but mbar persists in weather hobby and older atmospheric science publications. Both refer to the same physical quantity with no measurement difference.

How many pascals in 1 atmosphere?

Exactly 101 325 Pa. This is the standard atmosphere, fixed by the 10th CGPM in 1954 and codified in ISO 2533:1975. By definition, 1 atm also equals 1.01325 bar, 760 Torr, and approximately 14.6959 psi — the anchor for every atmospheric pressure conversion.

How many psi in 1 bar?

Approximately 14.50377 psi. The exact arithmetic is 100 000 Pa/bar ÷ 6 894.757293168361 Pa/psi = 14.503773773020924… psi/bar — a mathematically irrational decimal, even though both endpoints are exact rationals. Most casual references quote 14.5 psi or 14.504; this calculator will show up to 15 decimal digits if you ask.

How much pressure does a typical car tyre run at?

Most passenger-car tyres run at 32–35 psi cold, equivalent to 2.20–2.41 bar or 220–241 kPa. The owner's manual sticker (usually inside the driver's door jamb) shows the manufacturer-specified value, often in all three units. Light trucks and SUVs typically run higher (35–40 psi); high-load commercial tyres can exceed 100 psi (6.9 bar). Tyre pressure specs are gauge pressure (above ambient), not absolute.

What pressure is at the bottom of the Mariana Trench?

About 1 086 bar (15 750 psi or 108.6 MPa) at Challenger Deep, the deepest point of the Mariana Trench at roughly 10 935 m below sea level. The water column above exerts approximately 1 071.8 standard atmospheres of pressure, measured by the 1960 Trieste descent and re-verified by later expeditions. At this depth, seawater is about 5 % denser than at the surface.

What pressure setting do CPAP machines use?

CPAP machines for obstructive sleep apnoea deliver between 4 and 20 cmH₂O of continuous positive airway pressure, with most patients prescribed 6–14 cmH₂O depending on apnoea severity and BMI. The typical starting setting is around 8–10 cmH₂O; 10 cmH₂O converts to 980.665 Pa or 0.0097 atm. Mechanical-ventilator pressure-support modes start higher, typically 12–15 cmH₂O for adults with normal lung compliance.

Is 1 kgf/cm² the same as 1 atmosphere?

No. 1 kgf/cm² is the technical atmosphere = exactly 98 066.5 Pa (from CGPM 1901 standard gravity). The standard atmosphere = exactly 101 325 Pa (from 10th CGPM 1954 / ISO 2533). They differ by 3 258.5 Pa, about 3.21 %. The technical atmosphere shows up in older European industrial drawings (especially German, Russian and Czech hydraulic specs from before the 1990s metrication push), but modern engineering uses bar or pascal. If a spec just says "atm" with no qualifier, treat it as the standard atmosphere and flag the ambiguity.

Why is psi tagged exact but mmHg is not?

Because psi inherits its definition from two exact upstream constants: the IYP-1959 inch (0.0254 m exact) and the CGPM-1901 pound-force (4.4482216152605 N exact). The product 4.4482216152605 / 0.0254² = 6 894.757293168361 Pa is the exact IEEE-754 product of NIST-exact rationals. mmHg, by contrast, is a conventional value (133.322 387 415 Pa per NIST SP 811 / ISO 31-3) that depends on an empirical mercury density at 0 °C; that density is an idealisation, so the converter tags mmHg inexact even though the published value is fixed.

What is the difference between gauge pressure and absolute pressure?

Gauge pressure is measured relative to ambient atmospheric pressure (so a tyre at "32 psi gauge" is actually 32 psi above the surrounding air). Absolute pressure is measured relative to a perfect vacuum (so the same tyre is at ~46.7 psi absolute at sea level). This converter does pure unit math and does not add or subtract atmospheric pressure — if the input is gauge, the output is gauge; if the input is absolute, the output is absolute. Industrial sensors use the suffixes psig (gauge), psia (absolute), and barg / bara likewise. Convert thoughtfully when crossing between the two reference frames.

How much pressure does a steam boiler operate at?

Under ASME BPVC Section IV, a low-pressure heating boiler operates at or below 15 psig steam (103.4 kPa) or 160 psig hot water (1 103 kPa), with outlet temperature ≤ 250 °F (121 °C). Above those thresholds the boiler falls under Section I (high-pressure power boilers), where industrial units commonly run 150–600 psi (1–4 MPa) and power-generation supercritical units exceed 3 000 psi (≈ 21 MPa). The 15 psi threshold is the canonical low/high split in US boiler code.

Can I embed this pressure converter on my site?

Yes. The embeddable version lives at /en-US/widget/pressure-converter; copy the iframe snippet from the embed page. The iframe is ad-free, dependency-free, mobile-responsive, and inherits no third-party trackers — useful for mechanical-engineering and HVAC portals, scuba-diving training sites, weather-hobby pages, respiratory-therapy teaching materials, and physics-education content that needs a converter without ad noise.

Key Pressure Conversion Terms

Pascal (Pa)

The SI derived unit of pressure, defined as 1 Pa ≡ 1 N/m² ≡ 1 kg/(m·s²) — the pressure exerted by one newton of force distributed over a one-square-metre area. Named after Blaise Pascal and adopted as the SI unit of pressure by the 14th CGPM in 1971. Standard for all modern scientific publishing and engineering specifications. Wikidata: Q44395.

Kilopascal (kPa) and Megapascal (MPa)

SI multiples of the pascal equal to exactly 1 000 Pa and 1 000 000 Pa respectively. The kilopascal is the dominant atmospheric-pressure unit in metric weather reports outside meteorology (where hPa is standard), in automotive tyre-pressure specs in metric markets, and in cardiology when expressing blood pressure in SI form (16 kPa systolic). The megapascal is standard in materials science (yield strength), hydraulic engineering (system pressures), and high-pressure forming.

Bar (bar) and Millibar (mbar)

Non-SI units of pressure accepted by BIPM for use with the SI. 1 bar = exactly 100 000 Pa, chosen historically to approximate the standard atmosphere (1 atm = 1.01325 bar). 1 millibar = exactly 100 Pa, numerically identical to the hectopascal (1 hPa = 1 mbar) and standard in weather-hobby reporting; the WMO officially switched meteorology to the hectopascal in the 1980s but kept the same scale. Wikidata: Q103510 (bar).

Standard atmosphere (atm)

A non-SI unit of pressure equal to exactly 101 325 Pa, fixed by the 10th CGPM in 1954 and codified in ISO 2533:1975 as the reference sea-level pressure of the International Standard Atmosphere. Equivalent to exactly 1 013.25 hPa, exactly 1.01325 bar, exactly 760 Torr, and approximately 14.6959 psi. The anchor for every atmospheric pressure conversion. Wikidata: Q177974.

Torr

A non-SI unit of pressure defined as exactly 1/760 of a standard atmosphere = 101 325/760 = 133.32236842105263… Pa. Named after Evangelista Torricelli, the Italian physicist who built the first mercury barometer in 1644. The 101 325/760 definition is an exact rational, but its IEEE-754 binary representation is irrational, so the converter tags it inexact. Differs from mmHg by ~5 × 10⁻⁵ Pa (< 0.000015 %). Standard in vacuum technology and historical chemistry. Wikidata: Q185648.

Millimetre of mercury (mmHg)

A non-SI unit of pressure equal to exactly 133.322387415 Pa, defined by NIST SP 811 and ISO 31-3 as the pressure at the base of a mercury column 1 mm high at mercury density 13.5951 g/cm³ (the reference value at 0 °C) under standard gravity. The dominant unit for arterial blood pressure measurement (a systolic reading of 120 mmHg is the upper bound of normal per the 2017 ACC/AHA guideline). Distinct from Torr by an empirical convention; the two are interchangeable in clinical work but not in vacuum metrology.

Pound-force per square inch (psi, lbf/in²)

A US customary and imperial unit of pressure equal to exactly 6 894.757293168361 Pa — the IEEE-754 product of the IYP-1959 inch (0.0254 m exact) and the CGPM-1901 pound-force (4.4482216152605 N exact). Dominant in US engineering, automotive (tyre pressure 32 psi cold), aerospace (cabin altitude in psi), and process industries. The variants psig (gauge), psia (absolute), and psid (differential) are common on industrial pressure transducers and instrumentation labels. Wikidata: Q626299.

Kilogram-force per square centimetre (kgf/cm², technical atmosphere)

A non-SI unit of pressure equal to exactly 98 066.5 Pa — the pressure exerted by one kilogram of mass spread over one square centimetre under standard gravity (CGPM 1901, gn = 9.80665 m/s²). Also called the technical atmosphere (at). Differs from the standard atmosphere by 3.21 %; common in older European hydraulic and pneumatic catalogues (German, Russian, Czech industrial drawings before the 1990s metrication push).

Centimetre of water (cmH₂O) and inch of water (inH₂O)

Conventional liquid-column units of pressure. 1 cmH₂O ≈ 98.0665 Pa, 1 inH₂O ≈ 249.0889 Pa, both based on a water density of exactly 1 000 kg/m³ at 4 °C under standard gravity. Both are tagged inexact because real water density at room temperature is 999.972 kg/m³ — the conventional value drifts by ~28 ppm versus the physical reality. cmH₂O is the standard unit for CPAP / ventilator airway pressure (4–20 cmH₂O range) and clinical respiratory-mechanics measurements. inH₂O is standard for HVAC duct static pressure (typical residential range 0.3–0.6 inH₂O) and low-pressure pneumatic-instrumentation work.

Pressure

The physical quantity defined as force per unit area, with SI dimension M·L⁻¹·T⁻². Always a scalar in continuum mechanics (one of the three principal invariants of the stress tensor in an isotropic fluid). The recommended SI unit is the pascal (1 Pa = 1 N/m²); the kilopascal and megapascal cover the everyday and high-pressure engineering ranges, while bar, atmosphere and the liquid-column units survive for atmospheric, medical and HVAC applications. Wikidata: Q39552.

Standard gravity (g_n)

The conventional gravitational acceleration on Earth's surface, fixed at exactly 9.80665 m/s² by the 3rd CGPM in 1901. Not a measured value at any particular point (real values range from 9.764 to 9.834 m/s² over Earth's surface), but an exact defined constant that anchors every gravitational pressure unit (kgf/cm², cmH₂O, inH₂O, mmHg) to the SI pascal. Wikidata: Q13400897.

ISO 2533:1975 (Standard Atmosphere)

International Organization for Standardization document that codifies the International Standard Atmosphere (ISA), fixing sea-level reference pressure at exactly 101 325 Pa, sea-level reference temperature at 288.15 K (15 °C), and the lapse-rate / pressure-altitude profile used in aviation, ballistics and aerospace engineering. The downstream definition of atm, mbar and hPa as 1.01325 bar / 1 013.25 mbar / 1 013.25 hPa all flow from this document.

Exact factor

A conversion factor fixed by international definition or international agreement, with zero rounding error. For pressure, the exact factors in this calculator are 1 Pa = 1 (SI base), 1 kPa = 1 000 Pa, 1 MPa = 1 000 000 Pa, 1 bar = 100 000 Pa, 1 mbar = 100 Pa, 1 atm = 101 325 Pa, 1 psi = 6 894.757293168361 Pa, 1 kgf/cm² = 98 066.5 Pa. The conventional (inexact) factors are 1 Torr = 133.32236842105263… Pa, 1 mmHg = 133.322387415 Pa, 1 cmH₂O = 98.0665 Pa, 1 inH₂O = 249.0889 Pa.

Significant figures

The digits in a numerical result that carry meaning about its precision. Auto-precision in this calculator targets 6 significant figures, the engineering-tables default. NIST SP 811 pressure factors are typically quoted to 7–9 significant figures so that any rational rounding of an exact factor is unambiguous; the calculator exposes up to 15 decimal digits via the precision selector for metrology and vacuum-standards work.

Kilder og referencer

  1. NIST SP 811 Appendix B.8 — Factors for units listed alphabetically (pressure section: standard atmosphere 101 325 Pa exact, kgf/cm² 98 066.5 Pa exact, psi 6 894.757 Pa as rounded NIST printing of the exact IEEE-754 product 6 894.757293168361 Pa, mmHg 133.322 387 415 Pa conventional)
  2. NIST Guide to the SI, Appendix B — Conversion factors (landing page with the canonical bold-is-exact convention used across pressure, force, length and torque units)
  3. BIPM — The International System of Units (SI Brochure, 9th edition 2019): the pascal (1 Pa ≡ 1 N/m²) is the SI derived unit of pressure; the bar (= 10⁵ Pa exact) and the millimetre of mercury are listed as non-SI units accepted for use with the SI in specific application domains
  4. BIPM — Resolution of the 3rd CGPM (1901): declaration that standard gravity g_n = 9.80665 m/s² exact, the upstream constant that anchors every gravitational pressure unit (kgf/cm² = 98 066.5 Pa exact, cmH₂O = 98.0665 Pa conventional, inH₂O = 249.0889 Pa conventional, mmHg = 133.322 387 415 Pa conventional)
  5. ISO 2533:1975 — Standard Atmosphere: fixes sea-level reference pressure at exactly 101 325 Pa, sea-level reference temperature at 288.15 K, and the lapse-rate / pressure-altitude profile used in aviation, ballistics and aerospace engineering. The downstream definitions of 1 atm = 1.01325 bar = 1 013.25 hPa = 760 Torr all flow from this document
  6. Wikipedia — Pascal (unit): SI derived unit of pressure equal to 1 N/m²; covers the relationship with the hectopascal (hPa = mbar) for meteorology and the kilopascal/megapascal multiples used in engineering (Wikidata Q44395)
  7. Wikipedia — Bar (unit): non-SI unit of pressure equal to exactly 100 000 Pa = 10⁵ Pa; widely used in meteorology, oceanography (1 bar ≈ 10 m seawater depth) and process engineering; the millibar (mbar) is numerically identical to the hectopascal (hPa) (Wikidata Q103510)
  8. Wikipedia — Pound per square inch: US customary unit of pressure equal to exactly 6 894.757293168361 Pa, the IEEE-754 product of the IYP-1959 inch (0.0254 m exact) and the CGPM-1901 pound-force (4.4482216152605 N exact); explains psig vs psia vs psid variants used on US industrial sensors (Wikidata Q626299)
  9. Wikipedia — Torr: non-SI unit of pressure defined as exactly 1/760 of a standard atmosphere = 101 325/760 ≈ 133.3223684 Pa; named after Evangelista Torricelli (1644 mercury barometer); explains the < 0.000015 % gap versus mmHg (the two units are interchangeable in clinical work but distinct in vacuum metrology) (Wikidata Q185648)
  10. Wikipedia — Standard atmosphere (unit): atm = exactly 101 325 Pa (10th CGPM 1954, codified in ISO 2533:1975); equivalent to exactly 760 Torr, exactly 1.01325 bar, exactly 1 013.25 hPa, and approximately 14.6959 psi — the anchor for every atmospheric pressure conversion (Wikidata Q177974)

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