Unit Converter

Convert between length, mass, volume, area, temperature, pressure, energy, power, force, torque, and angle units with NIST-grade exact factors.

0.3048 m

1 ft= 0.3048 m

Decimals

All units

Unit	Value
Millimeter (mm)	304.8
Centimeter (cm)	30.48
Meter (m)	0.3048
Kilometer (km)	3.048E-4
Inch (in)	12
Foot (ft)	1
Yard (yd)	0.333333
Mile (mi)	1.89394E-4
Nautical mile (nmi)	1.64579E-4
Ångström (Å)	3,048,000,000
Astronomical unit (AU)	2.03746E-12
Light-year (ly)	3.22174E-17
Parsec (pc)	9.8779E-18

Mass Converter Length Converter

Updated: May 20, 2026

Unit converter. Ten measurement categories on one page with NIST-exact factors and shareable URL state.

A unit converter switches a value across ten measurement categories (length, volume, area, temperature, pressure, energy, power, force, torque, angle). It uses NIST and BIPM exact factors and tags every result as exact or approximate so engineers, students and casual users can tell definitionally exact conversions from ones rounded by IEEE-754 doubles.

What Is a Unit Converter?

A unit converter is a tool that takes a value expressed in one unit and returns the equivalent value in any other supported unit. This calculator covers ten measurement categories on one canonical URL: length (13 units, metre to parsec), volume (12 units, millilitre to UK gallon), area (10 units, square millimetre to square mile), temperature (5 scales, kelvin to Réaumur), pressure (12 units, pascal to inch of water), energy (11 units, joule to electronvolt), power (9 units, watt to mechanical horsepower), force (8 units, newton to short ton-force), torque (7 units, newton-metre to ounce-inch) and angle (7 units, radian to arcsecond). Every conversion routes through a single SI base unit per category — metre for length, cubic metre for volume, kelvin for temperature, pascal for pressure, joule for energy, watt for power, newton for force, newton-metre for torque, radian for angle — using factors sourced directly from NIST SP 811 Appendix B.8, the BIPM SI Brochure 9th edition, ISO 80000-3 through 80000-5, the 1959 International Yard and Pound Agreement, and the relevant IAU resolutions.

Most casual converters merge all categories into one giant dropdown of 2,000 units and treat every factor as equally trustworthy. This converter does the opposite: it groups units by physical quantity, exposes the underlying SI pivot, and labels every result with an exact or approximate badge. A length-to-length conversion between metre and inch is exact (both factors are integer-friendly rationals defined by the 1959 IYP); a temperature conversion between Celsius and Fahrenheit is tagged approximate because the 5/9 slope is non-terminating in IEEE-754 binary even though it is symbolically exact by the 13th CGPM 1967 Resolution 4. An angle conversion between radian and degree is approximate because π/180 is irrational and any 64-bit double of it is rounded at the 16th significant digit. The honesty about exact-versus-approximate is what separates a converter built for engineers from one built for ad impressions.

The other thing that separates a serious converter from a marketing widget is shareable state. Every choice you make in this tool — category, source unit, target unit, input value, precision — is encoded in the URL query string as ?cat=length&from=meter&to=foot&v=1&p=auto. Click the result card to copy the converted value to the clipboard; copy the browser URL to share the exact conversion you are looking at. There is no per-pair URL explosion (no /miles-to-kilometers route, no /celsius-to-fahrenheit route); the canonical URL is /tools/unit-converter and every conversion is a query-string permutation of it. Engineering portals, school materials, technical wikis, and stackoverflow comments can link to the exact conversion the reader needs without sending them through a search funnel.

How to Convert Between Any Units

Every conversion in this tool is two operations through a single SI base unit per category. For purely multiplicative quantities (length, volume, area, pressure, energy, power, force, torque, angle) the formula is:

y = x \cdot \frac{a_{\text{from}}}{a_{\text{to}}}

where x is your input value, a_from is the source unit's factor to the SI base, and a_to is the target unit's factor to the same base. For temperature — the only quantity with non-zero zero points relative to its absolute scale — the formula adds an offset and becomes affine:

T_{\text{out}} = \frac{(T_{\text{in}} \cdot a_{\text{from}} + b_{\text{from}}) - b_{\text{to}}}{a_{\text{to}}}

To do it by hand, follow the same four steps regardless of category:

1. Look up the source-to-base factor. For feet to metres, a_from = 0.3048. For psi to pascals, a_from = 6894.757293168361. For Btu (IT) to joules, a_from = 1055.05585262.

2. Multiply the input by that factor to get the value in the SI base unit. 100 ft × 0.3048 = 30.48 m. 50 psi × 6894.757293168361 = 344,737.86 Pa. 1 Btu × 1055.05585262 = 1055.05585262 J.

3. Look up the target-to-base factor. For metres to inches, a_to = 0.0254. For pascals to bar, a_to = 100,000. For joules to kilowatt-hours, a_to = 3,600,000.

4. Divide the base value by the target factor. 30.48 m ÷ 0.0254 = 1200 in. 344,737.86 Pa ÷ 100,000 = 3.4474 bar. 1055.05585262 J ÷ 3,600,000 = 0.00029307 kWh.

The same procedure works for every supported unit in every category. The one trap is temperature: a 5 °C change is not 41 °F (which would be the value at 5 °C); it is a 9 °F change, because the offset cancels in differences but not in absolute readings. The calculator handles this correctly by routing through the offset-aware affine formula.

To use this calculator, pick a measurement category from the sticky category strip at the top — Length, Volume, Area, Temperature, Pressure, Energy, Power, Force, Torque or Angle. Pick a source unit from the From searchable dropdown, type a value, then pick a target unit from the To dropdown. The result updates on every keystroke; no Convert button. Click the hero result card to copy the converted value to the clipboard. Use the precision selector to switch between auto (6 significant figures, scientific notation outside the 10⁻³ to 10¹² range), or a fixed 0, 2, 4, 6, 10 or 15 decimals. The exact badge appears next to the result whenever both the source and target units are definitionally exact and the conversion does not lose information in IEEE-754; approximate fires when the slope or factor is irrational in binary (5/9 for Fahrenheit and Rankine, π/180 for degree, π/200 for gradian, π/648000 for arcsecond) or when one endpoint is too large to fit a 64-bit double without rounding (light-year, parsec).

Unit Conversion Formulas

y = x \cdot \frac{a_{\text{from}}}{a_{\text{to}}}

$y$ = The converted value, expressed in the target unit.
$x$ = The input value, expressed in the source unit.
$a_{\text{from}}$ = Factor that converts the source unit to its category's SI base unit (e.g. 0.3048 for foot to metre, 6894.757293168361 for psi to pascal, 1055.05585262 for Btu_IT to joule).
$a_{\text{to}}$ = Factor that converts the target unit to the same SI base unit (e.g. 1000 for kilometre to metre, 100000 for bar to pascal, 3.6e6 for kilowatt-hour to joule).

The general multiplicative formula above covers nine of the ten supported categories (length, volume, area, pressure, energy, power, force, torque, angle). The tenth — temperature — uses an affine form because Celsius, Fahrenheit and Réaumur have non-zero zero points relative to the absolute (Kelvin) scale: T_out = ((T_in × a_from + b_from) − b_to) / a_to. Every factor table this calculator uses is sourced from primary metrology references:

Length: NIST SP 811 Appendix B.8 (inch 0.0254 m, foot 0.3048 m, mile 1609.344 m, all exact via the 1959 International Yard and Pound Agreement) + IAU Resolution B2 2012 for the astronomical unit (149,597,870,700 m exact).
Volume: NIST SP 811 Appendix B.8 (US gallon 3.785411784e-3 m³, US fluid ounce 2.95735295625e-5 m³, both exact) + UK Weights and Measures Act 1985 (UK gallon 4.54609e-3 m³ exact).
Area: derived from length (acre 4046.8564224 m² exact, square mile 2589988.110336 m² exact).
Temperature: 13th CGPM 1967 Resolution 4 (0 °C = 273.15 K exact) + BIPM SI Brochure §2.3.1 (kelvin) + ISO 80000-5:2019.
Pressure: NIST SP 811 (psi 6894.757293168361 Pa exact, atmosphere 101325 Pa exact via ISO 2533) + 3rd CGPM 1901 (g_n = 9.80665 m/s² exact, anchoring kgf/cm² and water-column units).
Energy: NIST SP 811 (Btu_IT 1055.05585262 J exact, calorie_th 4.184 J exact, calorie_IT 4.1868 J exact) + 2019 SI redefinition (electronvolt 1.602176634e-19 J exact via fixed elementary charge).
Power: NIST SP 811 (mechanical hp 745.6998715822702 W exact, metric hp 735.49875 W exact via CGPM 1901, electric hp 746 W exact via IEEE).
Force: NIST SP 811 (pound-force 4.4482216152605 N exact, kilogram-force 9.80665 N exact, ounce-force 0.27801385095378125 N exact).
Torque: derived from force × length (pound-foot 1.3558179483314003 N·m exact, kgf·m 9.80665 N·m exact).
Angle: BIPM SI Brochure §2.3.4 (radian) + ISO 80000-3:2019 (degree π/180 rad, gradian π/200 rad, arcsecond π/648000 rad — all symbolically exact, all irrational in IEEE-754).

The exact badge fires only when both endpoints are factors representable bit-perfectly as 64-bit doubles. So metre ↔ inch is exact (both factors are finite binary fractions), psi ↔ kPa is exact, joule ↔ kilowatt-hour is exact. But degree ↔ radian is approximate (π/180 is irrational), Celsius ↔ Fahrenheit is approximate (5/9 is non-terminating in binary), and light-year ↔ kilometre is approximate (the integer metre value of a light-year is too large to fit a double without rounding the last digit). The badge is informational honesty, not a quality complaint.

Worked Conversion Examples Across All Ten Categories

Length: 1 mile to kilometres (exact since 1959)

Pick Length, set From = Mile, To = Kilometer, Value = 1. The formula gives 1 × 1609.344 / 1000 = 1.609344 km exact. Both factors are fixed by the 1 July 1959 International Yard and Pound Agreement (1 mi = 1609.344 m exact, 1 km = 1000 m exact), so the conversion earns the exact badge. The 1.61 km approximation you see in driving handbooks accumulates 224 m of drift over 1000 km — fine for road trips, fatal for surveying.

Volume: 1 US gallon to litres

Pick Volume, set From = US gallon, To = Liter, Value = 1. The US gallon is defined as exactly 231 cubic inches (NIST SP 811 Appendix B.8), which works out to 3.785411784e-3 m³ exact. Dividing by 1e-3 m³/L gives 3.785411784 L exact. Note this is not the UK (imperial) gallon, which is 4.54609 L exact by the UK Weights and Measures Act 1985 — the two differ by ~20%, a constant source of confusion in cross-Atlantic recipes.

Area: 1 acre to square metres

Pick Area, set From = Acre, To = Square meter, Value = 1. The international acre is 4840 square yards exact, and the international yard is 0.9144 m exact, so 1 acre = 4840 × 0.9144² = 4046.8564224 m² exact. This is the value you find on every official land-survey reference; the 4047 m² rounding common in real-estate listings is off by 0.0035%, irrelevant for property tax but visible in cadastral GIS work.

Temperature: 100 °C to Fahrenheit (the boiling-water anchor)

Pick Temperature, set From = Celsius, To = Fahrenheit, Value = 100. The affine formula gives 100 × 9/5 + 32 = 180 + 32 = 212 °F in symbolic form. At 15-decimal precision the IEEE-754 representation reads 211.99999999999997 °F — a single ulp short of 212, the visible signature of the irrational 5/9 slope. The result is honestly tagged approximate because the slope is non-terminating in binary, even though the legal Fahrenheit anchor 32 °F = 0 °C is exact by the 13th CGPM 1967.

Pressure: 1 atm to psi (the sea-level anchor)

Pick Pressure, set From = Atmosphere, To = Pound per square inch, Value = 1. The standard atmosphere is 101325 Pa exact (ISO 2533:1975, 10th CGPM 1954). Dividing by the psi factor 6894.757293168361 Pa gives 14.69594877551345 psi — the value you find on every pressure regulator and SCUBA tank gauge worldwide. The conversion is tagged exact in this calculator because both endpoints are exact rationals in IEEE-754. Typical engineering shorthand 14.696 psi is a 5-significant-figure rounding.

Energy: 1 kWh to joules and Btu

Pick Energy, set From = Kilowatt-hour, To = Joule, Value = 1. By definition 1 kWh = 1 kW × 3600 s = 3,600,000 J = 3.6 MJ exact. Switch To = Btu (IT) and the result is 3,600,000 / 1055.05585262 = 3412.142 Btu — the value on the back of every US gas meter. The Btu_IT factor of 1055.05585262 J is exact per NIST SP 811 Appendix B.8, even though it was originally derived from the International Table calorie (4.1868 J × 251.99576... heat capacity); NIST fixes the rounded value as the canonical definition.

Power: 250 mechanical hp to kilowatts (US car spec)

Pick Power, set From = Mechanical horsepower, To = Kilowatt, Value = 250. Mechanical hp = 550 ft·lbf/s = 745.6998715822702 W exact (NIST SP 811, derived from the 1959 IYP and CGPM 1901 standard gravity). The result is 250 × 745.6998715822702 / 1000 = 186.4250 kW. Note this is NOT the same as 250 metric hp (PS) — that would be 250 × 735.49875 / 1000 = 183.875 kW. The 1.5% difference is why US, European and Japanese car specs use different hp variants; misusing them is a common engineering error this calculator surfaces.

Force: 1 kgf to newtons and pound-force

Pick Force, set From = Kilogram-force, To = Newton, Value = 1. The kilogram-force is defined as the gravitational pull on a 1 kg mass at standard gravity g_n = 9.80665 m/s² (3rd CGPM 1901, exact), so 1 kgf = 9.80665 N exact. Switch To = Pound-force and the result is 9.80665 / 4.4482216152605 = 2.20462 lbf — the same numerical value as the kg-to-pound mass conversion (because both are anchored to the same kg ↔ lb mass ratio of 0.45359237). Engineers use kgf and lbf interchangeably with kg and lb in many older datasheets; the calculator treats them as their own units to keep the dimensional check honest.

Torque: 100 N·m to pound-foot (wrench-spec conversion)

Pick Torque, set From = Newton-meter, To = Pound-foot, Value = 100. The pound-foot is defined as lbf × ft = 4.4482216152605 × 0.3048 = 1.3558179483314003 N·m exact (NIST SP 811, IEEE-754 product of two NIST-exact rationals). The result is 100 / 1.3558179483314003 = 73.7562 lb·ft — the value on every torque-wrench manual that lists both SI and US-customary settings. Use this for fastener-spec sheets, axle nuts, and engine head bolts where misreading the torque can crack a casting.

Angle: 1 turn to degrees, radians and arcseconds

Pick Angle, set From = Turn, To = Degree, Value = 1. By definition 1 turn = 2π rad = 360° exact in symbolic form. The calculator returns 360 ° at 0 decimals; at 15 decimals it shows 359.99999999999994 because the conversion goes through 2π in IEEE-754 doubles. Switch To = Radian and the result is 6.283185307179586 rad (the IEEE-754 double of 2π). Switch To = Arcsecond and the result is 1,296,000 arcsec — useful for astronomers comparing CCD plate scales (typical seeing is 1 arcsec, the Moon's angular diameter is 1864 arcsec, the Sun's is 1920 arcsec).

Comparative table: representative units across all ten categories

Category	Unit	Factor to SI base	Exact?
Length	foot (ft)	0.3048 m	yes
Length	mile (mi)	1609.344 m	yes
Volume	US gallon (gal)	3.785411784e-3 m³	yes
Volume	UK gallon	4.54609e-3 m³	yes
Area	acre	4046.8564224 m²	yes
Temperature	Celsius (°C)	slope 1, offset 273.15 K	yes
Temperature	Fahrenheit (°F)	slope 5/9, offset 459.67×5/9 K	no (5/9 irrational in binary)
Pressure	psi	6894.757293168361 Pa	yes
Pressure	atmosphere (atm)	101325 Pa	yes
Energy	Btu (IT)	1055.05585262 J	yes
Energy	kWh	3,600,000 J	yes
Power	mechanical hp	745.6998715822702 W	yes
Power	metric hp (PS)	735.49875 W	yes
Force	pound-force (lbf)	4.4482216152605 N	yes
Torque	pound-foot (lb·ft)	1.3558179483314003 N·m	yes
Angle	degree (°)	π/180 rad	no (π irrational)

Use this table to do conversions by hand: divide source-in-base by target-in-base. For example, 1 mile in feet = 1609.344 / 0.3048 = 5280 ft exact.

Most common conversions people look up

These are the unit pairs people search for most often, with the exact NIST/BIPM values. Paste them into the calculator for full 15-decimal precision.

Metres to feet: 1 m = 3.28084 ft (irrational; 1 ft = 0.3048 m exact)
Inches to centimetres: 1 in = 2.54 cm exact
Kilometres to miles: 1 km = 0.621371 mi
Litres to US gallons: 1 L = 0.264172 gal
Square metres to square feet: 1 m² = 10.7639 ft²
Celsius to Fahrenheit: 0 °C = 32 °F, 100 °C = 212 °F, −40 °C = −40 °F
Bar to psi: 1 bar = 14.5038 psi
Joules to calories (thermochemical): 1 J = 0.239006 cal
Kilowatts to mechanical hp: 1 kW = 1.34102 hp
Newtons to pound-force: 1 N = 0.224809 lbf
Newton-metres to pound-foot: 1 N·m = 0.737562 lb·ft
Degrees to radians: 1° = 0.0174533 rad (π/180)

Unit Conversion Tips

Pivot through the SI base. Every conversion in this tool is implemented as input × from-factor ÷ to-factor through the category's SI base (metre, square metre, cubic metre, kelvin, pascal, joule, watt, newton, newton-metre, radian). Memorising the factor of each unit to its base gives you every cross-conversion for free — and lets you spot-check a result the calculator hands back.
Use the exact and approximate badges to gate decisions. The exact tag means both endpoints are bit-perfectly representable in IEEE-754; the result has zero conversion error and any imprecision is in your input. The approximate tag means at least one factor is irrational in binary (5/9 for Fahrenheit, π for degree, the integer metre value of a light-year) or the legal definition is conventional rather than absolute (torr, mmHg, cmH₂O). For surveying, aerospace and primary metrology, refuse to ship results tagged approximate without thinking about the bound.
Watch out for horsepower variants. Mechanical hp (745.6998715822702 W exact, US/UK car specs), metric hp / PS / CV (735.49875 W exact, European and Japanese car specs), and electric hp (746 W exact, US electric motors) differ by ~1.5%. A 250 hp car spec means 186 kW in the US and 184 kW in Europe — confuse them and you ship a torque calculation off by 2 kW. The calculator surfaces all three so you can pick the one your datasheet meant.
Don't trust unit picker autocomplete in other converters. unitconverters.net, easyunitconverter.com and convertunits.com offer 2,000+ units in one giant searchable list; many of those entries are duplicates with rounded factors (e.g. "international foot" vs "survey foot" — the survey foot was 1200/3937 m until 1 January 2023, when NIST officially retired it and aligned everything to the international foot 0.3048 m). This calculator only ships units that have a single agreed definition and a verifiable primary source.
Share the exact conversion via URL. Every choice — category, source, target, value, precision — is encoded in the query string as ?cat=length&from=meter&to=foot&v=1&p=auto. Copy the URL bar and paste it in a Stack Overflow comment, Slack DM, or technical blog: the recipient lands on the same conversion you were looking at. No login, no account, no waiting for a screenshot.
For 10⁻³ to 10¹² value range, auto-precision shows 6 significant figures; outside that range, the display switches to scientific notation. 1 light-year in millimetres rendered as 9,460,730,472,580,800,000 mm is unreadable; 9.46073e18 mm is parseable at a glance. Use a fixed-decimal setting (0, 2, 4, 6, 10, 15) when you need the same number of decimals across every conversion you copy-paste into a spreadsheet column.
The category strip is sticky on desktop and a Select on mobile, so switching categories doesn't scroll you away from the result. When you change category, the From and To units reset to that category's sensible defaults (metre↔foot for length in en-US, celsius↔fahrenheit for temperature, pascal↔bar for pressure) but the input value and precision setting persist. Use this to do quick cross-category sanity checks.
Use the 15-decimal precision setting to expose IEEE-754 drift, not as a sign the calculator is broken. 100 °C → Fahrenheit at 15 decimals reads 211.99999999999997 instead of 212 because 5/9 has no finite binary expansion. The same conversion at 6 decimals reads 212.000000. This is the same behaviour your MATLAB, Python, or Excel session would show — it is honesty about double-precision arithmetic, not a math bug.

Unit Converter — Frequently Asked Questions

Is this unit converter free?

Yes. The calculator runs entirely in your browser, requires no account, and has no signup wall. The embeddable iframe version is also ad-free, so you can drop it into engineering portals, school materials or technical blogs without exposing readers to third-party trackers. The 10 single-purpose sister converters (length, volume, area, temperature, pressure, energy, power, force, torque, angle) live at the same /tools/ root and are also free.

What measurement categories does it cover?

Length, volume, area, temperature, pressure, energy, power, force, torque and angle. Each category routes through a single SI base unit: metre, cubic metre, square metre, kelvin, pascal, joule, watt, newton, newton-metre and radian. Mass, time, speed, and electrical quantities are not yet on the mega-converter; for those, link to a dedicated single-purpose calculator.

How accurate are the conversion factors?

Every multiplicative factor is sourced from NIST SP 811 Appendix B.8, the BIPM SI Brochure 9th edition, ISO 80000-3 through 80000-5, the 1959 International Yard and Pound Agreement, or the relevant IAU resolution. Factors flagged exact (boldface in NIST SP 811) come through this calculator as definitionally exact rationals; the remainder are tagged approximate either because the legal definition is conventional (torr, mmHg, cmH₂O) or because the slope is irrational in IEEE-754 binary (5/9 for Fahrenheit, π/180 for degree). At 15-decimal precision you are reading the full IEEE-754 mantissa.

Why does the calculator show an exact or approximate badge?

Because not every conversion is mathematically clean. A conversion is tagged exact when both source and target factors are representable bit-perfectly as 64-bit doubles (e.g. metre ↔ inch, pascal ↔ bar, joule ↔ kilowatt-hour). It is tagged approximate when the slope is irrational in IEEE-754 (5/9 for Fahrenheit, π/180 for degree), when the legal definition is itself a conventional rounded value (mmHg = 133.322387415 Pa), or when one endpoint is too large to fit a double without rounding (light-year, parsec). The badge is informational honesty, not a quality complaint.

How does the unit converter formula work?

For purely multiplicative quantities (length, volume, area, pressure, energy, power, force, torque, angle): output = input × source-factor ÷ target-factor, where both factors are expressed relative to the category's SI base unit. For temperature, which has non-zero zero points: output = (input × source-slope + source-offset − target-offset) ÷ target-slope, an affine transformation through the Kelvin scale. Both forms are written out in the Formula section above.

Can I embed this converter on my site?

Yes. The embeddable version lives at /widget/unit-converter; copy the iframe snippet from the embed page. The iframe is ad-free, dependency-free, mobile-responsive, and inherits no third-party trackers — unlike most multi-category converter embeds available today, which serve ads or tracking pixels inside the iframe.

Can I share a specific conversion by URL?

Yes. Category, source, target, value, and precision encode into the query string (?cat=length&from=meter&to=foot&v=1&p=auto). Copy the URL bar — the recipient lands on the same conversion. One canonical URL: /tools/unit-converter.

Is 1 inch exactly 2.54 cm?

Yes. Since 1 July 1959 the inch has been defined as exactly 25.4 mm — equivalently 2.54 cm — by the International Yard and Pound Agreement. It is the only common imperial unit whose metric value is a finite decimal.

How many feet are in a metre?

One metre equals 3.280839895013… feet. The commonly quoted 3.28084 is a six-significant-figure rounding. The exact direction is the inverse: one foot is exactly 0.3048 metres, fixed by the 1959 International Yard and Pound Agreement. For most practical work, 3.28084 ft/m is enough; for surveying or aerospace, take more decimals — at 15-decimal precision this calculator shows 3.280839895013123 ft/m.

What is the difference between a US gallon and an Imperial gallon?

A US gallon is exactly 3.785411784 litres (NIST SP 811 Appendix B.8); an Imperial (UK) gallon is exactly 4.54609 litres (UK Weights and Measures Act 1985). They differ by about 20%, which is why US and UK fuel economy figures cannot be compared directly — 30 mpg (US) is 36 mpg (UK) for the same car. The calculator surfaces both as separate units to keep the dimensional check honest.

Why are there three horsepower units?

Because the industry never agreed on one. Mechanical horsepower (745.6998715822702 W exact) is the US/UK automotive standard, derived from 550 ft·lbf/s. Metric horsepower (735.49875 W exact, also known as PS in German, CV in Romance languages, л.с. in Russian) is the European and Japanese automotive standard, derived from 75 kgf·m/s. Electric horsepower (746 W exact) is the US electric-motor standard set by IEEE C57.12.00. A 250 hp car spec is 186 kW in mechanical hp, 184 kW in metric hp — the 1.5% difference is small but visible on a dyno trace.

How does the converter handle temperature offsets?

Unlike length or mass, temperature scales do not all start at zero relative to the absolute (Kelvin) scale. Celsius and Réaumur both have a 273.15 K offset; Fahrenheit has a 459.67 × 5/9 K offset; Rankine and Kelvin both start at zero (absolute scales). The calculator uses an affine formula — multiply by the slope, add the offset, then subtract the target offset and divide by the target slope — instead of the multiplicative formula used for every other category. This is why the Temperature row in the calculator UI looks identical but the math underneath is different.

Should I use the mega converter or one of the single-quantity ones?

Use the mega converter when you bounce between categories frequently — sketching a building (length + area + volume), tuning an engine (power + torque + force), or planning a trip (length + temperature). Use the single-quantity sister calculators (/tools/length-converter, /tools/temperature-converter, etc.) when you want a deeper dive on one category, including category-specific FAQs, history of the units, and longer worked examples. Both are free, both use the same NIST/BIPM factors, both deep-link by URL.

What precision should I pick?

Auto-precision (6 significant figures) is fine for everyday conversions — recipe rescaling, road-trip distances, weather. Engineers double-checking a datasheet usually want 10 decimals; metrologists comparing against a primary standard want 15 decimals (the full IEEE-754 double mantissa). Construction trades almost never need more than 4. The precision setting is persisted in the URL, so you can save a copy-paste-friendly preset (?p=10 or ?p=4) for your spreadsheet workflow.

Key Unit Conversion Terms

SI base unit

One of the seven units fixed by the International System of Units (BIPM, 2019 redefinition): metre (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount of substance), candela (luminous intensity). Every multiplicative conversion in this calculator pivots through the SI base of the relevant category.

Conversion factor

A multiplicative number that converts a value from one unit to another. NIST SP 811 Appendix B.8 prints exact factors in boldface (1 in = 2.54 cm, 1 lbf = 4.4482216152605 N) and approximate factors in regular type. This calculator labels every result with an exact or approximate badge based on whether both factors are bit-perfectly representable in IEEE-754.

Exact factor

A conversion factor fixed by international definition, with zero rounding error. Examples used in this calculator: 1 in = 0.0254 m (1959 IYP), 1 atm = 101,325 Pa (ISO 2533), 1 kWh = 3,600,000 J (SI), 1 AU = 149,597,870,700 m (IAU 2012 Resolution B2). NIST SP 811 prints these in boldface; this calculator surfaces them with the exact badge.

Approximate factor

A conversion factor whose value is either irrational in IEEE-754 binary (5/9 for Fahrenheit and Rankine slopes, π/180 for degree, π/200 for gradian), conventional rather than definitional (133.322387415 Pa for mmHg, 98.0665 Pa for cmH₂O), or too large to fit a 64-bit double without rounding the last digit (light-year, parsec). The calculator tags any conversion involving these factors as approximate.

IYP 1959

The International Yard and Pound Agreement of 1 July 1959, signed by the US, UK, Canada, Australia, New Zealand and South Africa. Defined the yard as exactly 0.9144 m and the pound as exactly 0.45359237 kg. From these two anchors derive the exact metric values of the inch, foot, mile, acre, pound-force, ounce-force, and dozens of other US-customary and imperial units used in this calculator.

IAU resolution

A resolution adopted by the International Astronomical Union General Assembly. IAU 2012 Resolution B2 fixed the astronomical unit as exactly 149,597,870,700 m. IAU 2015 Resolution B2 fixed the parsec as exactly 648,000/π AU and standardised the light-year as Julian year × c. Used in the astronomical group of the Length category.

CGPM 1901 standard gravity

The 3rd General Conference on Weights and Measures (1901) declared standard gravity g_n = 9.80665 m/s² exactly. This anchor underpins every gravitational-unit conversion in this calculator: kilogram-force (9.80665 N exact), pound-force (lb × g_n = 4.4482216152605 N exact), ton-force, kgf·m torque, and the metric horsepower (75 kgf·m/s = 735.49875 W exact).

Affine conversion

A conversion of the form y = a × x + b, used here for temperature because Celsius, Fahrenheit and Réaumur have non-zero zero points relative to the Kelvin scale. Unlike multiplicative conversions, the offset must be added before the slope is reversed. The calculator handles this automatically — pick Celsius and Fahrenheit and the formula switches to affine without any user intervention.

IEEE-754 double

The 64-bit floating-point format used by JavaScript and most calculators. It carries about 15-17 significant decimal digits. The 15-decimal precision setting in this tool exposes the full mantissa for double-checking against a primary metrology reference. Conversions involving irrational factors (π, 5/9) display IEEE-754 drift at 15 decimals (e.g. 100 °C → 211.99999999999997 °F).

Scientific notation

A way of writing very large or very small numbers as a coefficient times a power of ten (e.g. 1 light-year = 9.46073 × 10¹⁵ m). The calculator's auto-precision mode switches to scientific notation when results fall outside the 10⁻³ to 10¹² range, keeping cross-unit conversions readable (1 mile in ångström = 1.609344 × 10¹³ Å, not 16,093,440,000,000).

Shareable URL state

Every input choice in the calculator — category, source unit, target unit, input value, precision — is encoded in the query string as ?cat=length&from=meter&to=foot&v=1&p=auto. Copy-paste the URL to share the exact conversion. There is no per-pair URL explosion; one canonical /tools/unit-converter URL hosts every combination.

Sources & References

Content verified by the Smart Calculators Team