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Temperature Converter

Convert between Celsius, Fahrenheit, Kelvin, Rankine, and Réaumur temperatures. Handles non-multiplicative scales correctly using the SI offset definition (0 °C = 273.15 K exactly, 13th CGPM 1967).

°F

Result

-17.2222 °C

1 °F≈ -17.222222 °C

UnitValue
Kelvin (K)255.928
Celsius (°C)-17.2222
Fahrenheit (°F)1
Rankine (°R)460.67
Réaumur (°Ré)-13.7778
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Temperature converter. Celsius, Fahrenheit, Kelvin, Rankine and Réaumur with offset-aware formulas.

A temperature converter switches a value between Celsius, Fahrenheit, Kelvin, Rankine and Réaumur using offset-aware formulas anchored to 0 °C = 273.15 K. It tags each result exact or approximate so weather followers, scientists, engineers, cooks and travellers can tell finite-decimal conversions from the 5/9-slope ones rounded by IEEE-754 doubles.

What Is a Temperature Converter?

Most temperature scales put their zero somewhere arbitrary. Fahrenheit pinned 0 °F to the coldest brine of salt and ice he could mix in 1724 and set roughly 96 near body heat; Celsius hung his zero and hundred on water freezing and boiling. Only Kelvin starts at absolute zero, the genuine floor of temperature, which is why it is the one scale with no negative numbers. Those mismatched zeros are exactly why temperature is the odd one out among the converters on this site: length, mass, pressure and angle all pivot through a single multiplication, but temperature cannot. To turn 0 °C into Fahrenheit you do not scale by a factor — you scale and then shift by 32, because the two scales do not share a zero. The proof of how stubborn that shift is shows up at one strange point: −40 reads −40 on both the Celsius and Fahrenheit dials, the single temperature where the two scales agree. So every conversion here runs as an affine map — multiply by a slope, then add an offset — pivoting through the kelvin: base = value × slope + offset to go up to kelvins, then result = (base − target offset) / target slope to come back out. That is the same offset-aware machinery the BIPM SI Brochure §2.3.1, the 13th CGPM 1967 Resolution 4, and ISO 80000-5:2019 describe.
This converter supports five scales across three groups. The absolute group covers the kelvin (SI base unit since 1967, redefined in 2019 via the Boltzmann constant) and the Rankine scale (R. W. M. Rankine, 1859 — an absolute scale that keeps Fahrenheit's degree size, so 0 °R = 0 K and 491.67 °R = 273.15 K). The common group covers the Celsius degree (Anders Celsius, 1742; the modern definition fixes 0 °C = 273.15 K exact via the 13th CGPM 1967 Resolution 4) and the Fahrenheit degree (Daniel Gabriel Fahrenheit, 1724 — slope 5/9 K per degree, intercept 459.67 × 5/9 K, so 0 °F = 459.67 °R). The scientific group covers the Réaumur scale (René Antoine Ferchault de Réaumur, 1731 — slope 5/4 K per degree, same 273.15 K intercept as Celsius, so 0 °Ré = 0 °C and 80 °Ré = 100 °C exactly). Réaumur was standard in 18th- and 19th-century continental European cookbooks, brewing manuals, and Russian literature — Tolstoy's War and Peace quotes Moscow winter temperatures in degrees Réaumur — and survived in Italian and Swiss dairy and cheese-making operations into the early 20th century.
What distinguishes a serious temperature converter from a marketing widget is honesty about which conversions are exact and which carry floating-point drift. Three conversion pairs are bit-exact in this set: kelvin ↔ Celsius (slope 1, offset 273.15 — both representable exactly as IEEE-754 doubles), Celsius ↔ Réaumur (slope 5/4 = 1.25, offset cancels — 1.25 is a finite binary fraction), and kelvin ↔ Réaumur (slope 5/4, offset 273.15 — both exact). The two pairs that are NOT exact involve Fahrenheit or Rankine: their slope is 5/9 ≈ 0.5555…, which is irrational in IEEE-754 (non-terminating binary expansion of 1/9). So every conversion that touches °F or °R picks up a few ulps of representational drift even though the underlying math is symbolically exact. The (exact) badge fires only on the three bit-exact pairs above; anything involving Fahrenheit or Rankine is honestly tagged approximate.

How to Convert Between Temperature Units

Every temperature conversion is two affine operations through the kelvin. The general formula is:
Tout=(Tin⋅afrom+bfrom)−btoatoT_{\text{out}} = \frac{(T_{\text{in}} \cdot a_{\text{from}} + b_{\text{from}}) - b_{\text{to}}}{a_{\text{to}}}
where Tin is your input value, afrom and bfrom are the source scale's slope and offset (both in kelvins), and ato, bto are the target scale's slope and offset. To do it by hand:
1. Look up the source slope and offset. For Celsius, afrom = 1 and bfrom = 273.15 K.
2. Multiply the input by the slope and add the offset to get kelvins. 100 °C × 1 + 273.15 = 373.15 K.
3. Look up the target slope and offset. For Fahrenheit, ato = 5/9 and bto = 459.67 × 5/9 ≈ 255.3722 K.
4. Subtract the target offset from the kelvin value and divide by the target slope. (373.15 − 255.3722) / (5/9) = 117.7778 × 9/5 = 212 °F exact.
The same procedure works for every supported scale. Going from 0 °C to Réaumur: (0 × 1 + 273.15 − 273.15) / (5/4) = 0 / 1.25 = 0 °Ré exact. Going from 80 °Ré to Celsius: (80 × 1.25 + 273.15 − 273.15) / 1 = 100 °C exact (Réaumur's defining anchor — water boils at 80 °Ré). Going from −40 °F to Celsius: (−40 × 5/9 + 459.67 × 5/9 − 273.15) / 1 = (5/9)(−40 + 459.67) − 273.15 = (5/9)(419.67) − 273.15 = 233.15 − 273.15 = −40 °C — the unique fixed point where Celsius and Fahrenheit cross.
To use this calculator, pick the source scale from the "From" dropdown, type a value, then pick the target scale 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 hot-plasma kelvin temperature to Rankine still renders readably. The (exact) badge appears on three pairs only — kelvin ↔ Celsius, Celsius ↔ Réaumur, and kelvin ↔ Réaumur — because these are the conversions whose slope (1 or 5/4) and offset (0 or 273.15) are all representable exactly as IEEE-754 doubles. Any conversion involving Fahrenheit or Rankine is tagged approximate to honestly report the irrational-in-binary 5/9 slope, not because the underlying definition is fuzzy.

Most-Searched Temperature Conversions at a Glance

FromToValueExact?
0 K (absolute zero)Celsius−273.15 °Cyes
0 °C (water freezing)Fahrenheit32 °Fno (approx.)
20 °C (room temperature)Fahrenheit68 °Fno (approx.)
37 °C (body temperature)Fahrenheit98.6 °Fno (approx.)
100 °C (water boiling, 1 atm)Fahrenheit212 °Fno (approx.)
−40 °C (the fixed point)Fahrenheit−40 °Fno (approx.)
−18 °C (freezer)Fahrenheit−0.4 °Fno (approx.)
350 °F (US baking oven)Celsius176.67 °Cno (approx.)
425 °F (high-heat oven)Celsius218.33 °Cno (approx.)
0 °C (water freezing)Kelvin273.15 Kyes
100 °C (water boiling)Kelvin373.15 Kyes
80 °Ré (Réaumur boiling)Celsius100 °Cyes

Temperature Conversion Formula

Tout=(Tin⋅afrom+bfrom)−btoatoT_{\text{out}} = \frac{(T_{\text{in}} \cdot a_{\text{from}} + b_{\text{from}}) - b_{\text{to}}}{a_{\text{to}}}
  • ToutT_{\text{out}} = The converted temperature, expressed in the target scale (°C, °F, K, °R or °Ré).
  • TinT_{\text{in}} = The input temperature, expressed in the source scale.
  • afroma_{\text{from}} = Slope of the source scale in kelvins per degree (1 for K and °C, 5/9 for °F and °R, 5/4 for °Ré).
  • bfromb_{\text{from}} = Offset of the source scale in kelvins at zero (0 for K and °R, 273.15 for °C and °Ré, 459.67 × 5/9 ≈ 255.3722 for °F).
  • atoa_{\text{to}} = Slope of the target scale in kelvins per degree.
  • btob_{\text{to}} = Offset of the target scale in kelvins at zero.
Unlike length, mass, energy or angle, temperature is not a purely multiplicative quantity — Celsius, Fahrenheit and Réaumur have non-zero zero points relative to the absolute (Kelvin) base. The formula is therefore an affine map: multiply by the slope, add the offset, then subtract the target offset and divide by the target slope. The factor table this calculator uses is sourced from the BIPM SI Brochure 9th edition, the 13th CGPM 1967 Resolution 4, and ISO 80000-5:2019:
  • Kelvin (K): slope = 1, offset = 0 (SI base unit, redefined in 2019 via the Boltzmann constant k = 1.380649 × 10⁻²³ J/K exact)
  • Celsius (°C): slope = 1, offset = 273.15 K exact (13th CGPM 1967 Resolution 4 fixes 0 °C = 273.15 K)
  • Fahrenheit (°F): slope = 5/9 K per degree, offset = 459.67 × 5/9 K ≈ 255.3722 K (anchors 32 °F = 0 °C = 273.15 K; slope is irrational in IEEE-754)
  • Rankine (°R): slope = 5/9 K per degree, offset = 0 (absolute scale with Fahrenheit slope; 0 °R = 0 K, 491.67 °R = 273.15 K)
  • Réaumur (°Ré): slope = 5/4 K per degree, offset = 273.15 K exact (0 °Ré = 0 °C, 80 °Ré = 100 °C)
For the Celsius → Fahrenheit direction the calculator computes TF = (TC × 1 + 273.15 − 459.67 × 5/9) / (5/9) = TC × 9/5 + 32, the familiar one-line formula every chemistry textbook prints. For Celsius → kelvin: TK = TC + 273.15 exact. For kelvin → Rankine: TR = TK × 9/5 exact in symbolic form, approximate in IEEE-754 because 5/9 and 9/5 are non-terminating in binary. For Celsius → Réaumur: TRé = TC × 4/5 = TC × 0.8 exact (0.8 is a finite binary fraction in IEEE-754). The three conversion pairs that earn the (exact) badge are kelvin ↔ Celsius, Celsius ↔ Réaumur, and kelvin ↔ Réaumur — every other pair touches Fahrenheit or Rankine, picks up the irrational 5/9 slope, and is honestly tagged approximate.

Worked Temperature Conversion Examples

100 °C to Fahrenheit (water-boiling, the canonical sanity check)

Set From = Celsius, To = Fahrenheit, Value = 100. The formula gives 100 × 9/5 + 32 = 180 + 32 = 212 °F exact in symbolic form. Set precision to 15 decimals to see the IEEE-754 representation 211.99999999999997 °F — a single ulp short of 212, the visible signature of the irrational 5/9 slope. This is the textbook anchor: water freezes at 0 °C = 32 °F and boils at 100 °C = 212 °F at 1 atm; a Celsius degree is exactly 1.8 (= 9/5) Fahrenheit degrees. The result is tagged approximate because the conversion runs through the 5/9 slope, even though the underlying definition is exact by the 13th CGPM 1967 Resolution and the legal Fahrenheit anchor 32 °F = 0 °C.

−40 °C to Fahrenheit (the unique fixed point)

Set From = Celsius, To = Fahrenheit, Value = −40. The formula gives −40 × 9/5 + 32 = −72 + 32 = −40 °F exact. This is the *only* temperature at which Celsius and Fahrenheit read the same number — the famous fixed point where the two scales cross. Algebraically: T = T × 9/5 + 32 simplifies to T(1 − 9/5) = 32, so T(−4/5) = 32, T = −40. Every other temperature reads a different number on the two scales. The fixed point is useful as a memory anchor for cold-climate reporting: Edmonton, Yellowknife, Yakutsk and the Antarctic interior routinely hit −40 in winter, and it's the only temperature you can report without specifying the scale. The result is tagged approximate (5/9 slope), even though the symbolic answer is the exact integer −40.

0 K to Celsius and Fahrenheit (absolute zero)

Set From = Kelvin, To = Celsius, Value = 0. The formula gives 0 − 273.15 = −273.15 °C exact (badge: yes). Set To = Fahrenheit: (0 − 459.67 × 5/9) / (5/9) = −459.67 °F (badge: no — touches 5/9). Set To = Rankine: 0 / (5/9) = 0 °R exact in symbolic form. Absolute zero is the floor of the thermodynamic temperature scale: it's the temperature at which classical kinetic energy of every particle vanishes (quantum zero-point energy remains, by Heisenberg uncertainty). The third law of thermodynamics says it is approachable but not reachable; the current record is about 38 picokelvin held by a Bose-Einstein condensate experiment. The Celsius answer earns the exact badge; the Fahrenheit and Rankine answers carry the IEEE-754 5/9 drift.

37 °C to Fahrenheit (human body temperature)

Set From = Celsius, To = Fahrenheit, Value = 37. The formula gives 37 × 9/5 + 32 = 66.6 + 32 = 98.6 °F. This is the canonical normal-body-temperature anchor that every English-speaking medical textbook prints. A clinical caveat that does *not* affect the conversion: modern thermometry shows actual mean oral body temperature is closer to 36.5–36.8 °C (97.7–98.2 °F), with the original 98.6 °F figure tracing back to Carl Wunderlich's 1868 axillary measurements which read a few tenths high by modern standards. The conversion 37 °C ↔ 98.6 °F is mathematically exact (rounds cleanly because 37 × 1.8 = 66.6 is a finite-decimal product), but the result is still tagged approximate because the calculator routes through the irrational 5/9 slope in IEEE-754.

77 K to Celsius and Fahrenheit (liquid nitrogen)

Set From = Kelvin, To = Celsius, Value = 77. The formula gives 77 − 273.15 = −196.15 °C exact. Switch To = Fahrenheit: (77 − 459.67 × 5/9) / (5/9) = 77 × 9/5 − 459.67 = 138.6 − 459.67 = −321.07 °F. Liquid nitrogen boils at 77.36 K = −195.79 °C = −320.42 °F at 1 atm — the workhorse cryogen for biology cell-line storage, superconducting magnet cooling (high-Tc YBCO at LN₂ temperatures), and metallurgical shrink-fitting. Liquid helium boils at 4.22 K = −268.93 °C, used to cool MRI magnets and dilution refrigerators to millikelvin range. The Celsius answer earns the (exact) badge; the Fahrenheit answer is honestly tagged approximate.

80 °Ré to Celsius (Réaumur's defining anchor)

Set From = Réaumur, To = Celsius, Value = 80. The formula gives (80 × 5/4 + 273.15 − 273.15) / 1 = 80 × 1.25 = 100 °C exact (badge: yes, slope 1.25 = 5/4 is a finite binary fraction). Réaumur's 1731 design divided the interval from water freezing (0 °Ré) to water boiling (80 °Ré) into 80 equal parts. The scale was standard in 18th- and 19th-century continental European cooking — period French and German cookbooks routinely specify oven temperatures in degrés Réaumur — and in Russian literature: Tolstoy's *War and Peace* uses Réaumur for Moscow winter (e.g., −20 °Ré = −25 °C, a typical Moscow January night). It survived in Italian and Swiss dairy and cheese-making operations (and in Dutch sugar-syrup production) into the early 20th century before being displaced by Celsius. Switch To = Fahrenheit: 80 × 9/4 + 32 = 180 + 32 = 212 °F — same as 100 °C, by construction.

Comparative table: common reference temperatures across five scales

ReferenceKelvinCelsiusFahrenheitRankineRéaumur
Absolute zero0 K−273.15 °C−459.67 °F0 °R−218.52 °Ré
LN₂ boiling77.36 K−195.79 °C−320.42 °F139.25 °R−156.63 °Ré
Dry-ice CO₂ sublim.194.65 K−78.5 °C−109.3 °F350.37 °R−62.8 °Ré
Water freezing273.15 K0 °C32 °F491.67 °R0 °Ré
Cel-Fahr fixed point233.15 K−40 °C−40 °F419.67 °R−32 °Ré
Room temperature293.15 K20 °C68 °F527.67 °R16 °Ré
Body temperature310.15 K37 °C98.6 °F558.27 °R29.6 °Ré
Water boiling (1 atm)373.15 K100 °C212 °F671.67 °R80 °Ré
Oven 350 °F (US baking)449.82 K176.67 °C350 °F809.67 °R141.33 °Ré
Iron melting1811 K1538 °C2800 °F3260 °R1230 °Ré
Use this table as a sanity check when converting unfamiliar temperatures by hand. The −40 row is the unique fixed point where Celsius and Fahrenheit cross; the absolute-zero row anchors the two absolute scales (Kelvin and Rankine) at the same floor; the 0 °Ré and 80 °Ré rows anchor the Réaumur historical scale on water's freezing and boiling points.

Most common temperature conversions

These are the unit pairs people look up most often. Use them as a quick reference, or paste them into the calculator for an exact result to 15 decimals.
  • 0 °C to °F: 32 °F (water freezing, definitional)
  • 100 °C to °F: 212 °F (water boiling at 1 atm, definitional)
  • 37 °C to °F: 98.6 °F (normal body temperature)
  • 20 °C to °F: 68 °F (room temperature)
  • 350 °F to °C: 176.67 °C (US baking oven temperature)
  • 375 °F to °C: 190.56 °C (US roasting oven temperature)
  • 425 °F to °C: 218.33 °C (US high-heat oven temperature)
  • −40 °C to °F: −40 °F (the unique fixed point)
  • 0 °C to K: 273.15 K (water freezing, definitional)
  • 100 °C to K: 373.15 K (water boiling at 1 atm)
  • −273.15 °C to K: 0 K (absolute zero, definitional)
  • 0 K to °R: 0 °R (absolute zero in Rankine)
  • 491.67 °R to K: 273.15 K (water freezing in Rankine)
  • 80 °Ré to °C: 100 °C (Réaumur's defining boiling anchor)
  • 1 °C change = 1 K change = 1.8 °F change = 1.8 °R change = 0.8 °Ré change (degree-size ratios)

Temperature Converter — Frequently Asked Questions

Is this temperature converter free?

Yes. The calculator is free, requires no account, runs entirely in your browser, and is ad-free. The embeddable iframe version at /widget/temperature-converter is also free and ad-free, so you can drop it into weather sites, science teaching materials, cooking blogs, travel guides, or engineering portals without exposing readers to third-party trackers.

How do I convert Celsius to Fahrenheit?

Multiply by 9/5 (= 1.8) and add 32. So 20 °C × 1.8 + 32 = 68 °F, 37 °C × 1.8 + 32 = 98.6 °F, and 100 °C × 1.8 + 32 = 212 °F. Going the other way: subtract 32 and multiply by 5/9. The formula is legally exact (the 13th CGPM 1967 fixed 0 °C = 273.15 K and the Fahrenheit anchor 32 °F = 0 °C is definitional) but carries a few ulps of IEEE-754 drift because 5/9 is irrational in binary.

How do I convert Fahrenheit to Celsius?

Subtract 32 and multiply by 5/9 (≈ 0.5556). So (68 − 32) × 5/9 = 20 °C, (98.6 − 32) × 5/9 = 37 °C, and (212 − 32) × 5/9 = 100 °C. A faster mental shortcut: subtract 32, then halve and add a tenth (e.g. 68 − 32 = 36; 36/2 = 18; 18 + 1.8 = 19.8 ≈ 20). Useful in conversation when you don't have the calculator handy.

Why does −40 °C equal −40 °F?

They have different degree sizes (1 °C = 1.8 °F) and different zeros (0 °C = 32 °F), so they cross exactly once. Solving T = T × 9/5 + 32 gives T = −40 — the only temperature both scales report identically.

What is absolute zero in each scale?

Absolute zero is the lower limit of the thermodynamic temperature scale, the point at which classical kinetic energy of every particle vanishes. It reads 0 K (definitional, SI base), −273.15 °C exact (via the 13th CGPM 1967), −459.67 °F exact (definitional anchor of the Fahrenheit scale to Rankine), 0 °R exact (Rankine is an absolute scale with Fahrenheit slope), and −218.52 °Ré (= −273.15 × 4/5). The third law of thermodynamics says absolute zero is approachable but not reachable; the current low-temperature record is about 38 picokelvin held by a Bose-Einstein condensate experiment.

How accurate are the conversion factors?

The conversion definitions are legally exact: 0 °C = 273.15 K exact (13th CGPM 1967 Resolution 4), 32 °F = 0 °C exact (Fahrenheit anchor), 0 °Ré = 0 °C exact and 80 °Ré = 100 °C exact (Réaumur's 1731 definition), 0 °R = 0 K exact (Rankine absolute anchor). Three conversion pairs are also bit-exact in IEEE-754 doubles: kelvin ↔ Celsius (slope 1, offset 273.15), Celsius ↔ Réaumur (slope 5/4 = 1.25), and kelvin ↔ Réaumur. Every pair involving Fahrenheit or Rankine carries a few ulps of representational drift because the 5/9 slope is irrational in binary — the calculator labels those conversions approximate to be honest with you.

Why is kelvin written without a degree symbol?

The 13th CGPM (1967) dropped the degree symbol from the kelvin: write 100 K, never 100 °K. Celsius, Fahrenheit, Rankine and Réaumur keep °C, °F, °R, °Ré because they are scale-based units, while the kelvin is thermodynamic temperature itself.

What temperature is 350 °F in Celsius (for baking)?

350 °F = (350 − 32) × 5/9 = 318 × 5/9 = 176.67 °C, the canonical US home-baking oven temperature. The nearest EU recipe step is 180 °C (350 °F = 176.67 °C is close enough for most baking; within ±10 °C the difference rarely matters except for delicate items like custards or soufflés). Other US baking conversions: 325 °F = 162.8 °C (cookies, low-and-slow roasting), 375 °F = 190.6 °C (most roasting), 425 °F = 218.3 °C (high-heat roasting, pizza).

What is the Réaumur scale and where is it still used?

The Réaumur scale, defined by René Antoine Ferchault de Réaumur in 1731, divides the interval from water freezing (0 °Ré) to water boiling (80 °Ré) into 80 equal parts. 1 °Ré = 1.25 °C exact. It was the dominant temperature scale across continental Europe through the 18th and early 19th centuries — Tolstoy's *War and Peace* reports Moscow winters in °Ré, period French and German cookbooks specify oven temperatures in °Ré — and survived in Italian and Swiss dairy and cheese-making operations into the early 20th century before being fully displaced by Celsius. Today it is essentially obsolete in active use, but the calculator includes it because historical sources still cite it.

What is the Rankine scale and where is it used?

The Rankine scale, proposed by Scottish engineer William John Macquorn Rankine in 1859, is an absolute thermodynamic temperature scale that keeps the Fahrenheit degree size. 0 °R = 0 K (absolute zero), 491.67 °R = 273.15 K = 0 °C (water freezing), and 1 °R change equals 1 °F change. It survives in US heat-transfer and thermodynamic engineering textbooks (Cengel/Boles, Moran), in legacy industrial pyrometry, and in some US aerospace propulsion calculations. ASHRAE handbooks moved fully to SI decades ago; outside these niches, Rankine is rarely encountered.

Is normal body temperature really 98.6 °F?

98.6 °F (= 37 °C exact) is the canonical textbook value, traceable to Carl Wunderlich's 1868 axillary measurements. Modern thermometry shows the actual mean oral body temperature is closer to 36.5–36.8 °C (97.7–98.2 °F), with significant individual and diurnal variation (typically 0.5 °C cooler in the morning, 0.5 °C warmer in late afternoon). The conversion 37 °C ↔ 98.6 °F is mathematically clean and remains the value printed in most clinical references, but for medical screening, 36.1–37.2 °C (97–99 °F) is the modern accepted range.

Can I embed this converter on my site?

Yes. The embeddable version lives at /widget/temperature-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 weather sites, cooking blogs, science teaching materials, travel guides, engineering portals, and historical-source annotation projects that want a clean temperature tool without sending readers off-site.

How big is 1 degree Celsius compared to 1 degree Fahrenheit?

1 °C is exactly 1.8 °F (= 9/5). So a 10 °C swing equals an 18 °F swing, a 100 °C interval equals a 180 °F interval, and a 1 K change equals a 1 °C change equals a 1.8 °F change. A Fahrenheit thermometer with 1-degree resolution is finer than a Celsius thermometer with 1-degree resolution; this is why US weather forecasts often quote temperatures to the whole degree while EU forecasts quote to the tenth.

Sources & References

  1. BIPM SI Brochure (9th edition, §2.3.1) — defines the kelvin as the SI base unit for thermodynamic temperature, anchored via the fixed Boltzmann constant k = 1.380649 × 10⁻²³ J/K exact
  2. 13th CGPM (1967) Resolution 4 — fixed 0 °C = 273.15 K exact and formally dropped the degree symbol from the kelvin ("100 K", not "100 °K")
  3. BIPM — 2019 revision of the SI (in force 20 May 2019): anchored the kelvin to the fixed Boltzmann constant k = 1.380649 × 10⁻²³ J/K exact, replacing the triple-point-of-water definition
  4. ISO 80000-5:2019 — Quantities and units, Part 5: Thermodynamics (codifies the kelvin, Celsius, Fahrenheit, Rankine and Réaumur unit definitions and conversion conventions)
  5. NIST Guide to the SI, Appendix B — Conversion factors (kelvin/Celsius/Fahrenheit/Rankine conversion equations and engineering-default rounding conventions)
  6. Wikipedia — Kelvin (SI base unit; 2019 SI redefinition via Boltzmann constant; absolute zero at 0 K; written without degree symbol per 13th CGPM 1967)
  7. Wikipedia — Celsius (1742 Anders Celsius; modern definition 0 °C = 273.15 K exact via 13th CGPM 1967; standard temperature scale outside the United States and a few US-affiliated territories)
  8. Wikipedia — Fahrenheit (1724 Daniel Gabriel Fahrenheit; legal anchors 32 °F = 0 °C, 212 °F = 100 °C; slope 5/9 K per degree; everyday scale in the United States)
  9. Wikipedia — Rankine scale (1859 William John Macquorn Rankine; absolute scale with Fahrenheit degree size; 0 °R = 0 K, 491.67 °R = 273.15 K; used in US heat-transfer textbooks and legacy industrial pyrometry)
  10. Wikipedia — Réaumur scale (1731 René Antoine Ferchault de Réaumur; 0 °Ré = 0 °C, 80 °Ré = 100 °C exact; dominant in 18th-19th C continental Europe — cited in Tolstoy's War and Peace and period French/German cookbooks)

Content verified by the Smart Calculators Team