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).

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From

Value

°F

Result

-17.2222 °C

1 °F≈ -17.222222 °C

Decimals

All units

Unit	Value
Kelvin (K)	255.928
Celsius (°C)	-17.2222
Fahrenheit (°F)	1
Rankine (°R)	460.67
Réaumur (°Ré)	-13.7778

Ažurirano: 20. svibnja 2026.

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?

A temperature converter is a tool that takes a temperature value in one scale and returns the equivalent value in any other supported scale. Unlike every other converter in this family, it cannot pivot through a simple multiplication: Celsius, Fahrenheit and Réaumur all have non-zero zero points relative to the absolute (Kelvin) scale, so the engine handles each unit with two numbers — a slope (factor to kelvins) and an offset (intercept in kelvins). The formula is base = value × slope + offset to go to kelvins, then result = (base − target offset) / target slope to come back out. This is the same offset-aware machinery used by NIST SP 811, BIPM SI Brochure §2.3.1, and ISO 80000-5:2019.

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 rad-of-temperature 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:

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

where $T_\text{in}$ is your input value, $a_\text{from}$ and $b_\text{from}$ are the source scale's slope and offset (both in kelvins), and $a_\text{to}$, $b_\text{to}$ are the target scale's slope and offset. To do it by hand:

1. Look up the source slope and offset. For Celsius, $a_\text{from} = 1$ and $b_\text{from} = 273.15$ 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, $a_\text{to} = 5/9$ and $b_\text{to} = 459.67 \times 5/9 \approx 255.3722$ 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.

Temperature Conversion Formula

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

$T_{\text{out}}$ = The converted temperature, expressed in the target scale (°C, °F, K, °R or °Ré).
$T_{\text{in}}$ = The input temperature, expressed in the source scale.
$a_{\text{from}}$ = Slope of the source scale in kelvins per degree (1 for K and °C, 5/9 for °F and °R, 5/4 for °Ré).
$b_{\text{from}}$ = 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).
$a_{\text{to}}$ = Slope of the target scale in kelvins per degree.
$b_{\text{to}}$ = 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 T_F = (T_C × 1 + 273.15 − 459.67 × 5/9) / (5/9) = T_C × 9/5 + 32, the familiar one-line formula every chemistry textbook prints. For Celsius → kelvin: T_K = T_C + 273.15 exact. For kelvin → Rankine: T_R = T_K × 9/5 exact in symbolic form, approximate in IEEE-754 because 5/9 and 9/5 are non-terminating in binary. For Celsius → Réaumur: T_Ré = T_C × 4/5 = T_C × 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 and Rankine answers earn the exact badge; the Fahrenheit answer carries 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-T_c 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

Reference	Kelvin	Celsius	Fahrenheit	Rankine	Réaumur
Absolute zero	0 K	−273.15 °C	−459.67 °F	0 °R	−218.52 °Ré
LN₂ boiling	77.36 K	−195.79 °C	−320.42 °F	139.25 °R	−156.63 °Ré
Dry-ice CO₂ sublim.	194.65 K	−78.5 °C	−109.3 °F	350.37 °R	−62.8 °Ré
Water freezing	273.15 K	0 °C	32 °F	491.67 °R	0 °Ré
Cel-Fahr fixed point	233.15 K	−40 °C	−40 °F	419.67 °R	−32 °Ré
Room temperature	293.15 K	20 °C	68 °F	527.67 °R	16 °Ré
Body temperature	310.15 K	37 °C	98.6 °F	558.27 °R	29.6 °Ré
Water boiling (1 atm)	373.15 K	100 °C	212 °F	671.67 °R	80 °Ré
Oven 350 °F (US baking)	449.82 K	176.67 °C	350 °F	809.67 °R	141.33 °Ré
Iron melting	1811 K	1538 °C	2800 °F	3260 °R	1230 °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 Conversion Tips

Affine, not multiplicative. Every conversion in this tool runs T_K = T × slope + offset to get kelvins, then unwinds to the target with (T_K − target offset) / target slope. Memorising five slopes and three offsets gives you every cross-conversion: K (slope 1, offset 0), °C (1, 273.15), °F (5/9, 459.67 × 5/9 ≈ 255.37), °R (5/9, 0), °Ré (5/4, 273.15). The familiar °F = °C × 9/5 + 32 is just this affine map collapsed.
Read the (exact) badge as a statement about IEEE-754, not mathematics. Only three pairs earn it: kelvin ↔ Celsius (slope 1, offset 273.15 — both representable exactly), Celsius ↔ Réaumur (slope 5/4 = 1.25, offsets cancel), and kelvin ↔ Réaumur (slope 5/4, offset 273.15). Anything touching Fahrenheit or Rankine is tagged approximate because their slope is 5/9 ≈ 0.555… — irrational in binary because 1/9 has an infinite non-terminating expansion. The badge is honest about a few-ulp drift, not a definitional fuzziness.
For the −40 mnemonic: −40 °C = −40 °F is the only temperature where the two scales read the same number. Memorise it as the cold-climate sanity check — Edmonton, Yakutsk, the Antarctic interior all hit −40 in winter and you don't have to specify the scale. The proof is one line: solve T = T × 9/5 + 32, get T(1 − 9/5) = 32, so T = 32 × (−5/4) = −40.
Convert oven temperatures with the 25 °F = ~14 °C step rule. US recipes step at 25 °F increments (300, 325, 350, 375, 400, 425, 450); EU recipes step at 10 °C increments (150, 160, 170, 180, 190, 200, 210, 220, 230). The 350 °F = 177 °C anchor is the most-baked temperature in US home cooking; 180 °C is its closest EU step. Within ±10 °C the difference rarely matters for baking; for delicate items (custards, soufflés) use the calculator for the exact value.
Use the kelvin for any physics or thermodynamics calculation. Ideal-gas law (PV = nRT), Stefan-Boltzmann (P = σT⁴), Wien displacement (λ_max T = constant), Planck radiation — every formula in classical and statistical thermodynamics requires absolute temperature, not Celsius. Plugging 0 °C into PV = nRT gives PV = 0, which is wrong; plugging 273.15 K gives the correct result. Climate-anomaly figures in K and °C are numerically identical because the slope is 1 — a +1.5 °C warming and a +1.5 K warming describe the same change.
Rankine is the absolute-scale companion to Fahrenheit, used in some US heat-transfer and thermodynamic engineering textbooks (chemical engineering Cengel/Boles, mechanical engineering Moran, aerospace propulsion). 0 °R = 0 K, 459.67 °R = 0 °F, 491.67 °R = 32 °F = 0 °C, 671.67 °R = 100 °C = 212 °F. ASHRAE handbooks moved to SI decades ago; Rankine survives mainly in legacy industrial pyrometry and a handful of US university curricula.
Réaumur was the dominant European scale through the 18th and early 19th centuries — useful when reading historical sources. Tolstoy's *War and Peace* reports Moscow winter temperatures in °Ré (a −20 °Ré night is −25 °C, brutally cold but not extreme by Moscow standards). 18th-century French and German cookbooks specify oven temperatures in °Ré; Italian and Swiss dairy and cheese-making operations used it into the early 20th century. To convert quickly: multiply Réaumur by 1.25 to get Celsius, by 2.25 to get Fahrenheit (then add 32), or by 1.25 and add 273.15 to get kelvin.
Watch the 1.8 vs 1.25 vs 1 degree-size ratio. A change of 1 K equals exactly 1 °C, exactly 1.8 °F, exactly 1.8 °R, and exactly 0.8 °Ré. So a Fahrenheit thermometer with 1-degree resolution is finer than a Celsius thermometer with 1-degree resolution (1 °F ≈ 0.56 °C); a Réaumur thermometer with 1-degree resolution is coarser (1 °Ré = 1.25 °C). When reporting climate anomalies or material thermal coefficients, always specify the scale to avoid the 1.8× and 0.8× confusion.
When the result is bigger than 10¹² (one trillion) or smaller than 10⁻³ at auto-precision, the display switches to scientific notation. Useful for high-temperature plasma physics (a 10⁸ K tokamak plasma renders as 1.8 × 10⁸ °F in scientific notation, unreadable as 180000032 °F) and for cryogenic millikelvin work.

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.

Key Temperature Conversion Terms

Kelvin (K)

The SI base unit of thermodynamic temperature. Originally defined by the 13th CGPM (1967) as 1/273.16 of the thermodynamic temperature of the triple point of water; redefined in 2019 via the fixed value of the Boltzmann constant k = 1.380649 × 10⁻²³ J/K exact. Written without the degree symbol: 100 K, not 100 °K. Absolute zero is 0 K by definition.

Celsius (°C)

An SI-derived unit of temperature, defined since the 13th CGPM (1967) by the relation 0 °C = 273.15 K exact, with the same degree size as the kelvin. Originally proposed by Anders Celsius in 1742 as a 100-degree scale between water boiling (then 0) and freezing (then 100); the scale was inverted later that same year by Carl Linnaeus to the modern convention (0 = freezing, 100 = boiling at 1 atm). Standard in every country except the United States, Liberia, Myanmar and a few US-affiliated territories.

Fahrenheit (°F)

A non-SI unit of temperature defined by Daniel Gabriel Fahrenheit in 1724. Modern legal anchors: 32 °F = 0 °C exact (water freezing) and 212 °F = 100 °C exact (water boiling at 1 atm). The slope is 5/9 K per degree (irrational in IEEE-754) and the offset is 459.67 × 5/9 ≈ 255.3722 K. Still the everyday temperature scale in the United States and a handful of US-affiliated territories.

Rankine (°R)

An absolute thermodynamic temperature scale proposed by Scottish engineer William John Macquorn Rankine in 1859. Keeps the Fahrenheit degree size (slope 5/9 K per degree) but starts at absolute zero (offset 0). So 0 °R = 0 K exact, 491.67 °R = 273.15 K = 0 °C, and 1 °R change = 1 °F change exactly. Used in US heat-transfer and thermodynamic engineering textbooks and in legacy industrial pyrometry.

Réaumur (°Ré)

A historical temperature scale defined by René Antoine Ferchault de Réaumur in 1731. Anchors at 0 °Ré = 0 °C (water freezing) and 80 °Ré = 100 °C (water boiling) — slope 5/4 K per degree, same 273.15 K offset as Celsius. Dominant across continental Europe through the 18th and early 19th centuries; quoted in Tolstoy's *War and Peace* for Moscow winters and in period French and German cookbooks for oven temperatures. Survived in Italian and Swiss dairy and cheese-making operations into the early 20th century. Essentially obsolete in active use today, but valuable for reading historical sources.

Absolute zero

The theoretical lower limit of the thermodynamic temperature scale: 0 K exact = −273.15 °C exact = −459.67 °F exact = 0 °R exact = −218.52 °Ré. The point at which classical kinetic energy of every particle vanishes (quantum zero-point energy remains). The third law of thermodynamics states absolute zero is approachable but not reachable; the current low-temperature record is about 38 picokelvin held by a Bose-Einstein condensate experiment.

13th CGPM 1967 Resolution 4

The General Conference on Weights and Measures resolution that defined the kelvin in terms of the triple point of water and fixed 0 °C = 273.15 K exact. Also formally dropped the degree symbol from the kelvin ("100 K", not "100 °K"). Superseded by the 2019 SI redefinition, which anchors the kelvin to the fixed Boltzmann constant k = 1.380649 × 10⁻²³ J/K exact — but the 0 °C = 273.15 K relation remains exact.

BIPM SI Brochure

The defining text of the International System of Units, maintained by the Bureau International des Poids et Mesures. The 9th edition (2019, with corrections to 2025) lists the kelvin as one of the seven SI base units in §2.3.1 and anchors it to the Boltzmann constant. ISO 80000-5:2019 mirrors the same definitions for thermodynamic units.

2019 SI redefinition

The major revision to the International System of Units that took effect 20 May 2019, fixing the values of seven defining constants. For temperature, the redefinition anchored the kelvin to the fixed Boltzmann constant k = 1.380649 × 10⁻²³ J/K exact — replacing the triple-point-of-water definition that had been in force since 1954. The 0 °C = 273.15 K relation remains exact under the new definition.

Triple point of water

The unique pressure-temperature point at which water coexists in solid, liquid and vapour phases simultaneously: 273.16 K = 0.01 °C, at 611.657 Pa. Before the 2019 SI redefinition, this point defined the kelvin (1 K = 1/273.16 of the triple-point temperature). After 2019 it is a measured quantity with a tiny uncertainty inherited from the new Boltzmann-anchored definition.

Affine conversion

A linear map of the form y = ax + b — multiplication by a slope plus addition of an offset. Temperature conversion is affine, not purely multiplicative, because Celsius, Fahrenheit and Réaumur all have non-zero zero points relative to the absolute kelvin scale. Every other quantity in this converter family (length, mass, area, volume, energy, power, angle, force, torque, pressure) is purely multiplicative (b = 0 for every unit); temperature is the only outlier.

IEEE-754 double

The 64-bit floating-point format used by JavaScript and most calculators. It carries about 15–17 significant decimal digits. Slopes 1, 5/4 (= 1.25), and offsets 0, 273.15 are representable exactly. Slope 5/9 ≈ 0.555… is NOT representable exactly because 1/9 has an infinite non-terminating binary expansion — which is why every conversion involving Fahrenheit or Rankine is tagged approximate even though the underlying mathematical definition is exact.

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