Temperature Conversion

Supported scales

Temperature is the one quantity on this site that does not convert by a factor. Each scale differs from the others in both where its zero sits and how large one degree is, so conversion is affine — multiply, then offset — rather than a single multiplication.

Celsius (°C)

The everyday temperature scale in most of the world, and the SI-derived scale for practical use. Zero is the freezing point of water and 100 the boiling point, both at standard atmospheric pressure. A Celsius degree is the same size as a kelvin, so the two differ only by an offset of 273.15.

°C = (°F − 32) × 5/9
°C = K − 273.15
°C = °R × 5/9 − 273.15
°C = °Ré × 5/4

Weather

The reported temperature in most countries.

Science and medicine

Body temperature, laboratory conditions and material properties.

Cooking

Oven temperatures outside the United States.

Fahrenheit (°F)

The everyday scale in the United States. Water freezes at 32 °F and boils at 212 °F, so the interval between the two is 180 degrees rather than 100 — a Fahrenheit degree is five-ninths the size of a Celsius degree. The two scales cross at −40, which reads the same on both.

°F = °C × 9/5 + 32
°F = (K − 273.15) × 9/5 + 32
°F = °R − 459.67

Weather

The reported temperature in the United States.

Cooking

Oven temperatures in US recipes.

Heating and cooling

Thermostat settings in US buildings.

Kelvin (K)

The SI base unit of thermodynamic temperature. Its zero is absolute zero, the point at which thermal motion is minimal, so kelvin values are never negative. Since the 2019 SI redefinition the kelvin is fixed by the Boltzmann constant at exactly 1.380 649 × 10⁻²³ J/K. Note that kelvin takes no degree sign: it is written 300 K, not 300 °K.

K = °C + 273.15
K = (°F − 32) × 5/9 + 273.15
K = °R × 5/9

Physics and chemistry

The scale used in thermodynamics and the gas laws.

Astronomy

Stellar surface temperatures and cosmic background radiation.

Lighting

Colour temperature of lamps and displays.

Rankine (°R)

An absolute scale that starts at absolute zero but uses Fahrenheit-sized degrees, so it stands to Fahrenheit as kelvin stands to Celsius. It survives in some US engineering work, particularly thermodynamics done in imperial units.

°R = °F + 459.67
°R = K × 9/5
°R = (°C + 273.15) × 9/5

Thermodynamics

Absolute temperature in US engineering calculations.

Aerospace

Some propulsion and gas-dynamics work in imperial units.

Réaumur (°Ré)

An eighteenth-century scale placing the freezing point of water at 0 and the boiling point at 80. It is largely obsolete, but survives in a few traditional contexts, notably some European cheese and syrup making.

°Ré = °C × 4/5
°Ré = (°F − 32) × 4/9
°Ré = (K − 273.15) × 4/5

Food production

Traditional cheese making and sugar syrup recipes.

Historical records

European scientific and weather records from the 18th and 19th centuries.

The scales explained

Celsius

Zero at the freezing point of water and 100 at its boiling point at standard pressure — a hundred degrees spanning the range in which most human activity happens. It is the everyday scale almost everywhere and the standard for science outside of thermodynamics.

Fahrenheit

Water freezes at 32 and boils at 212, giving 180 degrees between the two. Because its degrees are smaller than Celsius degrees, it offers finer resolution without decimals, which is part of why it persists for weather reporting in the United States.

Kelvin

The SI base unit. It starts at absolute zero, the point at which no further thermal energy can be extracted, and uses degrees the same size as Celsius degrees. Note that kelvin takes no degree sign: it is written 300 K, not 300 °K.

Rankine

An absolute scale like Kelvin, but with Fahrenheit-sized degrees. It exists so that engineers working in imperial units can do thermodynamic calculations without switching systems, and it still appears in some US engineering practice.

Réaumur

Zero at freezing and 80 at boiling. Largely historical now, it survives in a few specialist corners — some European cheese and syrup production still quotes it — and it turns up when reading older scientific literature.

Why temperature conversion is different

Most unit conversions are a single multiplication: a kilometer is always 1,000 meters, whatever the value. Temperature scales are not like that, because they disagree about both the size of a degree and where zero sits.

That means converting requires an offset as well as a factor. Going from Celsius to Fahrenheit is multiply by 9/5, then add 32 — omit the offset and every answer is wrong by 32 degrees. It also means ratios are meaningless on the relative scales: 20 °C is not "twice as hot" as 10 °C, because zero is an arbitrary point rather than an absence of heat.

On absolute scales, ratios do work. 300 K really does carry twice the thermal energy of 150 K, which is exactly why Kelvin and Rankine exist and why gas law calculations must use them.

Two conventions worth following, both from NIST SP 811: write a space between the number and the unit (25 °C, not 25°C), and write the kelvin without a degree sign — 300 K, never 300 °K, because the kelvin is not a degree of anything.

Reference points

• Absolute zero— −273.15 °C, −459.67 °F, 0 K, 0 °R.
• Water freezes— 0 °C, 32 °F, 273.15 K.
• Typical room temperature— 20 °C, 68 °F, 293.15 K.
• Human body temperature— about 37 °C, 98.6 °F.
• Water boils— 100 °C, 212 °F, 373.15 K, at standard pressure.
• The scales cross— −40 °C is exactly −40 °F, the one value where they agree.

Common uses

• Cooking with foreign recipes— oven temperatures quoted in the other scale.
• Weather— making sense of a forecast while traveling.
• Science and laboratory work— converting to Kelvin for any calculation involving gas laws or thermodynamics.
• Engineering— Rankine for thermodynamic work carried out in imperial units.
• Medical readings— comparing body temperature between scales.
• Industrial specifications— equipment datasheets that state operating ranges in an unfamiliar unit.

Quick mental approximations

• Celsius to Fahrenheit— double it and add 30. For 20 °C that gives 70, against a true 68 °F: close enough for deciding on a jacket.
• Fahrenheit to Celsius— subtract 30 and halve. For 70 °F that gives 20, against a true 21.1 °C.
• Celsius to Kelvin— add 273. This one is nearly exact; the precise offset is 273.15.
• Use the exact conversion for anything that matters. The shortcuts drift further apart the further you get from ordinary temperatures.