Temperature Converter

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About this tool

Converts between Celsius, Fahrenheit, Kelvin and Rankine. Unlike every other quantity here, these scales do not share a zero, so each conversion needs an offset as well as a factor.

That is the whole difficulty. °F = °C × 1.8 + 32, and the +32 is not optional: multiplying alone is right only for a difference between two temperatures, never for a reading. A rise of 10 °C is a rise of 18 °F, but 10 °C is 50 °F. Getting those two cases mixed up is the classic temperature error, and it produces answers that look plausible.

Kelvin is the exception that proves the point. It shares Celsius's degree size but starts at absolute zero, so the conversion is a pure offset of 273.15 - and because the scale starts at the true zero, a temperature in kelvin can be doubled or halved meaningfully, which is not true of Celsius or Fahrenheit. A kelvin value is written without a degree sign for that reason: it is an absolute unit, not a position on an arbitrary scale.

The reference points worth memorising: water freezes at 0 °C, 32 °F, 273.15 K and boils at 100 °C, 212 °F, 373.15 K at sea level. −40 is the same in Celsius and Fahrenheit, which is the only place the two scales cross. Body temperature is 37 °C or 98.6 °F, and a fever starts around 38 °C.

Rankine is included for completeness: it is to Fahrenheit what Kelvin is to Celsius, an absolute scale with Fahrenheit-sized degrees, and it still appears in some American thermodynamics work.

Good to know

Questions people actually ask

Why can't I just multiply to convert temperature?

Because the scales do not share a zero. 0 °C is 32 °F and 273.15 K, so every conversion needs an offset as well as a factor: °F = °C × 1.8 + 32. Multiplying alone is correct only for a difference between two temperatures, never for a reading.

What is absolute zero?

The coldest possible temperature: 0 K, which is −273.15 °C or −459.67 °F. It is where a substance holds the minimum energy quantum mechanics permits. It has never been reached, though laboratories have come within a billionth of a degree.

Why is Kelvin written without a degree sign?

Because it is an absolute unit rather than a position on an arbitrary scale - 300 K, not 300 °K. That also makes ratios meaningful: 300 K really is twice as hot as 150 K in terms of thermal energy, whereas 20 °C is not twice 10 °C.

At what temperature are Celsius and Fahrenheit the same?

−40. It is the only point where the two scales cross, which is why −40 degrees needs no unit - it is the same reading either way.

What is a normal body temperature?

About 37 °C or 98.6 °F, though it varies by a degree over the day and between people. A fever is usually reckoned from 38 °C (100.4 °F). The famous 98.6 is a conversion artefact: the original nineteenth-century figure was 37 °C, and converting it produced a precision the measurement never had.