Temperature Converter

A temperature converter changes Celsius to Fahrenheit, Fahrenheit to Celsius, and either to Kelvin: 0 Celsius is 32 Fahrenheit, 100 Celsius is 212 Fahrenheit, and 100 Fahrenheit is 37.78 Celsius. Temperature is the one conversion on this site that is not simple multiplication, because the scales do not share a zero — water freezes at 0 Celsius and at 32 Fahrenheit. Any converter that only multiplies gets this wrong, and reports 100 Fahrenheit as 55.6 Celsius, which is a temperature no weather forecast has ever produced. This page applies the offset as well as the ratio, and runs entirely in your browser.

How to use it

1. Choose the scale you have and the scale you want.
2. Type the temperature — negatives are fine, and are exactly where offset mistakes show up.
3. The conversion applies the ratio and the offset together: Celsius to Fahrenheit multiplies by 9/5 and then adds 32, in that order.
4. Kelvin is offered because it is what scientific work uses; it shares the size of the Celsius degree but starts at absolute zero.

Examples

• 0 Celsius = 32 Fahrenheit (water freezes).
• 100 Celsius = 212 Fahrenheit (water boils at sea level).
• 100 Fahrenheit = 37.7778 Celsius.
• -40 Celsius = -40 Fahrenheit — the one point where the scales agree.

Frequently asked questions

How do I convert Celsius to Fahrenheit?

Multiply by 9/5 — that is 1.8 — and then add 32. So 20 Celsius is 20 x 1.8 = 36, plus 32, which is 68 Fahrenheit. Going the other way you must reverse both steps in the opposite order: subtract 32 first, then multiply by 5/9. The order matters. Subtracting after dividing instead of before gives a wrong answer every time, and it is the single most common temperature mistake.

What is 100 Fahrenheit in Celsius?

37.78 Celsius, which is roughly a mild fever in a human. The working is (100 - 32) x 5/9 = 68 x 5/9 = 37.78. It is worth doing this one by hand once, because it is the example that exposes a broken converter: if a tool tells you 100 Fahrenheit is 55.6 Celsius, it has multiplied by 5/9 and skipped the subtraction, and every other answer it gives you is wrong in the same way.

Why can I not just multiply to convert temperature?

Because Celsius and Fahrenheit do not start counting from the same place. Every other conversion on this site is a ratio — a kilometre is always 1,000 metres, whatever the distance — so doubling the input doubles the output. Temperature scales have different zero points: zero Celsius is the freezing point of water, while zero Fahrenheit was set near the coldest temperature Fahrenheit could reliably reproduce in his laboratory. Because of that offset, 20 Celsius is not twice 10 Celsius in Fahrenheit terms: they are 68 and 50, not 68 and 34.

What is Kelvin, and when would I use it?

Kelvin is the scale that starts at absolute zero, the point where there is no thermal energy left to remove — which is -273.15 Celsius. A Kelvin is exactly the same size as a Celsius degree, so converting between them is only ever adding or subtracting 273.15, with no multiplication involved. It is used throughout science and engineering because it makes the physics work: gas laws and thermal calculations need a scale with a true zero, and doubling the Kelvin figure really does mean twice the thermal energy. There is no degree symbol with Kelvin — it is 300 K, not 300 degrees K.

At what temperature are Celsius and Fahrenheit the same?

At -40. It is the one point where the two scales cross, so -40 Celsius and -40 Fahrenheit are the same physical temperature and no conversion is needed. It falls out of the arithmetic: setting C equal to (C x 9/5) + 32 and solving gives C = -40. It is also a genuinely useful check on any converter, including this one — feed it -40 in either direction and a correct tool hands the same number straight back.