UnitBench

Temperature

Celsius, Fahrenheit and Kelvin — the one conversion that needs an offset as well as a multiplier.

Click any tile to make it the input — the value carries across.

Why temperature is the odd one out

Almost every conversion is a single multiplication, because the units share a zero: zero metres is zero feet, zero kilograms is zero pounds. Temperature scales do not. Zero Celsius is 32 Fahrenheit, so converting requires scaling and shifting — multiply by 9/5, then add 32.

This has a practical consequence people trip over constantly: you cannot convert a temperature difference the same way you convert a temperature. A rise of 10°C is a rise of 18°F, not 50°F — for a difference you apply the factor and not the offset. Weather reports and engineering specs both get this wrong from time to time.

Reference points worth memorising

CelsiusFahrenheitKelvinWhat it is
−273.15−459.670Absolute zero
−40−40233.15Where the two scales cross
032273.15Water freezes
2068293.15Comfortable room
3798.6310.15Body temperature
100212373.15Water boils at sea level
180356453.15Moderate oven

The rough mental conversion for weather is double the Celsius and add 30 — 20°C becomes about 70°F, which is close enough to decide on a jacket. It drifts at the extremes, where the exact 9/5 matters.

Also here

Everything elseLength, weight, volume, area, speedCooking measuresOven temperatures and ingredient weightsData sizesSI versus binary

Questions

Why can't I just multiply to convert temperature?
Because the scales do not share a zero. Celsius and Fahrenheit put their zero points in different places, so converting needs both a scale factor and an offset: multiply by 9/5, then add 32. Every other conversion on this site is a single multiplication because those units do share a zero — zero metres is zero feet — which is exactly why temperature gets its own page.
What is the -40 thing?
Minus forty is the one temperature where Celsius and Fahrenheit read the same number, because it is where the two lines cross. It is a genuinely useful sanity check: if a converter does not return -40°F for -40°C, its offset is wrong.
When would I use Kelvin?
Whenever a calculation involves a ratio of temperatures — gas laws, thermodynamics, colour temperature. Kelvin starts at absolute zero, so 200 K really is twice as hot as 100 K, whereas 20°C is not twice as hot as 10°C in any meaningful sense. Kelvin has no degree symbol, by convention.
Is body temperature really 98.6°F?
It is 37°C converted to one decimal place, and the precision is an artefact of the conversion rather than a measurement. The original 19th-century figure was 37°C — a round number in Celsius — and the oddly exact 98.6 is what that becomes in Fahrenheit. Modern studies put the average somewhat lower, and it varies by person, time of day and measurement site.