Temperature Converter

The three temperature scales

Celsius, also called centigrade, was designed around water. Zero degrees Celsius is the freezing point of water at standard pressure; 100 degrees Celsius is the boiling point. This makes it intuitive for everyday weather and cooking: a 20°C room is comfortable, a 200°C oven is hot enough to roast meat.

Fahrenheit was developed earlier and uses a different pair of reference points. It sets 32°F as the freezing point of water and 212°F as the boiling point. The 180-degree span between those points (versus 100 degrees in Celsius) means one Fahrenheit degree is smaller than one Celsius degree. Fahrenheit is still the standard unit for weather and oven temperatures in the United States.

Kelvin is the SI unit of thermodynamic temperature. It uses the same degree size as Celsius but starts at absolute zero, the coldest possible temperature, which is -273.15°C. So 0 K = -273.15°C and 273.15 K = 0°C. Kelvin has no degree symbol; you write 300 K, not 300°K. It is the required unit in thermodynamics, gas laws, and physics equations because negative temperatures cannot exist on the Kelvin scale.

Unlike length or mass conversions, temperature conversions are not simple multiplication. You must account for an additive offset. To convert Celsius to Fahrenheit: multiply by 9/5, then add 32. To convert Fahrenheit to Celsius: subtract 32, then multiply by 5/9. Kelvin to Celsius is simply subtract 273.15, with no scaling step needed because the degree sizes are equal.

Reference points worth memorising

A few reference points are useful anchors. Human body temperature is 37°C / 98.6°F / 310.15 K. A comfortable room is around 20-22°C / 68-72°F. Water freezes at 0°C / 32°F / 273.15 K and boils at 100°C / 212°F / 373.15 K at sea level. Absolute zero, the point where all molecular motion theoretically stops, is 0 K / -273.15°C / -459.67°F.

For science and engineering, Kelvin is the only scale where ratios are meaningful. Saying a gas is "twice as hot" only makes physical sense in Kelvin. Doubling a Celsius or Fahrenheit value is mathematically meaningless in thermodynamic context.

Frequently asked questions