Why temperature is not a simple multiplier
Length units share a common zero: zero metres and zero feet both mean no length. Celsius and Fahrenheit do not share a zero point. Their degree sizes also differ, so converting requires both multiplication and an offset.
Kelvin and Celsius have the same interval size, but their zeros are separated by 273.15. Absolute zero is 0 K, equal to −273.15 °C.
The three core formulas
Use the complete formula rather than converting only the degree size.
- Celsius to Fahrenheit: °F = (°C × 9/5) + 32
- Fahrenheit to Celsius: °C = (°F − 32) × 5/9
- Celsius to kelvin: K = °C + 273.15
Useful reference points
Reference points make quick estimates and error checks easier.
- −40 °C = −40 °F (the scales meet)
- 0 °C = 32 °F (water freezes at standard atmospheric pressure)
- 20 °C = 68 °F (mild room temperature)
- 37 °C = 98.6 °F (approximate normal body temperature)
- 100 °C = 212 °F (water boils at standard atmospheric pressure)
Temperature versus temperature difference
An absolute temperature and a temperature change are not converted in exactly the same way. A rise of 1 °C equals a rise of 1 K and a rise of 1.8 °F. Do not add 32 when converting a temperature difference because offsets cancel when subtracting two temperatures.
Sources and standards
Definitions were checked against the following primary references: