Celsius to Fahrenheit

Multiply the Celsius value by 9/5 (that is 1.8) and add 32: 20 °C × 1.8 + 32 = 68 °F. To go from Fahrenheit to Celsius, subtract 32 and multiply by 5/9: (98.6 − 32) × 5/9 = 37 °C. For a quick estimate in your head, double the Celsius value and add 30, which puts 20 °C at about 70 °F.

Kelvin is the SI temperature scale that starts at absolute zero. One kelvin is the same size as one Celsius degree, so K = °C + 273.15 and °C = K − 273.15. Water freezes at 273.15 K and boils at 373.15 K at standard pressure. Kelvin is written without a degree sign, for example 300 K.

Nothing can be colder than absolute zero, which is 0 K, −273.15 °C or −459.67 °F. Rather than returning a negative kelvin value, the converter reports an error for anything below that point. Absolute zero itself, such as −273.15 °C, converts normally to 0 K.

At −40 degrees: −40 °C × 9/5 + 32 = −72 + 32 = −40 °F, which makes it a handy check that a conversion is right. Other reference points are body temperature at 37 °C = 98.6 °F, a 180 °C oven at 356 °F and boiling water at 100 °C = 212 °F.

Results are rounded to two decimal places with trailing zeros removed, which is finer than any household thermometer. The formulas themselves are exact, but many conversions do not come out even: 350 °F is 176.67 °C, which you would round to 177 °C for cooking.