Multiply the Celsius value by 9/5 (or 1.8) and then add 32. For example, 100°C × 1.8 + 32 = 212°F. To reverse it, subtract 32 and multiply by 5/9: (212°F − 32) × 5/9 = 100°C.

Absolute zero, the coldest theoretically possible temperature, is 0 Kelvin, which equals −273.15°C or −459.67°F. At this temperature all molecular motion theoretically ceases. It has never been achieved experimentally, though scientists have come extremely close.

Fahrenheit was established in the early 18th century by Daniel Gabriel Fahrenheit and was widely adopted in the British Empire. The United States retained the system after independence. Most of the world switched to Celsius (also called centigrade) as part of the metric system, which anchors 0° to the freezing point of water and 100° to the boiling point.

Kelvin is the SI base unit for thermodynamic temperature and uses the same degree size as Celsius, but starts at absolute zero instead of the freezing point of water. To convert Celsius to Kelvin, simply add 273.15. Room temperature (~22°C) is 295.15 K.

Celsius and Fahrenheit are equal at −40°. Both scales cross at −40°C = −40°F. This is a useful sanity check: below this point Fahrenheit readings are numerically lower than Celsius; above it they are higher.