Temperature Converter

What is temperature?

Temperature measures how hot or cold something is: physically, it reflects the average kinetic energy of the particles in a substance. Unlike length or mass, most temperature scales do not start at zero amount, so converting between them usually means multiplying and then adding or subtracting an offset. That is why 0 °C is 32 °F rather than 0 °F.

The SI unit of temperature is the kelvin, which starts at absolute zero, the coldest possible temperature (−273.15 °C or −459.67 °F). The Celsius scale uses the same size of degree as the kelvin but puts its zero at the freezing point of water. The Fahrenheit scale, still used for everyday temperatures in the United States, has smaller degrees: a change of 1 °C equals a change of 1.8 °F.

To convert a temperature difference (for example "the temperature rose by 10 degrees"), use the temperature difference converter, because differences convert without the offset.

Temperature formulas

• Celsius to Fahrenheit: °F = °C × 9/5 + 32
• Fahrenheit to Celsius: °C = (°F − 32) × 5/9
• Celsius to Kelvin: K = °C + 273.15
• Kelvin to Celsius: °C = K − 273.15
• Fahrenheit to Kelvin: K = (°F + 459.67) × 5/9
• Fahrenheit to Rankine: °R = °F + 459.67

A quick mental shortcut from Celsius to Fahrenheit: double it and add 30. It is within 2 °F between about 0 °C and 20 °C and drifts further off outside that range.

A short history of temperature scales

Early thermometers of the 1600s had no agreed scale. In 1724 Daniel Gabriel Fahrenheit, a German-Dutch instrument maker, published a scale with 0 at the temperature of a brine mixture and 96 at body temperature; water then froze at 32 and boiled at 212. In 1742 the Swedish astronomer Anders Celsius proposed a scale with 100 degrees between the freezing and boiling points of water; it was originally inverted, with 0 for boiling, and was flipped after his death.

In 1848 William Thomson, later Lord Kelvin, proposed an absolute scale starting from the lowest possible temperature. The kelvin became an SI base unit, and since 2019 it has been defined by fixing the Boltzmann constant, so that the Celsius scale is now formally defined from the kelvin rather than from water.