Temperature converters

Convert between the four temperature scales still in use.

• Celsius to Fahrenheit
• Celsius to Kelvin
• Fahrenheit to Celsius
• Fahrenheit to Kelvin
• Kelvin to Celsius
• Kelvin to Fahrenheit

The scales

Celsius (°C)

The Celsius scale puts the freezing point of water at 0° and its boiling point at 100° at standard pressure.

Fahrenheit (°F)

The Fahrenheit scale puts water’s freezing point at 32° and its boiling point at 212°, and is used mainly in the United States.

Kelvin (K)

The kelvin is the SI base unit of temperature, starting at absolute zero; it uses the same degree size as Celsius.

Rankine (°R)

The Rankine scale starts at absolute zero but uses Fahrenheit-sized degrees, and appears in US thermodynamics.

Why temperature is not like other conversions

Every other unit conversion is a single multiplication, because the scales share a zero point: zero metres and zero feet are the same length, so one factor covers it. Temperature scales disagree about where zero is and how big a degree is, so a conversion needs a multiplication and an offset. That is why you cannot convert a temperature difference the same way as a temperature: a rise of 10 °C is a rise of 18 °F, not 50 °F, because the offset applies to the reading and not to the interval.

The one point where two scales agree

Celsius and Fahrenheit cross at −40°, where both scales give the same number. It falls out of the arithmetic rather than being designed in, and it makes a convenient sanity check: any conversion that does not map −40 to −40 has the offset wrong. Kelvin and Rankine both start at absolute zero, so they never go negative and never cross the other two.

Absolute zero and why Kelvin has no degree sign

Kelvin measures from absolute zero, the point at which a system holds no thermal energy that can be removed. It is written 295 K rather than 295 °K, because a kelvin is a unit in its own right rather than a position on a graduated scale. A kelvin is the same size as a degree Celsius, so converting between them is pure addition: subtract 273.15 to get Celsius. Rankine does the same job for Fahrenheit-sized degrees and appears mainly in US thermodynamics.

Reference points worth knowing

Body temperature is the one people misremember. 98.6 °F looks like a precise measurement but is simply 37 °C converted, and the original nineteenth-century figure was never that exact. Modern studies put the average slightly lower, nearer 36.6 °C.

Doing it in your head

To go from Celsius to Fahrenheit, double it and add 30. That is exact at 10 °C and within 4 degrees anywhere between −10 °C and 30 °C, which covers ordinary weather. It drifts badly at the extremes — at 100 °C it is out by 18 degrees — so use the real formula for anything that matters. Going the other way, subtract 30 and halve.