Kelvin Converter

Kelvin is the SI base unit of temperature. It begins at absolute zero, the coldest temperature possible, and its degree size is identical to the Celsius degree, so 0 °C is 273.15 K exactly.

Kelvin is used in science and engineering, especially thermodynamics, astronomy, cryogenics and gas calculations, where temperatures must be measured from absolute zero rather than from a water reference point.

Kelvin shares the Celsius degree size, so a temperature difference of one kelvin equals a difference of one degree Celsius. The scales differ only by the offset of 273.15.

A common error is writing a Celsius value with the kelvin symbol; 25 °C is 298.15 K, and 25 K is far colder. Kelvin is written without a degree symbol, and ordinary temperatures are never negative on this scale.

Kelvin is the outlier in its dimension because its zero is absolute zero, while Celsius and Fahrenheit place their zeros at chosen reference points. Weather, cooking and body temperature use those two, and laboratory work uses Kelvin.

Celsius to Kelvin is the only temperature pair that behaves like a simple conversion: same degree size, different zero, so the value shifts by 273.15 and no multiplication is involved. Every pair involving Fahrenheit is affine, meaning a scale factor and an offset applied together.

That is why this dimension is the exception to the rest of the converter. Multiplying a Celsius figure by 9/5 gives a number that is not a Fahrenheit temperature, because the 32-degree offset is still missing. The result only becomes correct when the addition is made in the right order.

Checking a temperature conversion is easiest at a value whose answer you know: water freezing and boiling, body temperature, or the single reading of −40 where the Celsius and Fahrenheit scales agree. The error to watch for is dropping the offset when converting a temperature difference, where it genuinely does not apply.