Temperature Converter

The formulas

Celsius to Fahrenheit is C times 9/5 plus 32. Fahrenheit to Celsius is F minus 32, times 5/9. Celsius to Kelvin is simply C plus 273.15, because the scales share a degree size and differ only in where zero sits.

Why the offset exists

Celsius sets zero at the freezing point of water and 100 at its boiling point at standard pressure. Fahrenheit's zero came from the freezing point of a brine solution, with body temperature originally intended to land near 96. The two scales therefore differ in both zero point and degree size, which is why conversion needs a multiply and an add rather than one operation.

A temperature and a difference convert differently

This catches people constantly. To convert a temperature, apply the full formula including the offset. To convert a temperature difference, use only the ratio: a rise of 10 degrees Celsius is a rise of 18 degrees Fahrenheit, not 50. Applying the offset to a difference is wrong and produces answers that are badly off. The same applies to Kelvin, where a difference of 1 K equals a difference of 1 degree Celsius exactly.

Reference points worth memorising

Minus 40 is the same in both Celsius and Fahrenheit, which is the only point where the scales cross. Water freezes at 0 C, 32 F, 273.15 K and boils at 100 C, 212 F, 373.15 K. Normal human body temperature is about 37 C or 98.6 F, though that figure comes from nineteenth-century measurements and modern averages run slightly lower. A comfortable room is around 20 to 22 C, which is 68 to 72 F.

Absolute zero and why Kelvin has no degrees

Kelvin starts at absolute zero, minus 273.15 C, the point at which molecular motion reaches its minimum. Because the scale is absolute rather than relative to an arbitrary reference, values are written as kelvin without a degree sign: 300 K, not 300 degrees K. Kelvin is what scientific work uses precisely because ratios on it are meaningful, which they are not on Celsius or Fahrenheit.

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