Guide

Converting temperature correctly

Temperature is the only quantity in this collection with an offset zero point. Treating it as a pure factor produces wrong results — although at minus 40 degrees the two scales do meet.

Two scales, two zero points

Celsius puts the freezing point of water at 0 °C and the boiling point at 100 °C. Fahrenheit puts freezing at 32 °F and boiling at 212 °F. That yields the conversion °F = °C × 1.8 + 32. The factor 1.8 describes the different degree size, the added 32 the shifted zero point.

Kelvin has no offset against Celsius

Kelvin starts at absolute zero, which sits at −273.15 °C. The conversion is therefore a pure addition: K = °C + 273.15. A temperature difference of 10 °C equals a difference of 10 K, but the same difference equals 18 °F. For differences and for ratios such as the thermal expansion coefficient only the degree size matters, not the zero point.

The one point where both scales agree

At −40 °C Fahrenheit also reads −40 °F. That is the only temperature at which both scales share the same number, and it makes a good check calculation. Other fixed points for verification: 0 °C = 32 °F, 37 °C = 98.6 °F, 100 °C = 212 °F.

Putting everyday temperature figures in context

Body temperature is about 37 °C, that is 98.6 °F. An oven at 180 °C equals about 356 °F. Fahrenheit figures look considerably larger for weather because one Fahrenheit degree is only five ninths of a Celsius degree. Take care with cold figures from North America: −10 °F is already −23.3 °C.

Matching conversions

Matching tables

Unit family: Temperature

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