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Thermometer (photo by bucklava - CC-BY)

Fahrenheit, Celsius, Centigrade, Kelvin and Rankine are all units of measurement for temperature scales, but some of them have unusual characteristics. Here’s what they mean and how to convert between them.

We can easily deal with the degree Centigrade (abbreviation °C) because the unit is no longer in use. Since 1948 the equivalent term degree Celsius (°C) has been adopted internationally to avoid confusion with the centigrade, which is an angular unit in French and Spanish.

The Celsius scale was originally defined with the melting point of water being zero degrees, and the boiling point of water being 100 degrees – but these physical characteristics of water vary with atmospheric pressure. Nowadays the Celsius scale is defined precisely with absolute zero (the coldest possible temperature) being -273.15 °C and the triple point of water being 0.01 °C.

The Fahrenheit scale is defined with the freezing point of water at 32 degrees Fahrenheit (32 °F) and the boiling point at 212 °F. To convert from Fahrenheit to Celsius, subtract 32 (to align the freezing points) then multiply by five-ninths (to account for the different sizes of the degrees). To convert from Celsius to Fahrenheit, multiply by nine-fifths then add 32.

The Kelvin scale has degrees the same size as those of the Celsius scale, but starts with absolute zero being zero Kelvin (0K). A degree sign is not used for the Kelvin scale. There cannot be negative temperatures on the Kelvin scale. To convert from Kelvin to Celsius, subtract 273.15. To convert from Celsius to Kelvin, add 273.15.

The Rankine scale has degrees the same size as those of the Fahrenheit scale, but starts with absolute zero being zero degrees Rankine (0R). To convert from Rankine to Fahrenheit, subtract 459.67. To convert from Fahrenheit to Rankine, add 459.67.

Here are the two most common conversions, expressed mathematically:

F = C × 9/5 + 32
C = (F – 32) × 5/9

A change of one degree on the Celsius or Kelvin scale is the same temperature change as a change of 1.8 degrees on the Fahrenheit or Rankine scales.

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