Temperature conversion

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Temperature, like many quantities, can be measured in different units, many of which have particular uses. Temperature conversion refers to the process of transforming values measured in one unit to values in another unit. The most widely used scales are those of Kelvin, Celsius, Fahrenheit and Rankine, for which conversion tables are provided below.
(CC) Photo: Jan Schwartz
A thermometer dial displaying temperatures in degrees Celsius and in degrees Fahrenheit.

Table 1 compares the values for three major reference points in the Kelvin, Celsius, Fahrenheit and Rankine temperature scales.

Table 1: Comparison of Temperature Scales
Reference points Kelvin Celsius Fahrenheit Rankine
Absolute zero 0 K −273.15 °C −459.67 °F 0 °R
Melting point of water (a) 273.15 K 0 °C 32 °F 491.67 °R
Normal boiling point of water (b) 373.15 K 100 °C 212 °F 671.67 °R
(a) The melting point of water (i.e., ice) used to be defined as 0°C (32°F) under an atmospheric pressure of 101.325 kPa. 0 °C is taken to be 273.15 K but does not exactly equal the melting point of water which is now defined as 273.152519 K. Note that the freezing point of water is ill-defined as water usually freezes a few degrees below 0°C and the actual temperature is not reproducible.

(b) The normal boiling point of water used to be defined as 100°C at an atmospheric pressure of 101.325 kPa, but it now been estimated at 99.9839°C.

Table 2 presents the factors required to convert temperatures from any one of those four scales to the other three.

Table 2: Temperature Conversions
Scale To Kelvin From Kelvin
Celsius K = °C + 273.15 °C = K − 273.15
Fahrenheit K = (°F + 459.67) ÷ 1.8 °F = (K × 1.8) − 459.67
Rankine K = °R ÷ 1.8 °R = 1.8 × K
Scale To Fahrenheit From Fahrenheit
Celsius °F = (1.8 × °C) + 32 °C = (°F − 32) ÷ 1.8
Rankine °F = °R − 459.67 °R = °F + 459.67
Kelvin °F = (1.8 × K) − 459.67 K = (°F + 459.67) ÷ 1.8
Scale To Celsius From Celsius
Fahrenheit °C = (°F − 32) ÷ 1.8 °F = (1.8 × °C) + 32
Rankine °C = (°R ÷ 1.8) − 273.15 °R = 1.8 × (°C + 273.15)
Kelvin °C = K − 273.15 K = °C + 273.15
Scale To Rankine From Rankine
Celsius °R = 1.8 × (°C + 273.15) °C = (°R ÷ 1.8) − 273.15
Fahrenheit °R = °F + 459.67 °F = °R − 459.67
Kelvin °R = 1.8 × K K = °R ÷ 1.8