By definition, **absolute temperature** is a temperature reading reported using a temperature scale where 0 is absolute zero. In other words, it is the temperature of an object using an **absolute temperature scale**. The two absolute temperature scales are Kelvin (metric) and Rankine (English). Absolute temperature is also known as thermodynamic temperature.

The Kelvin scale is the International System of Units (SI) temperature scale. It is an absolute temperature scale defined where the Boltzmann constant equals 1.380649 x 10^{–23} joule per kelvin. The unit of the Kelvin scale is the kelvin (K), named for William Thompson (Lord Kelvin). Lord Kelvin described an absolute temperature scale in 1848 and estimated the value of absolute zero as -273^{o} C.

### What You Should Know About Absolute Temperature

- Temperature values on an absolute scale do not have degree symbols. Celsius and Fahrenheit are relative scales, based on the freezing point of water, so they use degree symbols. So, while you might say body temperature is 98.6 °F or 37 °C, the absolute temperature is 310.5 K or 558.27 R. Sometimes you’ll see a Rankine temperature reported with a degree symbol. This distinguishes it from other types of “R” used in science. Rankine temperatures also use °Ra, to differentiate between Rankine and the Rømer and Réaumur scales.
- Except for absolute zero, every temperature on an absolute scale is a positive value. This is the main reason many equations require absolute temperatures.
- Absolute temperature and relative temperature use similar scales (for the primary metric and English scales). The Celsius degree is the same size as the interval between Kelvin units. So, a temperature increase of 1 °C is the same as a 1 K increase. The Fahrenheit degree is the same size as the interval between Rankine units.
- At absolute zero, the kinetic energy of atoms and molecules is its minimum value. Some sources say atoms and molecules have zero energy, but that isn’t technically correct. At zero on an absolute scale, molecules have zero thermal (heat) energy, but they still have enthalpy and still vibrate. Also, the definition of absolute temperature is based on ideal gas behavior. A solid can have multiple stable crystal structures at absolute zero, yet only one of them has minimal energy.

### Important Absolute Temperature Values

Here are some important absolute temperature values:

Kelvin | Rankine | Celsius | Fahrenheit | |

Absolute Zero | 0 | 0 | -273.15 | -459.67 |

Freezing Point of Water | 273.15 | 491.67 | 0 | 32 |

Room Temperature | 298.15 | 536.67 | 25 | 77 |

Body Temperature | 310.15 | 558.27 | 37 | 98.6 |

Boiling Point of Water | 373.15 | 671.67 | 100 | 212 |

The Celsius and Fahrenheit scales are equal at -40°, which is 233.15K. The Fahrenheit and Kelvin scales are equal at 574.59°.

### Metric and English Temperature Conversions

Many equations require absolute temperature values. So, if you have a temperature in Celsius or Fahrenheit, convert them to Kelvin or Rankine before plugging them into equations. Convert from Kelvin or Rankine into Celsius or Fahrenheit after performing the calculation.

Here are the four most common temperature conversion formulas:

#### Metric Conversions Between Celsius and Kelvin

- Celsius to Kelvin: K = C + 273.15 (no degree symbol)
- Kelvin to Celsius: C = K – 273.15 (include degree symbol)

#### English Conversions Between Fahrenheit and Rankine

- Fahrenheit to Rankine: R = F + 459.67 (no degree symbol)
- Rankine to Fahrenheit: F = R – 459.67 (include degree symbol)

### References

- Balmer, Robert (2011).
*Modern Engineering Thermodynamic*s. Oxford: Elsevier Inc. ISBN 978-0-12-374996-3. - Pauken, Michael (2011).
*Thermodynamics For Dummies*. Indianapolis: Wiley Publishing Inc. ISBN 978-1-118-00291-9. - Thompson, Ambler; Taylor, Barry N. (2008).
*Guide for the use of the International System of Units (SI)*. National Institute of Standards and Technology (NIST). doi:10.6028/nist.sp.811e2008