The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius.[1][2][3][4] It is named after the Belfast-born and University of Glasgow-based engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907). The Kelvin scale is an absolute thermodynamic temperature scale, meaning it uses absolute zero as its zero point.[2][5]

Thermometer with markings in degrees Celsius and in kelvins
General information
Unit systemSI
Unit oftemperature
Named afterWilliam Thomson, 1st Baron Kelvin
x K in ...... corresponds to ...
   Celsius   (x − 273.15) °C
   Fahrenheit   (1.8x − 459.67) °F
   Rankine   1.8x °Ra

Historically, the Kelvin scale was developed by shifting the starting point of the much-older Celsius scale down from the melting point of ice to absolute zero, and its increments still closely approximate the historic definition of a degree Celsius, but since 2019 the scale has been defined by fixing the Boltzmann constant k to be exactly 1.380649×10−23 J⋅K−1.[1] Hence, one kelvin is equal to a change in the thermodynamic temperature T that results in a change of thermal energy kT by 1.380649×10−23 J. The temperature in degree Celsius is now defined as the temperature in kelvins minus 273.15,[2] meaning that a change or difference in temperature has the same value when expressed in degrees Celsius as in kelvins, and that 0 °C is equal to 273.15 K.

The kelvin is the primary unit of temperature for engineering and the physical sciences, while in most countries Celsius remains the dominant scale outside of these fields. In the United States, outside of the physical sciences the Fahrenheit scale predominates, with the kelvin or Rankine scale employed for absolute temperature. Those are defined using the kelvin.[6][7]

According to SI convention, the kelvin is never referred to nor written as a degree. The word kelvin is not capitalised when used as a unit, but is pluralised as appropriate. The unit symbol K is a capital letter. For example, "It is 50 degrees Fahrenheit outside" vs "It is 10 degrees Celsius outside" vs "It is 283 kelvins outside".[8] It is common convention to capitalize Kelvin when referring to the Kelvin scale.[9]



An ice water bath offered a practical calibration point for thermometers in a time before the physical nature of heat was well understood.

During the 18th century, multiple temperature scales were developed,[10] notably Fahrenheit and centigrade (later Celsius). These scales predated much of the modern science of thermodynamics, including atomic theory and the kinetic theory of gases which underpin the concept of absolute zero. Instead, they chose defining points within the range of human experience that could be reproduced easily and with reasonable accuracy, but lacked any deep significance in thermal physics. In the case of the Celsius scale (and the long since defunct Newton scale and Réaumur scale) the melting point of water served as such a starting point, with Celsius being defined, from the 1740s up until the 1940s, by calibrating a thermometer such that:

This definition assumes pure water at a specific pressure chosen to approximate the natural air pressure at sea level. Thus an increment of 1 °C equals 1/100 of the temperature difference between the melting and boiling points. This temperature interval would go on to become the template for the kelvin.

Charles's law

From 1787 to 1802, it was determined by Jacques Charles (unpublished), John Dalton,[11] and Joseph Louis Gay-Lussac[12] that, at constant pressure, ideal gases expanded or contracted their volume linearly (Charles's law) by about 1/273 parts per degree Celsius of temperature's change up or down, between 0° and 100° C. This suggested that the volume of a gas cooled at about −273 °C would reach zero.

Lord Kelvin

Lord Kelvin, the namesake of the unit of measure.

In 1848, William Thomson, who was later ennobled as Lord Kelvin, published a paper On an Absolute Thermometric Scale.[13][14][15] Using the soon-to-be-defunct caloric theory, he proposed an "absolute" scale based on the following parameters:

  • The melting point of water is 0 degrees.
  • The boiling point of water is 100 degrees.

"The arbitrary points which coincide on the two scales are 0° and 100°"

  • Any two heat engines whose heat source and heat sink are both separated by the same number of degrees will, per Carnot's theorem, be capable of producing the same amount of mechanical work per unit of "caloric" passing through.

"The characteristic property of the scale which I now propose is, that all degrees have the same value; that is, that a unit of heat descending from a body A at the temperature T° of this scale, to a body B at the temperature (T − 1)°, would give out the same mechanical effect, whatever be the number T. This may justly be termed an absolute scale, since its characteristic is quite independent of the physical properties of any specific substance."

As Carnot's theorem is understood in modern thermodynamics to simply describe the maximum efficiency with which thermal energy can be converted to mechanical energy and the predicted maximum efficiency is a function of the ratio between the absolute temperatures of the heat source and heat sink:

  • Efficiency ≤ 1 − absolute temperate of heat sink/absolute temperature of heat source

It follows that increments of equal numbers of degrees on this scale must always represent equal proportional increases in absolute temperature. The numerical value of an absolute temperature, T, on the 1848 scale is related to the absolute temperature of the melting point of water, Tmpw, and the absolute temperature of the boiling point of water, Tbpw, by:

  • T (1848 scale) = 100 (ln T/Tmpw) / (ln Tbpw/Tmpw)

On this scale, an increase of 222 degrees always means an approximate doubling of absolute temperature regardless of the starting temperature.

In a footnote Thomson calculated that "infinite cold" (absolute zero, which would have a numerical value of negative infinity on this scale) was equivalent to −273 °C using the air thermometers of the time. This value of "−273" was the negative reciprocal of 0.00366—the accepted coefficient of thermal expansion of an ideal gas per degree Celsius relative to the ice point, giving a remarkable consistency to the currently accepted value.

Within a decade, Thomson had abandoned caloric theory and superseded the 1848 scale with a new one[14][16] based on the 2 features that would characterise all future versions of the kelvin scale:

  • Absolute zero is the null point.
  • Increments have the same magnitude as they do in the Celsius scale.

In 1892, Thomson was awarded the noble title 1st Baron Kelvin of Largs, or more succinctly Lord Kelvin. This name was a reference to the River Kelvin which flows through the grounds of Glasgow University.

In the early decades of the 20th century, the Kelvin scale was often called the "absolute Celsius" scale, indicating Celsius degrees counted from absolute zero rather than the freezing point of water, and using the same symbol for regular Celsius degrees, °C.[17]

Triple point standard

A typical phase diagram. The solid green line applies to most substances; the dashed green line gives the anomalous behavior of water. The boiling line (solid blue) runs from the triple point to the critical point, beyond which further increases in temperature and pressure produce a supercritical fluid.

In 1873, William Thomson's older brother James coined the term triple point[18] to describe the combination of temperature and pressure at which the solid, liquid, and gas phases of a substance were capable of coexisting in thermodynamic equilibrium. While any two phases could coexist along a range of temperature-pressure combinations (e.g. the boiling point of water can be affected quite dramatically by raising or lowering the pressure), the triple point condition for a given substance can occur only at a single pressure and only at a single temperature. By the 1940s, the triple point of water had been experimentally measured to be about 0.6% of standard atmospheric pressure and very close to 0.01 °C per the historical definition of Celsius then in use.

In 1948, the Celsius scale was recalibrated by assigning the triple point temperature of water the value of 0.01 °C exactly and allowing the melting point at standard atmospheric pressure to have an empirically determined value (and the actual melting point at ambient pressure to have a fluctuating value) close to 0 °C. This was justified on the grounds that the triple point was judged to give a more accurately reproducible reference temperature than the melting point.[19]

In 1954, with absolute zero having been experimentally determined to be about −273.15 °C per the definition of °C then in use, Resolution 3 of the 10th General Conference on Weights and Measures (CGPM) introduced a new internationally standardised Kelvin scale which defined the triple point as exactly: 273.15 + 0.01 = 273.16 "degrees Kelvin".[20][21]

In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K.[8][22] The 13th CGPM also held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."[4][23][24]

After the 1983 redefinition of the metre, this left the kelvin, the second, and the kilogram as the only SI units not defined with reference to any other unit.

In 2005, noting that the triple point could be influenced by the isotopic ratio of the hydrogen and oxygen making up a water sample and that this was "now one of the major sources of the observed variability between different realizations of the water triple point", the International Committee for Weights and Measures (CIPM), a committee of the CGPM, affirmed that for the purposes of delineating the temperature of the triple point of water, the definition of the kelvin would refer to water having the isotopic composition specified for Vienna Standard Mean Ocean Water.[4][25][26]

2019 redefinition

The kelvin is now fixed in terms of the Boltzmann constant and the joule, itself defined by the caesium-133 hyperfine transition frequency and the Planck constant. Both k and kB are accepted shorthand for the Boltzmann constant.

In 2005, the CIPM began a programme to redefine the kelvin (along with the other SI units) using a more experimentally rigorous method. In particular, the committee proposed redefining the kelvin such that the Boltzmann constant takes the exact value 1.3806505×10−23 J/K.[27] The committee had hoped that the program would be completed in time for its adoption by the CGPM at its 2011 meeting, but at the 2011 meeting the decision was postponed to the 2014 meeting when it would be considered as part of a larger program.[28]

The redefinition was further postponed in 2014, pending more accurate measurements of the Boltzmann constant in terms of the current definition,[29] but was finally adopted at the 26th CGPM in late 2018, with a value of k = 1.380649×10−23 J⋅K−1.[30][27][1][2][4][31]

For scientific purposes, the main advantage is that this allows measurements at very low and very high temperatures to be made more accurately, as the techniques used depend on the Boltzmann constant. It also has the philosophical advantage of being independent of any particular substance. The unit J/K is equal to kg⋅m2⋅s−2⋅K−1, where the kilogram, metre and second are defined in terms of the Planck constant, the speed of light, and the duration of the caesium-133 ground-state hyperfine transition respectively.[2] Thus, this definition depends only on universal constants, and not on any physical artifacts as practiced previously. The challenge was to avoid degrading the accuracy of measurements close to the triple point. For practical purposes, the redefinition was unnoticed; water still freezes at 273.15 K (0 °C),[2][32] and the triple point of water continues to be a commonly used laboratory reference temperature.

The difference is that, before the redefinition, the triple point of water was exact and the Boltzmann constant had a measured value of 1.38064903(51)×10−23 J/K, with a relative standard uncertainty of 3.7×10−7.[33] Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now 273.1600(1) K.

The new definition officially came into force on 20 May 2019, the 144th anniversary of the Metre Convention.[31][1][2][4]

Practical uses

Colour temperature

The kelvin is often used as a measure of the colour temperature of light sources. Colour temperature is based upon the principle that a black body radiator emits light with a frequency distribution characteristic of its temperature. Black bodies at temperatures below about 4000 K appear reddish, whereas those above about 7500 K appear bluish. Colour temperature is important in the fields of image projection and photography, where a colour temperature of approximately 5600 K is required to match "daylight" film emulsions. In astronomy, the stellar classification of stars and their place on the Hertzsprung–Russell diagram are based, in part, upon their surface temperature, known as effective temperature. The photosphere of the Sun, for instance, has an effective temperature of 5778 K.

Digital cameras and photographic software often use colour temperature in K in edit and setup menus. The simple guide is that higher colour temperature produces an image with enhanced white and blue hues. The reduction in colour temperature produces an image more dominated by reddish, "warmer" colours.

Kelvin as a unit of noise temperature

For electronics, the kelvin is used as an indicator of how noisy a circuit is in relation to an ultimate noise floor, i.e. the noise temperature. The so-called Johnson–Nyquist noise of discrete resistors and capacitors is a type of thermal noise derived from the Boltzmann constant and can be used to determine the noise temperature of a circuit using the Friis formulas for noise.

Derived units and SI multiples

The only SI derived unit with a special name derived from the kelvin is the degree Celsius. Like other SI units, the kelvin can also be modified by adding a metric prefix that multiplies it by a power of 10:

SI multiples of kelvin (K)
Submultiples Multiples
Value SI symbol Name Value SI symbol Name
10−1 K dK decikelvin 101 K daK decakelvin
10−2 K cK centikelvin 102 K hK hectokelvin
10−3 K mK millikelvin 103 K kK kilokelvin
10−6 K µK microkelvin 106 K MK megakelvin
10−9 K nK nanokelvin 109 K GK gigakelvin
10−12 K pK picokelvin 1012 K TK terakelvin
10−15 K fK femtokelvin 1015 K PK petakelvin
10−18 K aK attokelvin 1018 K EK exakelvin
10−21 K zK zeptokelvin 1021 K ZK zettakelvin
10−24 K yK yoctokelvin 1024 K YK yottakelvin
10−27 K rK rontokelvin 1027 K RK ronnakelvin
10−30 K qK quectokelvin 1030 K QK quettakelvin

Unicode character

The symbol is encoded in Unicode at code point U+212A KELVIN SIGN. However, this is a compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using U+004B K LATIN CAPITAL LETTER K instead; that is, a normal capital K. "Three letterlike symbols have been given canonical equivalence to regular letters: U+2126 OHM SIGN, U+212A KELVIN SIGN, and U+212B ANGSTROM SIGN. In all three instances, the regular letter should be used."[34]

See also


  1. BIPM (20 May 2019). "Mise en pratique for the definition of the kelvin in the SI". Retrieved 18 February 2022.
  2. "SI Brochure: The International System of Units (SI) – 9th edition". BIPM. Retrieved 21 February 2022.
  3. "SI base unit: kelvin (K)". BIPM. Retrieved 5 March 2022.
  4. "A Turning Point for Humanity: Redefining the World's Measurement System". Nist. 12 May 2018. Retrieved 21 February 2022.
  5. "Kelvin: Introduction". NIST. 14 May 2018. Retrieved 2 September 2022.
  6. Benham, Elizabeth (6 October 2020). "Busting Myths about the Metric System". Nist. Taking Measure (official blog of the NIST). Retrieved 21 February 2022.
  7. "Handbook 44 – 2022 – Appendix C – General Tables of Units of Measurement" (PDF). NIST. Retrieved 21 February 2022.
  8. "Resolution 3 of the 13th CGPM (1967)". BIPM. Retrieved 21 February 2022.
  9. Brady, James E.; Senese, Fred (28 January 2008). Chemistry, Student Study Guide: The Study of Matter and Its Changes. John Wiley & Sons. p. 15. ISBN 978-0-470-18464-6.
  10. "Kelvin: History". Nist. 14 May 2018. Retrieved 21 February 2022.
  11. J. Dalton (1802), "Essay II. On the force of steam or vapour from water and various other liquids, both in vacuum and in air" and Essay IV. "On the expansion of elastic fluids by heat," Memoirs of the Literary and Philosophical Society of Manchester, vol. 8, pt. 2, pp. 550–74, 595–602.
  12. Gay-Lussac, J. L. (1802), "Recherches sur la dilatation des gaz et des vapeurs", Annales de Chimie, XLIII: 137. English translation (extract).
  13. Thomson, William. "On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat, and calculated from Regnault's Observations". Philosophical Magazine. Retrieved 21 February 2022.
  14. Thomson, William. "On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat, and calculated from Regnault's Observations (1881 reprint)" (PDF). Philosophical Magazine. Retrieved 21 February 2022.
  15. Lord Kelvin, William (October 1848). "On an Absolute Thermometric Scale". Philosophical Magazine. Archived from the original on 1 February 2008. Retrieved 6 February 2008.
  16. Thomson, William. "On the Dynamical Theory of Heat, with numerical results deduced from Mr Joule's equivalent of a Thermal Unit, and M. Regnault's Observations on Steam (Excerpts)". Transactions of the Royal Society of Edinburgh and Philosophical Magazine. Retrieved 21 February 2022.
  17. For example, Encyclopaedia Britannica editions from the 1920s and 1950s, one example being the article "Planets".
  18. James Thomson (1873) "A quantitative investigation of certain relations between the gaseous, the liquid, and the solid states of water-substance", Proceedings of the Royal Society, 22 : 27–36. From a footnote on page 28: " … the three curves would meet or cross each other in one point, which I have called the triple point".
  19. "Resolution 3 of the 9th CGPM (1948)". BIPM. Retrieved 21 February 2022.
  20. "Resolution 3 of the 10th CGPM (1954)". BIPM. Retrieved 21 February 2022.
  21. "Resolution 3: Definition of the thermodynamic temperature scale". Resolutions of the 10th CGPM. Bureau International des Poids et Mesures. 1954. Archived from the original on 23 June 2007. Retrieved 6 February 2008.
  22. "Resolution 3: SI unit of thermodynamic temperature (kelvin)". Resolutions of the 13th CGPM. Bureau International des Poids et Mesures. 1967. Archived from the original on 21 April 2007. Retrieved 6 February 2008.
  23. "Resolution 4 of the 13th CGPM (1967)". BIPM. Retrieved 21 February 2022.
  24. "Resolution 4: Definition of the SI unit of thermodynamic temperature (kelvin)". Resolutions of the 13th CGPM. Bureau International des Poids et Mesures. 1967. Archived from the original on 15 June 2007. Retrieved 6 February 2008.
  25. "Resolution 10 of the 23rd CGPM (2007)". BIPM. Retrieved 21 February 2022.
  26. "Unit of thermodynamic temperature (kelvin)". SI Brochure, 8th edition. Bureau International des Poids et Mesures. 1967. pp. Section Archived from the original on 26 September 2007. Retrieved 6 February 2008.
  27. Ian Mills (29 September 2010). "Draft Chapter 2 for SI Brochure, following redefinitions of the base units" (PDF). CCU. Archived (PDF) from the original on 10 January 2011. Retrieved 1 January 2011.
  28. "General Conference on Weights and Measures approves possible changes to the International System of Units, including redefinition of the kilogram" (PDF) (Press release). Sèvres, France: General Conference on Weights and Measures. 23 October 2011. Archived (PDF) from the original on 9 February 2012. Retrieved 25 October 2011.
  29. Wood, B. (3–4 November 2014). "Report on the Meeting of the CODATA Task Group on Fundamental Constants" (PDF). BIPM. p. 7. Archived (PDF) from the original on 13 October 2015. [BIPM director Martin] Milton responded to a question about what would happen if ... the CIPM or the CGPM voted not to move forward with the redefinition of the SI. He responded that he felt that by that time the decision to move forward should be seen as a foregone conclusion.
  30. "2018 CODATA Value: Boltzmann constant". The NIST Reference on Constants, Units, and Uncertainty. NIST. 20 May 2019. Retrieved 20 May 2019.
  31. "Resolution 1 of the 26th CGPM (2018)". BIPM. Retrieved 21 February 2022.
  32. "Updating the definition of the kelvin" (PDF). International Bureau for Weights and Measures (BIPM). Archived (PDF) from the original on 23 November 2008. Retrieved 23 February 2010.
  33. Newell, D B; Cabiati, F; Fischer, J; Fujii, K; Karshenboim, S G; Margolis, H S; de Mirandés, E; Mohr, P J; Nez, F; Pachucki, K; Quinn, T J; Taylor, B N; Wang, M; Wood, B M; Zhang, Z; et al. (Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants) (29 January 2018). "The CODATA 2017 values of h, e, k, and NA for the revision of the SI". Metrologia. 55 (1): L13–L16. Bibcode:2018Metro..55L..13N. doi:10.1088/1681-7575/aa950a.
  34. "22.2". The Unicode Standard, Version 8.0 (PDF). Mountain View, CA, USA: The Unicode Consortium. August 2015. ISBN 978-1-936213-10-8. Archived (PDF) from the original on 6 December 2016. Retrieved 6 September 2015.


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