Thermometer with Fahrenheit (marked on outer bezel) and Celsius (marked on inner dial) degree units.
General information
Unit systemImperial/US customary
Unit ofTemperature
Named afterDaniel Gabriel Fahrenheit
x °F in ...... corresponds to ...
   SI base units   5/9(x +459.67) K
   SI derived units   5/9(x − 32) °C
   Imperial/US absolute scale   x + 459.67 °Ra

The Fahrenheit scale (/ˈfærənht, ˈfɑːr-/) is a temperature scale based on one proposed in 1724 by the European physicist Daniel Gabriel Fahrenheit (1686–1736).[1] It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he originally defined his scale exist, but the original paper suggests the lower defining point, 0 °F, was established as the freezing temperature of a solution of brine made from a mixture of water, ice, and ammonium chloride (a salt).[2][3] The other limit established was his best estimate of the average human body temperature, originally set at 90 °F, then 96 °F (about 2.6 °F less than the modern value due to a later redefinition of the scale).[2]

For much of the 20th century, the Fahrenheit scale was defined by two fixed points with a 180 °F separation: the temperature at which pure water freezes was defined as 32 °F and the boiling point of water was defined to be 212 °F, both at sea level and under standard atmospheric pressure. It is now formally defined using the Kelvin scale.[4][5]

It continues to be used in the United States (including its unincorporated territories), its freely associated states in the Western Pacific (Palau, the Federated States of Micronesia and the Marshall Islands), the Cayman Islands, and Liberia.

Fahrenheit is commonly still used alongside the Celsius scale in other countries that use the U.S. metrological service, such as Antigua and Barbuda, Saint Kitts and Nevis, the Bahamas, and Belize. A handful of British Overseas Territories, including the Virgin Islands, Montserrat, Anguilla, and Bermuda, also still use both scales.[6] All other countries now use Celsius ("centigrade" until 1948), which was invented 18 years after the Fahrenheit scale.[7]

Though the United Kingdom changed from Fahrenheit to Celsius in metrology, Fahrenheit is sometimes still used in newspaper headlines to sensationalise heatwaves.[8]

Definition and conversion

Fahrenheit temperature conversion formulae
from Fahrenheit to Fahrenheit
Celsius x °F ≘ (x − 32) × 5/9 °C x °C ≘ (x × 9/5 + 32) °F
Kelvin x °F ≘ (x + 459.67) × 5/9 K x K ≘ (x × 9/5 − 459.67) °F
Rankine x °F ≘ (x + 459.67) °R x °R ≘ (x − 459.67) °F
For temperature intervals rather than specific temperatures,
1 °F = 1 °R = 5/9 °C = 5/9 K
Conversion between temperature scales

Historically, on the Fahrenheit scale the freezing point of water was 32 °F, and the boiling point was 212 °F (at standard atmospheric pressure). This put the boiling and freezing points of water 180 degrees apart.[9] Therefore, a degree on the Fahrenheit scale was 1180 of the interval between the freezing point and the boiling point. On the Celsius scale, the freezing and boiling points of water were originally defined to be 100 degrees apart. A temperature interval of 1 °F was equal to an interval of 59 degrees Celsius. With the Fahrenheit and Celsius scales now both defined by the kelvin, this relationship was preserved, a temperature interval of 1 °F being equal to an interval of 59 K and of 59 °C. The Fahrenheit and Celsius scales intersect numerically at −40 in the respective unit (i.e, −40 °F ≘ −40 °C).

Absolute zero is 0 K, −273.15 °C, or −459.67 °F. The Rankine temperature scale uses degree intervals of the same size as those of the Fahrenheit scale, except that absolute zero is 0 °R – the same way that the Kelvin temperature scale matches the Celsius scale, except that absolute zero is 0 K.[9]

The combination of degree symbol (°) followed by an uppercase letter F is the conventional symbol for the Fahrenheit temperature scale. A number followed by this symbol (and separated from it with a space) denotes a specific temperature point (e.g., "Gallium melts at 85.5763 °F"). A difference between temperatures or an uncertainty in temperature is also conventionally written the same way as well, e.g., "The output of the heat exchanger experiences an increase of 72 °F" or "Our standard uncertainty is ±5 °F". However, some authors instead use the notation "An increase of 50 F°" (reversing the symbol order) to indicate temperature differences. Similar conventions exist for the Celsius scale, see Celsius § Temperatures and intervals.[10][11]

Conversion (specific temperature point)

For an exact conversion between degrees Fahrenheit and Celsius, and kelvins of a specific temperature point, the following formulas can be applied. Here, f is the value in degrees Fahrenheit, c the value in degrees Celsius, and k the value in kelvins:

There is also an exact conversion between Celsius and Fahrenheit scales making use of the correspondence −40 °F ≘ −40 °C. Again, f is the numeric value in degrees Fahrenheit, and c the numeric value in degrees Celsius:

Conversion (temperature difference or interval)

When converting a temperature interval between the Fahrenheit and Celsius scales, only the ratio is used, without any constant (in this case, the interval has the same numeric value in kelvins as in degrees Celsius):


Fahrenheit proposed his temperature scale in 1724, basing it on two reference points of temperature. In his initial scale (which is not the final Fahrenheit scale), the zero point was determined by placing the thermometer in "a mixture of ice, water, and salis Armoniaci[note 1] [transl. ammonium chloride] or even sea salt".[12] This combination forms a eutectic system, which stabilizes its temperature automatically: 0 °F was defined to be that stable temperature. A second point, 96 degrees, was approximately the human body's temperature.[12] A third point, 32 degrees, was marked as being the temperature of ice and water "without the aforementioned salts".[12]

According to a German story, Fahrenheit actually chose the lowest air temperature measured in his hometown Danzig (Gdańsk, Poland) in winter 1708–09 as 0 °F, and only later had the need to be able to make this value reproducible using brine.[13][failed verification]

According to a letter Fahrenheit wrote to his friend Herman Boerhaave,[14] his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer scale, brine freezes at zero, water freezes and melts at 7.5 degrees, body temperature is 22.5, and water boils at 60 degrees. Fahrenheit multiplied each value by 4 in order to eliminate fractions and make the scale more fine-grained. He then re-calibrated his scale using the melting point of ice and normal human body temperature (which were at 30 and 90 degrees); he adjusted the scale so that the melting point of ice would be 32 degrees, and body temperature 96 degrees, so that 64 intervals would separate the two, allowing him to mark degree lines on his instruments by simply bisecting the interval 6 times (since 64 = 26).[15][16]

Fahrenheit soon after observed that water boils at about 212 degrees using this scale.[17] The use of the freezing and boiling points of water as thermometer fixed reference points became popular following the work of Anders Celsius, and these fixed points were adopted by a committee of the Royal Society led by Henry Cavendish in 1776–77.[18][19] Under this system, the Fahrenheit scale is redefined slightly so that the freezing point of water was exactly 32 °F, and the boiling point was exactly 212 °F, or 180 degrees higher. It is for this reason that normal human body temperature is approximately 98.6 °F (oral temperature) on the revised scale (whereas it was 90° on Fahrenheit's multiplication of Rømer, and 96° on his original scale).[20]

In the present-day Fahrenheit scale, 0 °F no longer corresponds to the eutectic temperature of ammonium chloride brine as described above. Instead, that eutectic is at approximately 4 °F on the final Fahrenheit scale.[note 2]

The Rankine temperature scale was based upon the Fahrenheit temperature scale, with its zero representing absolute zero instead.



Countries by usage:
  Fahrenheit (°F)
  Fahrenheit (°F) and Celsius (°C)
  Celsius (°C)

The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in Anglophone countries until the 1960s. In the late 1960s and 1970s, the Celsius scale replaced Fahrenheit in almost all of those countries—with the notable exception of the United States and in certain cases, the United Kingdom—typically during their general metrication process.

Fahrenheit is used in the United States, its territories and associated states (all served by the U.S. National Weather Service), as well as the (British) Cayman Islands and Liberia for everyday applications. For example, U.S. weather forecasts, food cooking, and freezing temperatures are typically given in degrees Fahrenheit. Scientists, including meteorologists, use degrees Celsius or kelvin in all countries.[21]

United States

Early in the 20th century, Halsey and Dale suggested that reasons for resistance to use the centigrade (now Celsius) system in the U.S. included the larger size of each degree Celsius and the lower zero point in the Fahrenheit system; put another way, the Fahrenheit scale is more intuitive than Celsius for describing outdoor temperatures in temperate latitudes, with 100 °F being a hot summer day and 0 °F a cold winter day.[22]


Canada has passed legislation favoring the International System of Units, while also maintaining legal definitions for traditional Canadian imperial units.[23] Canadian weather reports are conveyed using degrees Celsius with occasional reference to Fahrenheit especially for cross-border broadcasts. Fahrenheit is still used on virtually all Canadian ovens.[24] Thermometers, both digital and analog, sold in Canada usually employ both the Celsius and Fahrenheit scales.[25][26][27]

European Union

European laundry symbol for "Wash at 40 °C"

In the European Union, it is mandatory to use kelvins or degrees Celsius when quoting temperature for "economic, public health, public safety and administrative" purposes, though degrees Fahrenheit may be used alongside degrees Celsius as a supplementary unit.[28] For example, the laundry symbols used in the United Kingdom follow the recommendations of ISO 3758:2005 showing the temperature of the washing machine water in degrees Celsius only.[29] The equivalent label in North America uses one to six dots to denote temperature with an optional temperature in degrees Celsius.[30][31]

United Kingdom

Although Fahrenheit was the most popular scale in the United Kingdom, for many years Celsius has been the primary scale used, and it has been taught in schools since the 1970s.[32] Within unregulated sectors, such as journalism, the use of Fahrenheit in the United Kingdom follows no fixed pattern, with degrees Fahrenheit sometimes appearing alongside degrees Celsius. The Daily Telegraph does not mention Fahrenheit on its daily weather page,[33] while The Times also has an all-metric daily weather page but has a Celsius-to-Fahrenheit conversion table.[34] When publishing news stories, some UK tabloids have adopted a tendency of using degrees Celsius in headlines and discussion relating to low temperatures and Fahrenheit for mid to high temperatures.[35][discuss] In February 2006, the writer of an article in The Times suggested that the rationale was one of emphasis: "−6 °C" sounds colder than "21 °F", and "94 °F" sounds more impressive than "34 °C".[36]

Unicode representation of symbol

Unicode provides the Fahrenheit symbol at code point U+2109 DEGREE FAHRENHEIT. However, this is a compatibility character encoded for roundtrip compatibility with legacy encodings. The Unicode standard explicitly discourages the use of this character: "The sequence U+00B0 ° DEGREE SIGN + U+0046 F LATIN CAPITAL LETTER F is preferred over U+2109 DEGREE FAHRENHEIT, and those two sequences should be treated as identical for searching."[37]

See also


  1. ^ "Sal Armoniac" was an impure form of ammonium chloride. The French chemist Nicolas Lémery (1645–1715) discussed it in his book Cours de Chymie (A Course of Chemistry, 1675), describing where it occurs naturally and how it can be prepared artificially. It occurs naturally in the deserts of northern Africa, where it forms from puddles of animal urine. It can be prepared artificially by boiling 5 parts of urine, 1 part of sea salt, and ½ part of chimney soot until the mixture has dried. The mixture is then heated in a sublimation pot until it sublimates; the sublimated crystals are sal Armoniac. See:
    • Nicolas Lémery, Cours de chymie […], 7th ed. (Paris, France: Estienne Michallet, 1688), Chapitre XVII : du Sel Armoniac, pp. 338–339.
      • English translation: Nicolas Lémery with James Keill, trans., A Course of Chymistry […], 3rd ed. (London, England: Walter Kettilby, 1698), Chap. XVII: of Sal Armoniack, p. 383, available on-line at Heinrich Heine University (Düsseldorf, Germany).
  2. ^ Eutectic temperature of ammonium chloride and water is listed as −15.9 °C (3.38 °F) and as −15.4 °C (4.28 °F) in (respectively)
    • Peppin, S. S.; Huppert, H. E.; Worster, M. G. (2008). "Steady-state solidification of aqueous ammonium chloride" (PDF). J. Fluid Mech. 159. Cambridge University Press: 472 (table 1). Bibcode:2008JFM...599..465P. doi:10.1017/S0022112008000219. S2CID 30271164. Archived (PDF) from the original on 19 January 2010.
    • Barman, N.; Nayak, A. K.; Chattopadhyay, H. (2021). "Solidification of a Binary Solution (NH4Cl + H2O) on an Inclined Cooling Plate: A Parametric Study" (PDF). Procedia Materials Science. 7: 456 (table 1). doi:10.1016/j.mspro.2014.07.288. Archived (PDF) from the original on 15 January 2019.


  1. ^ Balmer, Robert T. (2010). Modern Engineering Thermodynamics. Academic Press. p. 9. ISBN 978-0-12-374996-3. Retrieved 17 July 2011.
  2. ^ a b "Fahrenheit temperature scale". Encyclopædia Britannica Online. Retrieved 25 September 2015.
  3. ^ "Fahrenheit: Facts, History & Conversion Formulas". Live Science. Retrieved 9 February 2018.
  4. ^ Benham, Elizabeth (6 October 2020). "Busting Myths about the Metric System". US National Institute of Standards and Technology (NIST).
  5. ^ "Appendix C – General Tables of Units of Measurement" (PDF). NIST Handbook 44 -Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices - 2022. US National Institute of Standards and Technology (NIST). 29 November 2021. Archived (PDF) from the original on 27 December 2021.
  6. ^ "50 years of Celsius weather forecasts – time to kill off Fahrenheit for good? | Metric Views". Archived from the original on 2 October 2020. Retrieved 28 July 2019.
  7. ^ Celsius, Anders (1742) "Observationer om twänne beständiga grader på en thermometer" (Observations about two stable degrees on a thermometer), Kungliga Svenska Vetenskapsakademiens Handlingar (Proceedings of the Royal Swedish Academy of Sciences), 3: 171–180 and Fig. 1.
  8. ^ Barba, Vassia (14 July 2023). "Earth's hottest ever temperature of 131F to be recorded this weekend". The Mirror. Retrieved 24 October 2023.
  9. ^ a b Boyes, Walt (2009). Instrumentation Reference Book. Butterworth-Heinemann. pp. 273–274. ISBN 978-0-7506-8308-1. Retrieved 17 July 2011.
  10. ^ Dingman, S. L. (26 February 2009). Fluvial Hydraulics. Oxford University Press, USA. p. 522. ISBN 978-0-19-517286-7.
  11. ^ Sogin, Harold H.; Hassan, Kamal-Eldin (June 1956). A design manual for regenerative heat exchangers of the rotary type. Wright Air Development Center. p. xii.
  12. ^ a b c Fahrenheit, Daniele Gabr. (1724) Experimenta & observationes de congelatione aquæ in vacuo factæ a D. G. Fahrenheit, R. S. S (Experiments and observations on water freezing in the void by D. G. Fahrenheit, R. S. S.), Philosophical Transactions of the Royal Society of London, vol. 33, no. 382, page 78 (March–April 1724). Cited and translated in http://www.sizes.com:80/units/temperature_Fahrenheit.htm.
  13. ^ "Wetterlexikon - Lufttemperatur" (in German). Deutscher Wetterdienst. Archived from the original on 15 October 2013. Retrieved 13 December 2013.
  14. ^ Ernst Cohen and W. A. T. Cohen-De Meester. Chemisch Weekblad, volume 33 (1936), pages 374–393, cited and translated in http://www.sizes.com:80/units/temperature_Fahrenheit.htm.
  15. ^ Frautschi, Steven C.; Richard P. Olenick; Tom M. Apostol; David L. Goodstein (14 January 2008). The mechanical universe: mechanics and heat. Cambridge University Press. p. 502. ISBN 978-0-521-71590-4.
  16. ^ Adams, Cecil (15 December 1989). "On the Fahrenheit scale, do 0 and 100 have any special significance?". The Straight Dope.
  17. ^ Fahrenheit, Daniele Gabr. (1724) "Experimenta circa gradum caloris liquorum nonnullorum ebullientium instituta". Archived 29 June 2014 at the Wayback Machine (Experiments performed concerning the degree of heat of some boiling liquids), Philosophical Transactions of the Royal Society of London, 33 : 1–3. For an English translation, see: Le Moyne College (Syracuse, New York).
  18. ^ Hasok Chang, Inventing Temperature: Measurement and Scientific Progress, pp. 8–11, Oxford University Press, 2004, ISBN 0198038240.
  19. ^ Cavendish, Henry; Heberden, William; Aubert, Alexander; Luc, Jean Andre De; Maskelyne, Nevil; Horsley, Samuel; Planta, Joseph (28 December 1777). "XXXVII. The report of the Committee appointed by the Royal Society to consider of the best method of adjusting the fixed points of thermometers; and of the precautions necessary to be used in making experiments with those instruments". Philosophical Transactions of the Royal Society of London. 67: 816–857. doi:10.1098/rstl.1777.0038. ISSN 0261-0523.
  20. ^ Elert, Glenn; Forsberg, C.; Wahren, L.K. (2002). "Temperature of a Healthy Human (Body Temperature)". Scandinavian Journal of Caring Sciences. 16 (2): 122–8. doi:10.1046/j.1471-6712.2002.00069.x. PMID 12000664. Archived from the original on 26 September 2010. Retrieved 12 April 2008.
  21. ^ "782 - Aerodrome reports and forecasts: A user's handbook to the codes". World Meteorological Organization. Retrieved 23 September 2009.
  22. ^ Halsey, Frederick A.; Dale, Samuel S. (1919). The metric fallacy (2 ed.). The American Institute of Weights and Measures. pp. 165–166, 176–177. Retrieved 19 May 2009.
  23. ^ "Canadian Units of Measurement; Department of Justice, Weights and Measures Act (R.S.C., 1985, c. W-6)". 17 May 2011. Archived from the original on 13 May 2011. Retrieved 5 June 2011.
  24. ^ Pearlstein, Steven (4 June 2000). "Did Canada go metric? Yes - and no". The Seattle Times. Retrieved 5 June 2011.
  25. ^ "Example of analog thermometer frequently used in Canada". Archived from the original on 6 July 2011. Retrieved 6 June 2011.
  26. ^ "Example of digital thermometer frequently used in Canada". Archived from the original on 6 July 2011. Retrieved 6 June 2011.
  27. ^ Department of Justice (26 February 2009). "Canadian Weights and Measures Act". Federal Government of Canada. Archived from the original on 10 August 2011. Retrieved 17 July 2011.
  28. ^ Statutory Instrument 2009/3046 – Weights and Measures – The Units of Measurement Regulations 2009 (PDF), archived from the original (PDF) on 1 January 2017, "The Secretary of State, being a Minister designated(a) for the purposes of section 2(2) of the European Communities Act 1972(b) in relation to units of measurement to be used for economic, health, safety, or administrative purposes, in exercise of the powers conferred by that subsection, makes the following Regulations:
  29. ^ "Home Laundering Consultative Council – What Symbols Mean". Home Laundering Consultative Council. Retrieved 15 July 2013.
  30. ^ "Guide to Common Home Laundering & Drycleaning Symbols". Textile Industry Affairs. 2010. Retrieved 15 July 2013.
  31. ^ "Guide to Apparel and Textile Care Symbols". Office of Consumer Affairs, Government of Canada. 17 April 2003. Retrieved 15 July 2013.
  32. ^ "The media like Fahrenheit but most Brits think in Celsius". Opinium. 24 July 2013. Retrieved 21 June 2021.
  33. ^ "weather". The Daily Telegraph. 3 July 2013. p. 31.
  34. ^ "Weather". The Times. 3 July 2013. p. 55.
  35. ^ Roy Greenslade (29 May 2014). "Newspapers run hot and cold over Celsius and Fahrenheit". The Guardian.
  36. ^ "Measure for measure". The Times. 23 February 2006.
  37. ^ "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 5 September 2015. Retrieved 6 September 2015.