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Thermometer with Fahrenheit units on the outer scale and Celsius units on the inner scale

Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit (1686–1736). Today, the temperature scale has been replaced by the Celsius scale in most countries.[1] It is still in use in the United States and a few other nations, such as Belize.[2]


Definition and conversions

Fahrenheit temperature conversion formulae
from Fahrenheit to Fahrenheit
Celsius [°C] = ([°F] − 32) × 59 [°F] = [°C] × 95 + 32
Kelvin [K] = ([°F] + 459.67) × 59 [°F] = [K] × 95 − 459.67
Rankine [°R] = [°F] + 459.67 [°F] = [°R] − 459.67
For temperature intervals rather than specific temperatures,
1 °F = 1 °R = 59 °C = 59 K
Comparisons among various temperature scales

On the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point 212 °F (at standard atmospheric pressure), placing the boiling and freezing points of water exactly 180 degrees apart.[3] A degree on the Fahrenheit scale is 1180 of the interval between the freezing point and the boiling point. On the Celsius scale, the freezing and boiling points of water are 100 degrees apart, hence the unit of this temperature scale.[3] A temperature interval of 1 degree Fahrenheit is equal to an interval of 59 degrees Celsius. The Fahrenheit and Celsius scales converge at −40 degrees (i.e. −40 °F and −40 °C represent the same temperature).[3]

Absolute zero is −459.67 °F.[4] The Rankine temperature scale was created to use degree intervals the same size as those of the Fahrenheit scale, such that a temperature difference of one degree Rankine (1 °R) is equal to a difference of 1 °F, 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.


According to a journal article Fahrenheit wrote in 1724,[5] he based his scale on three reference points of temperature. The zero point is determined by placing the thermometer in brine: he used a mixture of ice, water, and ammonium chloride, a salt. This is a frigorific mixture which automatically stabilizes its temperature at 0 °F. A mixture of ice and water stabilizes, either freezing or melting at 32 °F,[6] though Fahrenheit did not use this point in defining his temperature scale. The third point, 96 degrees, was the level of the liquid in the thermometer when held in the mouth or under the armpit of his wife. Fahrenheit noted that, using this scale, mercury boils at around 600 degrees.

Later, work by other scientists observed that water boils about 180 degrees higher than the freezing point and decided to redefine the degree slightly to make it exactly 180 degrees higher.[5] It is for this reason that normal body temperature is 98.6 on the revised scale (whereas it was 96 on Fahrenheit's original scale).[7]

According to a letter Fahrenheit wrote to his friend Herman Boerhaave,[8] his scale was built on the work of Ole Rømer, whom he had met earlier. In Rømer’s scale, the two fixed reference points are that brine also freezes at 0 degrees and water boils at 60 degrees. He observed that, on this scale, water freezes at 7.5 degrees. Fahrenheit multiplied each value by four in order to eliminate fractions and increase the granularity of the scale (resulting in 30 and 240 degrees, respectively). He then re-calibrated his scale between the freezing point of water and normal human body temperature (which he observed to be 96 degrees); he adjusted the scale so that the melting point of ice would be 32 degrees, so that 64 intervals would separate the two, allowing him to mark degree lines on his instruments by simply bisecting the interval six times (since 64 is 2 to the sixth power).[9]


The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in some English-speaking countries until the 1960s. In the late 1960s and 1970s, the Celsius (known until 1948 as centigrade) scale was adopted by most of these countries as part of the standardizing process called metrication.

Only in the United States and a few other countries (Belize[2], Burma, and Liberia[10]) does the Fahrenheit system continue to be used, and only for non-scientific use. Most other countries have adopted Celsius as the primary scale in all use, although Fahrenheit continues to be the scale of preference for a minority of people in the United Kingdom, particularly when referring to summer temperatures[citation needed].

In the U.S., weather forecasts, food cooking and freezing temperatures are commonly in degrees Fahrenheit.

Various reasons are given for the resistance to the Celsius system in the U.S., including the larger size of each degree Celsius (resulting in the need for decimals where integer Fahrenheit degrees were adequate for much technical work), or the lower zero point in the Fahrenheit system which reduces the number of negative signs when measurements such as weather data were averaged.[11] On the other hand, most countries using the Fahrenheit system, like New Zealand or Australia, have totally switched to the Celsius system making the Fahrenheit system obsolete there.[12]


The Fahrenheit symbol has its own Unicode character: "℉" (U+2109). This is a compatibility character encoded for roundtrip compatibility with legacy CJK encodings (which included it to conform to layout in square ideographic character cells) and vertical layout. Use of compatibility characters is discouraged by the Unicode Consortium. The ordinary degree sign (U+00B0) followed by the Latin letter F ("°F") is thus the preferred way of recording the symbol for degree Fahrenheit.

Temperatures and intervals

As with the Celsius scale, the same symbol, °, is used to denote both a point on the temperature scale, with a letter (C, F) indicating which scale is being used (e.g. "Gallium melts at 85.5763 °F"), and to denote a difference between temperatures or an uncertainty of temperature (e.g. "The output of the heat exchanger is hotter by 72 °F" and "Our standard uncertainty is ±5 °F").

See also


  1. ^ "Metric usage and metrication in other countries". Retrieved December 11, 2009. 
  2. ^ a b "Belize Weather Bureau". Retrieved May 9, 2008. 
  3. ^ a b c Conversion of Temperature
  4. ^ Temperature Units Converter
  5. ^ a b "Fahrenheit temperature scale". Sizes, Inc. December 12, 2006. Retrieved May 9, 2008. 
  6. ^ Muller, R.A. (2009). Physics for Future Presidents. pp. 2-5, 2-17. Retrieved 20 February 2010. 
  7. ^ Elert, Glenn (2002). "Temperature of a Healthy Human (Body Temperature)". Scandinavian Journal of Caring Sciences 16: 122. doi:10.1046/j.1471-6712.2002.00069.x. Retrieved 2008. 
  8. ^ Ernst Cohen and W. A. T. Cohen-De Meester. Chemisch Weekblad, volume 33 (1936), pages 374–393, cited and translated in
  9. ^ Cecil Adams. "On the Fahrenheit scale, do 0 and 100 have any special significance?". The Straight Dope. 
  10. ^ CIA Factbook: Weights and Measures
  11. ^ [1] Halsey, Frederick A., Dale, Sanuel S., "The metric fallacy," The American institute of weights and measures, Second Edition, 1919. Pages 165-166, 176-177. Retrieved May 19, 2009
  12. ^ [2] Information about New Zealand's money and metric measures


Up to date as of January 15, 2010
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Definition from Wiktionary, a free dictionary



  1. The symbol for degrees Fahrenheit.

See also

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