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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 physicist Daniel Gabriel Fahrenheit (1686–1736). Today, the scale has been replaced by the Celsius scale in most countries; it is still in use for non-scientific purposes in the United Statesmarker and a few other nations, such as Belizemarker.

Definition and conversions

On the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point (at standard atmospheric pressure), placing the boiling and freezing points of water exactly 180 degrees apart. A degree on the Fahrenheit scale is 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 scale. A temperature interval of 1 degree Fahrenheit is equal to an interval of degrees Celsius. The Fahrenheit and Celsius scales converge at −40 degrees (i.e. and represent the same temperature).

Absolute zero is . 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 Rankine is equal to a difference of , except that absolute zero is – the same way that the Kelvin temperature scale matches the Celsius scale, except that absolute zero is .


According to a journal article Fahrenheit wrote in 1724, 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 type of frigorific mixture. The mixture automatically stabilizes its temperature at . He then put a thermometer into the mixture and let the liquid in the thermometer descend to its lowest point. The second point is the 32 degree found by putting the thermometer in still water as ice is just forming on the surface. The third point, the 96 degree, was the level of the liquid in the thermometer when held in the mouth or under the armpit. 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. 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).

According to a letter Fahrenheit wrote to his friend Herman Boerhaave, his scale 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).


The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in most English-speaking countries until the 1960s. In the late 1960's and 1970's, 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 Statesmarker and a few other countries (such as Belizemarker) 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.

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

Resistance to the Celsius system was partly due to the larger size of each degree Celsius, resulting in the need for fractions, where integral Fahrenheit degrees were adequate for much technical work. The lower zero point in the Fahrenheit system reduced the number of negative signs when measurements such as weather data were averaged.


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 "), and to denote a difference between temperatures or an uncertainty of temperature (e.g. "The output of the heat exchanger is hotter by " and "Our standard uncertainty is ").

See also


  1. Ernst Cohen and W. A. T. Cohen-De Meester. Chemisch Weekblad, volume 33 (1936), pages 374–393, cited and translated in
  2. [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

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