# Fahrenheit

Fahrenheit temperature conversion formulas
To find From Formula
Celsius Fahrenheit °C = (°F − 32) ÷ 1.8
Fahrenheit Celsius °F = (°C × 1.8) + 32
Kelvin Fahrenheit K = (°F + 459.67) ÷ 1.8
Fahrenheit Kelvin °F = (K × 1.8) − 459.67
Rankine Fahrenheit R = °F + 459.67
Fahrenheit Rankine °F = R − 459.67
Conversion calculator for units of temperature

Fahrenheit is a temperature scale named after Daniel Gabriel Fahrenheit (1686–1736), a German physicist who did most of his work in the Netherlands. This temperature scale, which was in use long before the Celsius scale was proposed, continues to be used for everyday temperature measurements by the general population of the United States. In most other countries (and in scientific studies worldwide), temperature measurements are made primarily on the Celsius scale.

## Contents

On the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit (written "32 °F"), and the boiling point is 212 degrees, placing the boiling and freezing points of water exactly 180 degrees apart. Thus the unit of this scale, a degree Fahrenheit, is five-ninths (59ths) of a degree Celsius, and negative 40 degrees Fahrenheit is equal to negative 40 degrees Celsius. Absolute zero is at −459.67 °F.

## History

Did you know?
The Fahrenheit temperature scale was proposed in 1724 by Amsterdam-based physicist Daniel Gabriel Fahrenheit

There are several competing versions of the story of how Fahrenheit came to devise his temperature scale. According to one version, Fahrenheit established the zero (0 °F) and 100 °F points on his scale by recording the lowest outdoor temperatures he could measure, and his own body temperature. He took as his zero point the lowest temperature he measured in the harsh winter of 1708 through 1709 in his hometown of Danzig (now Gdańsk, Poland) (−17.8 °C). (He was later able to reach this temperature under laboratory conditions using a mixture of ice, ammonium chloride, and water.) Fahrenheit wanted to avoid the negative temperatures that Ole Rømer's scale had produced in everyday use. He fixed his own body temperature as 100 °F. He then divided his original scale into twelve parts, and later divided each of these into 8 equal subdivisions, produced a scale of 96 degrees. Fahrenheit noted that his scale placed the freezing point of water at 32 °F and the boiling point at 212 °F, a neat 180 degrees apart.

Another story holds that Fahrenheit established the zero of his scale (0 °F) as the temperature at which a mixture of equal parts of ice and salt melts (some say he took that fixed mixture of ice and salt that produced the lowest temperature); and 96 degrees as the temperature of blood (he initially used horse blood to calibrate his scale). Initially, his scale contained only 12 equal divisions, but he later subdivided each division into eight equal degrees, ending up with 96.

A third, well-known version of the story, as described in the popular physics television series The Mechanical Universe, maintains that Fahrenheit simply adopted Rømer's scale (in which water freezes at 7.5 degrees) and multiplied each value by four to eliminate the fractions and increase the granularity of the scale (giving 30 and 240 degrees). He then re-calibrated his scale between the melting point of water and normal human body temperature (which he took to be 96 degrees); the melting point of ice was adjusted to 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 two to the sixth power).

His measurements were not entirely accurate, though. By his original scale, the actual melting and boiling points would have been noticeably different from 32 °F and 212 °F. Some time after his death, it was decided to recalibrate the scale with 32 °F and 212 °F as the exact melting and boiling points of plain water. That change was made to easily convert from Celsius to Fahrenheit and vice versa, with a simple formula. This change also explains why the body temperature once taken as 96 or 100 °F by Fahrenheit is today taken by many as 98.6 °F (it is a direct conversion of 37 °C), although giving the value as 98 °F would be more accurate.

There are at least three other versions of this story, but they appear to be based less on evidence and more on speculation. They are therefore not recounted here.

## Conversions and key temperatures on different scales

On the Fahrenheit scale, the freezing point of water is 32 degrees Fahrenheit (°F) and the boiling point is 212 °F (at standard atmospheric pressure). This puts the boiling and freezing points of water exactly 180 degrees apart. Therefore, 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. A temperature interval of 1 °F is equal to an interval of 59 degrees Celsius. The Fahrenheit and Celsius scales intersect at −40° (i.e., −40 °F = −40 °C).

Absolute zero is −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.

The Fahrenheit scale uses the symbol ° to denote a point on the temperature scale (as does Celsius) and the letter F to indicate the use of the Fahrenheit scale (e.g. "Gallium melts at 85.5763 °F"), as well as to denote a difference between temperatures or an uncertainty in temperature (e.g. "The output of the heat exchanger experiences an increase of 72 °F" and "Our standard uncertainty is ±5 °F").

For an exact conversion, the following formulas can be applied. Here, f is the value in Fahrenheit and c the value in Celsius:

• f °Fahrenheit to c °Celsius : (f − 32) °F × 5°C/9°F = (f − 32)/1.8 °C = c °C
• c °Celsius to f °Fahrenheit : (c °C × 9°F/5°C) + 32 °F = (c × 1.8) °F + 32 °F = f °F

This is also an exact conversion making use of the identity -40 °F = -40 °C. Again, f is the value in Fahrenheit and c the value in Celsius:

• f °Fahrenheit to c °Celsius : ((f + 40) ÷ 1.8) − 40 = c.
• c °Celsius to f °Fahrenheit : ((c + 40) * 1.8) − 40 = f.

Some key temperatures relating the Fahrenheit scale to other temperature scales are shown in the table below.

 Kelvin Celsius Fahrenheit Absolute zero (precise, by definition) 0 K −273.15 °C −459.67 °F Melting point of ice 273.15 K 0 °C 32 °F Water’s triple point (precise, by definition) 273.16 K 0.01 °C 32.018 °F Water’s boiling point A 373.1339 K 99.9839 °C 211.9710 °F

A For Vienna Standard Mean Ocean Water (VSMOW) at a pressure of one standard atmosphere (101.325 kPa) when calibrated solely per the two-point definition of thermodynamic temperature.

## Usage

The Fahrenheit scale was the primary temperature standard for climatic, industrial and medical purposes in most English-speaking countries until the 1960s. In the 1960s and 1970s, governments phased in the Celsius (formerly centigrade) scale as part of the shift to the metric system of units.

Fahrenheit supporters assert its previous popularity was due to Fahrenheit's user-friendliness. The unit of measure, being only 59 the size of the Celsius degree, permits more precise communication of measurements without resorting to fractional degrees. Also, the ambient air temperature in most inhabited regions of the world tends not to go far beyond the range of 0 °F to 100 °F: therefore, the Fahrenheit scale would reflect the perceived ambient temperatures, following 10-degree bands that emerge in the Fahrenheit system:

• 0s Extremely cold. (0°F ~ -17.8°C)
• 10s Deep frost.
• 20s Light frost.
• 30s Very cold.
• 32 Freezing.
• 40s Cold. Heavy clothing needed.
• 50s Cool. Moderate Clothing required.
• 60s Tepid. Light clothing.
• 70s Warm. Summer clothing.
• 80s Hot. Breathable clothing.
• 90s Very hot. Minimal clothing.
• 98.6 Body Temperature
• 100s Extremely hot. Take precautions against overheating.

Some Celsius supporters argue that their system can be just as natural, for example they might say that 0–10 °C indicates cold, 10–20 °C mild, 20–30 °C warm and 30–40 °C hot.

In the United States, the Fahrenheit system continues to be the accepted standard for nonscientific use. On the other hand, the Celsius scale is widely used in most other parts of the world, including Europe, Canada, Australia, and New Zealand. Fahrenheit is sometimes used by older generations, especially for measurement of higher temperatures.