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The Fahrenheit scale is a temperature scale named after the German physicist Daniel Gabriel Fahrenheit (1686 − 1736), who proposed it in 1724. The symbol of a degree Fahrenheit is °F and the scale has now been largely been replaced by the Celsius scale although it is still in use for non-scientific purposes in the United States.

In the Fahrenheit scale, the melting point of water (i.e., ice) at an atmospheric pressure of 101.325 kPa is 32 degrees Fahrenheit (°F) and the normal boiling point of water is 212 °F, placing those two reference points exactly 180 °F apart. Absolute zero in the Fahrenheit scale is −459.67 °F.

For comparison, in the Celsius scale, the melting point of water is 0 degrees Celsius (°F) and the normal boiling point of water is 100 °C, placing those two reference points exactly 100 °C apart. Absolute zero in the Celsius scale is −273.15 °C.

A temperature interval of 1 degree Celsius is equal to an interval of 1.8 degrees Fahrenheit.

The Rankine scale

The Rankine scale is a temperature scale named after the Scottish physicist and engineer 1820 - 1872), Willam John Macquorn Rankine, who proposed it in 1839. The symbol for a degree Rankine is °R and it is based on one degree Rankine being equal to one degree Fahrenheit. As with the Kelvin scale, zero in the Rankine scale is absolute zero. A temperature of −459.67 °F is exactly 0 °R.

The melting point of water in the Rankine scale is 491.67 °R and the normal boiling point of water is 671.67 °R (i.e., 459.67 + 212).

Comparisons and conversions

Table 1 compares the values for three major reference point in the Kelvin, Celsius , Fahrenheit and Rankine temperature scales. Table 2 presents the factors required convert temperatures in any one of those four scales to the other three scales.

Table 1: Comparison of Temperature Scales

Reference points	Kelvin	Celsius	Fahrenheit	Rankine
Absolute zero	0 K	−273.15 °C	−459.67 °F	0 °R
Melting point of water (a)	273.15 K	0 °C	32 °F	491.67 °R
Normal boiling point of water (b)	373.15 K	100 °C	212 °F	671.67 °R
(a) The melting point of water (i.e., ice) used to be defined as 0°C (32°F) under an atmospheric pressure of 101.325 kPa. 0 °C is now taken to be 273.15 K but does not exactly equal the melting point of water which is now defined as 273.152519 K. Note that the freezing point of water is ill-defined as water usually freezes a few degrees below 0°C and the actual temperature is not reproducible. (b) The normal boiling point of water used to be defined as 100°C at an atmospheric pressure of 101.325 kPa, but it now been estimated at 99.9839°C.

Table 2: Temperature Conversions

Scale	To Kelvin	From Kelvin
Celsius	K = °C + 273.15	°C = K − 273.15
Fahrenheit	K = (°F + 459.67) ÷ 1.8	°F = (K × 1.8) − 459.67
Rankine	K = °R ÷ 1.8	°R = 1.8 × K
Scale	To Fahrenheit	From Fahrenheit
Celsius	°F = (1.8 × °C) + 32	°C = (°F − 32) ÷ 1.8
Rankine	°F = °R − 459.67	°R = °F + 459.67
Kelvin	°F = (1.8 × K) − 459.67	K = (°F + 459.67) ÷ 1.8
Scale	To Celsius	From Celsius
Fahrenheit	°C = (°F − 32) ÷ 1.8	°F = (1.8 × °C) + 32
Rankine	°C = (°R ÷ 1.8) − 273.15	°R = 1.8 × (°C + 273.15)
Kelvin	°C = K − 273.15	K = °C + 273.15
Scale	To Rankine	From Rankine
Celsius	°R = 1.8 × (°C + 273.15)	°C = (°R ÷ 1.8) − 273.15
Fahrenheit	°R = °F + 459.67	°F = °R − 459.67
Kelvin	°R = 1.8 × K	K = °R ÷ 1.8

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 late 1960's and 1970's, the Celsius (formerly Centigrade) scale was adopted by most of these countries as part of the standardizing process called metrication.

Only in the United States, Belize and perhaps a few other countries does the Fahrenheit system continue to be used, and mainly 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. Many British people are conversant with both the Celsius and Fahrenheit temperature scales.