| Fahrenheits (°F) | Celsius (°C) |
|---|---|
| 1 Fahrenheit | -17.2222222222 °C |
| 2 Fahrenheits | -16.6666666667 °C |
| 3 Fahrenheits | -16.1111111111 °C |
| 4 Fahrenheits | -15.5555555556 °C |
| 5 Fahrenheits | -15 °C |
| 10 Fahrenheits | -12.2222222222 °C |
| 20 Fahrenheits | -6.66666666667 °C |
| 25 Fahrenheits | -3.88888888889 °C |
| 50 Fahrenheits | 10 °C |
| 100 Fahrenheits | 37.7777777778 °C |
| Reference | Fahrenheits (°F) | Celsius (°C) |
|---|---|---|
| Absolute zero | -459.67 °F | -273.15 °C |
| Freezing point of water | 32 °F | 0 °C |
| Average room temperature | 68 °F | 20 °C |
| Human body temperature | 98.6 °F | 37 °C |
| Boiling point of water | 212 °F | 100 °C |
The degree Fahrenheit is a unit of temperature on a scale where water freezes at 32 and boils at 212 under standard atmospheric pressure. Its symbol is the degree sign followed by F. It is the everyday temperature unit of the United States and a handful of small territories, and it is used almost nowhere else.
Daniel Gabriel Fahrenheit devised the scale in 1724 using three reference points. Zero was the temperature of a freezing mixture of ice, water and ammonium chloride, the coldest he could reliably reproduce. Thirty-two was the freezing point of plain water, and ninety-six was intended as human body temperature, though later measurement put normal body temperature nearer 98.6. The odd-looking numbers are the residue of that construction rather than arbitrary choices.
The degree is smaller than the Celsius degree, five Fahrenheit degrees to every nine Celsius, which gives the scale finer resolution in whole numbers. Defenders argue this suits weather reporting, where the range from zero to a hundred covers most of the inhabited world's climate. Critics point out that the same resolution is available in Celsius by using one decimal place, and that the scale's reference points relate to nothing anyone encounters.
American usage is nearly total in daily life: forecasts, thermostats, ovens and body temperature are all in Fahrenheit. American science, medicine and engineering are not, which produces the routine domestic conversion of a laboratory result in Celsius into a clinical note in Fahrenheit. The two scales cross at minus forty, where both read the same number.
Its finer graduation is the one technical argument in its favour. Because a Fahrenheit degree is five ninths of a Celsius degree, ordinary weather can be described in whole numbers across a wider range without fractions, and the span from a cold night to a hot afternoon covers roughly a hundred units rather than forty. Defenders of the scale add that it is calibrated to human comfort rather than to water: zero and a hundred bracket the range a person is likely to meet outdoors, whereas nought and a hundred Celsius bracket the range a kettle meets. Neither argument has moved any country to adopt it, but both explain why American resistance to changing has been durable.
Thirty-two degrees Fahrenheit equals 0 degrees Celsius, and a change of one degree Fahrenheit equals a change of five ninths of a degree Celsius.
The degree Celsius is a unit of temperature on a scale where water freezes at 0 and boils at 100 under standard atmospheric pressure. Its symbol is the degree sign followed by C. It is the everyday temperature unit of almost every country in the world, and its degree is exactly the same size as the kelvin.
Anders Celsius proposed the scale in 1742 with the numbers the other way round: 0 for the boiling point of water and 100 for its freezing point. Whether Celsius himself, Carl Linnaeus or the instrument maker Daniel Ekström reversed it is disputed, but the inversion happened within a few years of his death and the scale has run upward ever since. It was called the centigrade scale for two centuries and renamed after Celsius in 1948, partly because centigrade already meant a hundredth of a right angle in French and Spanish surveying.
The modern definition no longer refers to water at all. The Celsius scale is defined from the kelvin, with zero degrees Celsius set at exactly 273.15 kelvin, and the kelvin itself fixed by the Boltzmann constant since 2019. The freezing and boiling points of water are now measured results rather than definitions, and neither falls at exactly 0 or 100 under all conditions.
The United States is the only major country that reports weather in Fahrenheit, and even there the sciences work in Celsius. Body temperature, cooking, weather and industrial process control all use the unit, and its two anchor points are memorable enough that most people can estimate an unfamiliar temperature without conversion.
A distinction of grammar carries a real meaning here. A temperature is written as a degree Celsius, but a difference between two temperatures is written as a kelvin, because the two scales share the same interval and differ only in where zero sits. Heat calculations therefore express specific heat capacity in joules per kilogram per kelvin even when the temperatures themselves are quoted in Celsius, and a rise of five degrees Celsius is a rise of five kelvin exactly. The convention looks pedantic until a calculation mixes an absolute temperature with a temperature difference, at which point keeping them apart is the only thing that prevents an offset of 273.15 from entering the arithmetic.
Zero degrees Celsius equals 273.15 kelvin or 32 degrees Fahrenheit, and a change of one degree Celsius equals a change of 1.8 degrees Fahrenheit.