Fahrenheit, Celsius and Kelvin: Where Temperature Scales Come From and Who Uses Them
Unlike length or weight, temperature scales don’t just differ in size, they start from different points. That’s why converting them takes a formula and not just a multiplication. Here’s where the three main scales came from and who uses each one.
Need a quick answer? Use the temperature converter.
Who uses what today
| Country or context | Everyday temperature scale |
|---|---|
| Almost every country | Celsius |
| United States | Fahrenheit for weather, cooking, body temperature and thermostats |
| A few other places | Fahrenheit is commonly reported as the everyday scale in the Bahamas, Belize, the Cayman Islands, Palau, and several other Pacific islands associated with the US |
| United Kingdom | Celsius officially, though some people (and newspaper headlines) still describe heat waves in Fahrenheit |
| Canada | Celsius for weather, but ovens, pools and cooking often use Fahrenheit |
| Science (worldwide) | Kelvin and Celsius |
Fahrenheit
Daniel Gabriel Fahrenheit was an instrument maker born in Danzig (now Gdańsk, Poland) who worked mostly in the Dutch Republic, famous for making reliable mercury thermometers. He published his scale in 1724.
For 0 °F he used the coldest temperature he could reliably reproduce in his workshop: a mixture of ice, water and a salt (ammonium chloride). He set the freezing point of plain water at 32 °F and placed human body temperature at around 96 °F. Having 64 degrees between freezing and body temperature made it easy to mark a thermometer by repeatedly halving the gap.
The scale was later recalibrated so that water boils at exactly 212 °F, 180 degrees above freezing. That’s why normal body temperature ended up at the less tidy 98.6 °F.
Fahrenheit became the standard scale in Britain and its colonies, which is how it came to the United States.
Celsius
Swedish astronomer Anders Celsius proposed his scale in 1742, using two fixed points: the freezing and boiling points of water, 100 degrees apart. Strangely to modern eyes, his original scale was upside down: water boiled at 0 and froze at 100.
Shortly after his death in 1744 the scale was flipped to the version we use today. Botanist Carl Linnaeus is often credited with the reversal, and Jean-Pierre Christin in Lyon independently proposed a similar scale around the same time.
For two centuries it was usually called “centigrade” (“a hundred steps”). In 1948 an international conference renamed it degrees Celsius, partly because “centigrade” also meant a unit of angle in some languages.
Most countries switched from Fahrenheit to Celsius during metrication in the 1960s and 70s.
Kelvin
In 1848, the Belfast-born physicist William Thomson, later Lord Kelvin, proposed an absolute temperature scale. Instead of water, it starts at absolute zero, the point at which nothing can get any colder because particles have the least possible energy. Absolute zero is −273.15 °C, or −459.67 °F.
A kelvin is the same size as a degree Celsius, so converting between them is just adding or subtracting 273.15. Since 1968 the unit has been simply “kelvin” (symbol K), not “degrees Kelvin”. Scientists use it because many laws of physics only work on an absolute scale, where 0 really means zero. In 2019 the kelvin, like the kilogram, was redefined in terms of a fundamental constant (the Boltzmann constant).
Converting between them
| From | To | Formula |
|---|---|---|
| Celsius | Fahrenheit | °F = °C × 9/5 + 32 |
| Fahrenheit | Celsius | °C = (°F − 32) × 5/9 |
| Celsius | Kelvin | K = °C + 273.15 |
| Kelvin | Celsius | °C = K − 273.15 |
A quick mental shortcut from Celsius to Fahrenheit: double it and add 30. At 20 °C that gives 70 °F, close to the real answer of 68 °F.
Temperatures worth knowing
- −40°: the one temperature that’s the same in Celsius and Fahrenheit
- 0 °C = 32 °F: water freezes
- 20–22 °C = 68–72 °F: comfortable room temperature
- 37 °C = 98.6 °F: typical body temperature
- 100 °C = 212 °F: water boils (at sea level)
- 180 °C ≈ 350 °F: a common oven temperature
Convert any temperature with the temperature converter.