What Celsius, Fahrenheit and Kelvin each anchor to

What zero means

Celsius sets water's freezing point at zero and its boiling point at one hundred. Fahrenheit was built on different anchors, with water freezing at 32 and boiling at 212. In both scales, zero is simply a point on the scale rather than an absence of temperature. Saying twenty degrees is twice as hot as ten therefore does not hold. Celsius is named after the Swedish astronomer Anders Celsius, and Fahrenheit after the German-born physicist Daniel Gabriel Fahrenheit. Celsius's original scale ran the other way, with water boiling at 0, and was later inverted.

Only Kelvin supports multiples

Kelvin sets zero at absolute zero, where molecular motion stops. Because zero really means none, 200K is twice 100K. Its degree size matches Celsius, so adding 273.15 to a Celsius value gives Kelvin. That property is why science uses Kelvin. Since 2019 the kelvin has been defined by fixing the value of the Boltzmann constant, so it no longer depends on the state of any particular substance. Kelvin takes no 'degree': you simply say so many kelvins.

The formulas

Confusing the order of subtracting and dividing produces badly wrong values. Thinking of it as aligning the zero point first, then the degree size, keeps it straight. Using 9/5 instead of 1.8 gives the same result. A body temperature of 37 °C is 98.6 °F, and absolute zero is −459.67 °F.

• Fahrenheit to Celsius: (F − 32) ÷ 1.8
• Celsius to Fahrenheit: C × 1.8 + 32
• Celsius to Kelvin: C + 273.15
• −40 is the single value where Celsius and Fahrenheit agree

Estimating in your head

Where precision is unnecessary, subtracting 30 from Fahrenheit and halving lands close to Celsius. 70°F is roughly 20°C and 90°F roughly 30°C. Going the other way, doubling Celsius and adding 30 gives a rough Fahrenheit. For reading a forecast this is enough. This shortcut drifts further the hotter or colder it gets, though. By the shortcut 0 °F is −15 °C, but in fact it is about −17.8 °C.

Differences convert differently

A temperature and a temperature difference convert by different rules. To express a five-degree rise in Fahrenheit, you multiply by 1.8 only and do not add 32, because the zero points already cancel. This is a common error in calculations dealing with differences, such as heating and cooling settings. For the same reason, a difference in kelvins equals the same difference in degrees Celsius. A gap of 10 K is 10 °C, and scientific papers often state temperature differences in kelvins.

Reference points worth remembering

Even without applying the formula each time, knowing a few pairs lets you estimate most temperatures quickly. It also helps to remember that every 10 °F step is about 5.6 °C, which makes filling in values between the pairs easy.

• 0 °C = 32 °F = 273.15 K (water freezes)
• 20 °C = 68 °F (a comfortable room)
• 37 °C = 98.6 °F (human body temperature)
• 100 °C = 212 °F = 373.15 K (water boils at one atmosphere)

Who uses which scale

Most countries use Celsius for weather and daily life. The United States mainly uses Fahrenheit day to day, so oven temperatures in American recipes and US forecasts need converting. An oven at 350 °F is about 177 °C, usually set as 180. Science and engineering use kelvin as standard, and kelvin appears in daily life too, as the colour temperature of lighting. A label such as 6500 K describes the tint of the light rather than how hot anything is.

Common mistakes

Temperature conversion errors fall into a few types. Recalling the list below before calculating avoids most of them. Where a few degrees matter, such as body or cooking temperatures, calculate exactly with the formula rather than estimating.

• Swapping the order of subtracting 32 and dividing by 1.8
• Adding or subtracting 32 when converting a temperature difference
• Adding 273 to a kelvin value (going from kelvin to Celsius you subtract)
• Reading a doubled Celsius value as 'twice as hot'