Three temperature scales are in common use. Daniel Gabriel Fahrenheit proposed his in 1724. Anders Celsius proposed a hundred-degree scale between freezing and boiling water in 1742, though his original version ran the other way, with water boiling at 0 and freezing at 100. The scale was reversed shortly after his death, and it was officially renamed from "centigrade" to "Celsius" in 1948. Lord Kelvin proposed an absolute scale starting at absolute zero in 1848, and the kelvin is now the SI unit, defined through the Boltzmann constant since 2019.
Gas marks and fan ovens
British cookery adds a fourth scale: the gas mark. Gas mark 4 is about 180°C or 350°F, gas mark 6 about 200°C or 400°F, and gas mark 1 about 140°C or 275°F. The steps are roughly 14°C apart, but not exactly, so use a conversion chart rather than a formula. Fan ovens add another wrinkle. Because circulating air cooks food faster, recipes written for conventional ovens usually need about 20°C less in a fan oven, so a recipe at 200°C becomes about 180°C fan.
The difference trap
Converting a temperature and converting a temperature change are different calculations. To convert a temperature from Celsius to Fahrenheit, multiply by 1.8 and add 32. To convert a change, just multiply by 1.8, with no 32 added. A rise of 10°C is a rise of 18°F, not 50°F. This trips people up in climate reporting, where a 1.5°C rise in average temperature is a 2.7°F rise, and in heating, where a thermostat turned up by 2°C is about 3.6°F warmer.
Worked conversions
The UK's record temperature of 40.3°C, set in July 2022 at Coningsby in Lincolnshire, is about 104.5°F. A fridge at 4°C is about 39°F, and a freezer at −18°C is about 0°F, a convenient coincidence that makes −18°C a common freezer setting. In kelvin, 20°C room temperature is 293.15 K. Kelvin has no degree sign, and no negative values.
Temperature and health
Body temperature is one of the most common conversions at home. A normal temperature is around 36 to 37°C, about 96.8 to 98.6°F. The NHS treats 38°C, about 100.4°F, or above as a high temperature. Older thermometers and some imported devices still show Fahrenheit, so it's worth knowing that 100°F, which sounds alarming, is about 37.8°C, just below the NHS fever threshold.
Checking your answer
Three reference points cover most conversions: water freezes at 0°C, 32°F, 273.15 K; water boils at 100°C, 212°F at sea level; and −40° is the same on both the Celsius and Fahrenheit scales. A quick mental approximation for weather is to double the Celsius figure and add 30: 15°C becomes about 60°F, against an exact 59°F. It's fine for everyday use, but drifts at higher temperatures, so use the exact formula for cooking or medical readings.
One more worked example
An American recipe asks for an oven at 375°F. That's about 190°C, or gas mark 5, and about 170°C in a fan oven. A recipe calling for a "moderate oven" in an old British cookbook usually means around 180°C, gas mark 4. When a recipe gives only Fahrenheit, convert, then round to the nearest 5 or 10 degrees your oven dial shows.
Temperatures on clothing labels
Washing symbols show a maximum water temperature in Celsius, typically 30, 40 or 60 degrees. A 30°C wash is about 86°F and a 60°C wash about 140°F. Washing at 30°C rather than 40°C uses noticeably less energy, because heating the water is most of what a washing machine spends power on, and modern detergents are designed to work at lower temperatures.