The formula
Celsius and Fahrenheit are both linear scales, so converting between them is one multiplication and one addition. The two scales put water's freezing point at 0°C and 32°F, and a Celsius degree is 1.8 times the size of a Fahrenheit degree, which gives:
Type in either box above and the other converts as you type. The Kelvin row is Celsius plus 273.15, the scale science uses because it starts at absolute zero, the coldest temperature possible. A kelvin is the same size as a Celsius degree, so 20°C is 293.15 K, and nothing can be colder than 0 K.
The 9/5 (or 1.8) in the formula is the only awkward part. It exists because Fahrenheit squeezed 180 degrees between water freezing (32°F) and boiling (212°F), while Celsius used a round 100. The add-32 part shifts the two zero points into line. Do the multiplication before the addition when going to Fahrenheit, and the subtraction before the multiplication when coming back, and the formula never bites.
Worked examples
Reference temperatures
A handful of anchor points make most conversions unnecessary. If you know that room temperature sits around 20°C (68°F) and that anything over 30°C (86°F) counts as a properly hot day in Britain, you can place an unfamiliar Fahrenheit figure by feel before reaching for the exact number.
- -40°C = -40°F (the point where the scales meet)
- 0°C = 32°F (water freezes)
- 10°C = 50°F (a cool British day)
- 25°C = 77°F (a warm British day)
- 37°C = 98.6°F (normal body temperature)
- 38°C = 100.4°F (the NHS fever threshold)
- 100°C = 212°F (water boils at sea level)
- 180°C = 356°F (moderate oven, roughly the American 350°F)
When you actually need this
Britain reports weather in Celsius, so the conversion mostly comes up with American content: recipes that bake at 350°F, weather reports of a 100°F heatwave (37.8°C), or a US thermometer reading 101°F that you want to sanity-check against NHS fever guidance (38°C and above counts). Older British recipe books and pre-1970s weather records also used Fahrenheit, which is why your grandmother's "very hot oven, 450 degrees" means 232°C, not something impossible.
Two mental shortcuts cover casual use. For Celsius to Fahrenheit, double it and add 30: 20°C becomes roughly 70°F, close to the true 68°F. For Fahrenheit to Celsius, subtract 30 and halve it: 80°F becomes roughly 25°C (the true figure is 26.7°C). Both drift as temperatures get more extreme, so use the exact converter when the number matters, and especially for anything medical. If it is oven settings specifically, our oven temperature converter also handles fan ovens and gas marks in one go.
Frequently asked questions
How do I convert Celsius to Fahrenheit?
Multiply by 9, divide by 5, then add 32. So 20°C is 20 × 9 ÷ 5 = 36, plus 32 gives 68°F. Going the other way, subtract 32 first, then multiply by 5 and divide by 9.
What is normal body temperature in both scales?
Around 37°C, which is 98.6°F. Anything from about 36.1°C to 37.2°C is considered normal, and the NHS treats 38°C (100.4°F) or above as a fever.
At what temperature are Celsius and Fahrenheit equal?
At minus 40. Set C = F in the conversion formula and the only solution is -40, so -40°C and -40°F are exactly the same temperature.
What is 180°C in Fahrenheit for baking?
356°F, which American recipes usually round to 350°F. In practice ovens are nowhere near precise enough for the 6 degree difference to matter, so treat 180°C and 350°F as the same moderate oven.
Why does Fahrenheit use 32 for freezing?
Fahrenheit set his zero at the coldest brine mixture he could reproduce, which left pure water freezing at 32 degrees on his scale. Celsius later defined a scale with water freezing at 0 and boiling at 100, which is the one most of the world adopted.