Temperature Converter
The Formula, and the Two Numbers Worth Remembering
To go from Fahrenheit to Celsius, subtract 32 and multiply by five ninths. To go back, multiply by nine fifths and add 32.
The two anchors most people already know are water freezing at 32F or 0C and boiling at 212F or 100C at sea level. A third is worth knowing because it surprises people: minus 40 is the same number on both scales. It is the one point where the two lines cross.
For rough mental conversion from Fahrenheit, subtract 30 and halve it. That gives 35C for 100F against a true 37.8C, so it is about two degrees out in the middle of the range, which is fine for deciding what to wear and not fine for cooking.
Gas Marks
British gas ovens are marked 1 to 9 rather than in degrees, which makes an American recipe genuinely hard to follow without a chart. Gas marks are not a formula, they are a fixed scale, so the mapping is a lookup:
- Gas mark 1: 140C, 275F
- Gas mark 2: 150C, 300F
- Gas mark 3: 170C, 325F
- Gas mark 4: 180C, 350F
- Gas mark 5: 190C, 375F
- Gas mark 6: 200C, 400F
- Gas mark 7: 220C, 425F
- Gas mark 8: 230C, 450F
- Gas mark 9: 240C, 475F
Gas mark 4 at 180C is the workhorse setting that most cakes and roasts are written for. Below gas mark 1 there are quarter and half marks used for slow cooking and meringues.
Fan Ovens Run Hotter Than the Dial Says
A fan oven, called convection in the US, circulates air and cooks faster than a conventional oven set to the same number. Most manufacturers advise reducing the temperature by about 20C, or roughly 35F, and checking earlier than the recipe says.
So a recipe written for 180C in a conventional oven becomes about 160C in a fan oven. UK recipes often print both, labelled as 180C and 160C fan. American recipes rarely do, so if you have a convection setting and the recipe does not mention it, the 20C reduction is the adjustment to make.
One more thing worth knowing: most domestic ovens are not accurate. A dial set to 180C can run 15 or 20 degrees either side, and they drift as they age. An oven thermometer costs very little and is the only way to find out what your oven is actually doing, which matters far more than converting the number precisely.
Kelvin
Kelvin is the scientific scale and uses the same degree size as Celsius, offset so that zero sits at absolute zero, the point where molecular motion stops. Converting is addition: Kelvin is Celsius plus 273.15.
Because nothing can be colder than absolute zero, a negative Kelvin figure means an impossible temperature. The converter flags it rather than showing the number, since the usual cause is picking the wrong input unit.
Frequently Asked Questions
Related Tools
American recipes also give quantities in cups. The cooking measurement converter turns those into grams and millilitres, adjusted for the ingredient.
Reference Points Outside the Kitchen
Useful anchors if you are reading a forecast or a thermostat in the other scale:
- 0F is minus 18C. Severely cold. This is also roughly the setting of a domestic freezer.
- 32F is 0C. Water freezes. Below this, rain becomes snow and roads ice.
- 50F is 10C. Cool. A jacket.
- 68F is 20C. Room temperature, and the figure most building standards are written around.
- 86F is 30C. Properly hot weather.
- 98.6F is 37C. The traditional figure for human body temperature, though normal actually ranges from about 36.1C to 37.2C and shifts through the day.
- 104F is 40C. Dangerous heat for sustained exposure.
- 212F is 100C. Water boils at sea level. It boils lower at altitude, which is why high-altitude cooking takes longer.
The scales also behave differently in conversation. A one degree change in Celsius is a 1.8 degree change in Fahrenheit, so Fahrenheit sounds more dramatic for the same shift. A forecast moving from 20C to 25C is a five degree change that an American hears as 68F to 77F, a nine degree change.
Why Fahrenheit Exists at All
Daniel Fahrenheit set his scale in 1724 using a freezing brine solution as zero and roughly human body temperature near 96, later adjusted. Anders Celsius proposed his scale in 1742 keyed to water freezing and boiling, which is why the round numbers land where they do.
The practical argument people make for Fahrenheit is resolution: it divides the range of everyday weather into finer steps, so you can describe conditions in whole numbers without decimals. The argument for Celsius is that its anchors are physically meaningful and the arithmetic with Kelvin is trivial. Both are true, which is roughly why the world has not converged.