How the four scales relate
British recipes use up to four temperature systems at once: conventional Celsius, fan oven Celsius, Fahrenheit (mostly in American recipes) and the gas mark scale printed on gas oven dials. Two simple rules connect them:
Fan ovens circulate hot air, which transfers heat more efficiently, so the convention is to set a fan oven 20 degrees Celsius lower than a conventional one. Gas marks come from an older Fahrenheit-based scale: gas mark 1 is 275F and each mark adds 25F. The Celsius values on charts are rounded conventions rather than exact conversions, which is why 180C sits next to 350F on a recipe even though 180C is strictly 356F.
The full conversion chart
- Gas 1 = 140°C = 120°C fan = 275°F (cool)
- Gas 2 = 150°C = 130°C fan = 300°F (cool)
- Gas 3 = 160°C = 140°C fan = 325°F (moderate)
- Gas 4 = 180°C = 160°C fan = 350°F (moderate)
- Gas 5 = 190°C = 170°C fan = 375°F (moderately hot)
- Gas 6 = 200°C = 180°C fan = 400°F (moderately hot)
- Gas 7 = 220°C = 200°C fan = 425°F (hot)
- Gas 8 = 230°C = 210°C fan = 450°F (hot)
- Gas 9 = 240°C = 220°C fan = 475°F (very hot)
Below gas mark 1 there are also the very slow settings, gas ¼ and gas ½, at roughly 110 to 120°C, used for meringues and long braises.
Worked examples
Ovens are approximate anyway
Do not agonise over a few degrees. A domestic oven thermostat cycles the heat on and off, so the real temperature typically swings 10 to 20 degrees around the number on the dial, and different shelves run hotter or cooler again. The conversions here are well inside that tolerance. What matters is matching the type of setting the recipe intended (cool, moderate or hot) and knowing your own oven's personality; a cheap oven thermometer is the quickest way to learn it. Preheating matters more than precision as well: put a cake into an oven still climbing to temperature and the first ten minutes happen at the wrong heat, whatever scale the recipe used. Give the oven a good ten to fifteen minutes before anything sensitive goes in, and resist opening the door early, since every peek drops the temperature by far more than any conversion rounding. Converting an American recipe usually means Fahrenheit plus cup measures, so keep our cups to grams converter open in the next tab, and the litres to pints converter covers the liquids.
Frequently asked questions
What is 180 degrees Celsius in a fan oven?
160 degrees. The standard rule is to knock 20 degrees Celsius off the conventional temperature for a fan oven, because the moving air transfers heat more efficiently. 180C conventional is also gas mark 4 and roughly 350F.
What temperature is gas mark 6?
Gas mark 6 is 200C in a conventional oven, 180C in a fan oven and 400F. It is the classic moderately hot setting used for roasting vegetables, pizzas and pastry.
How do I convert a fan oven temperature to conventional?
Add 20 degrees Celsius. If a recipe says 160C fan, that is 180C in a conventional oven. Going the other way, subtract 20. The rule is a convention rather than physics, but it works well in practice.
Why do Celsius and Fahrenheit not line up exactly on oven charts?
Gas marks were defined in Fahrenheit, 25 degrees apart from 275F at gas mark 1, and the Celsius values on charts are rounded conventions. Strictly 180C is 356F, but recipe books call it 350F. Ovens are not precise enough for the difference to matter.
How accurate does an oven temperature need to be?
Most domestic ovens swing 10 to 20 degrees around their set point as the thermostat cycles, so these conversions are well within normal oven tolerance. If baking results seem consistently off, an inexpensive oven thermometer will tell you what your oven really does.