How this works
Recipes from different countries use different temperature scales. American cookbooks default to Fahrenheit ("bake at 350°F"), continental European to Celsius ("180°C"), and many British cookbooks still use gas marks ("gas mark 4"). Without a quick converter you waste minutes Googling each step or risk over-baking when you eyeball the dial.
The Celsius–Fahrenheit conversion is the standard °F = °C × 9/5 + 32. Gas marks use a British scale: gas mark 1 ≈ 140 °C / 275 °F, with each mark adding 14 °C / 25 °F. Mark 4 (350 °F / 180 °C) is the workhorse "moderate" oven for most baking; mark 7 (425 °F / 220 °C) is "hot" for roasting and bread; mark 9 (475 °F / 245 °C) is "very hot" for pizza. Below mark 1 is a slow-cook range often labelled "warm" or "cool" for slow roasts and meringues.
One crucial caveat for fan-assisted (convection) ovens: they cook noticeably hotter than the dial indicates because moving air strips the boundary layer faster. Convert your recipe temperature, then drop it by 20 °C (35 °F) when using the fan setting. Modern combi ovens often do this conversion automatically when you switch modes.
The formula
C is Celsius, F is Fahrenheit, G is the British gas mark. Gas marks below 1 (½, ¼) extend down to ~110 °C / 225 °F for slow cooking. Marks above 9 are non-standard.
Example calculation
- A US recipe says "bake at 375 °F".
- C = (375 − 32) × 5/9 ≈ 191 °C; nearest gas mark = 1 + (191 − 140) / 14 ≈ 4.6 → use mark 5 (190 °C). With fan-forced, set 170 °C.
Frequently asked questions
How much hotter does a fan oven cook?
About 20 °C (35 °F) hotter at the same dial setting. Moving air strips the cooler boundary layer off the food faster, so heat transfers more aggressively. The standard fix is to lower the recipe temperature by 20 °C when using fan/convection. For delicate bakes (custards, cakes that should stay moist), some bakers go even further — drop 25 °C and check 5 minutes earlier than the recipe says.
What's "gas mark ½" or "gas mark ¼"?
British low-temperature settings used for slow cooking, baking meringues, and warming. Mark ¼ ≈ 225 °F / 110 °C, mark ½ ≈ 250 °F / 120 °C, mark 1 ≈ 275 °F / 140 °C. They're common in older British recipes for things like overnight slow-roast lamb shoulder or drying out meringues. On a modern oven, just set the equivalent °C — the result is the same.
My oven runs hot/cold — how do I calibrate?
Buy a £10–15 oven thermometer, place it on the middle rack, set the oven to 180 °C / 350 °F, let it preheat 20 minutes, and read the actual temperature. Most home ovens drift 10–20 °C from the dial. If yours runs 20 °C hot, set 160 °C when the recipe asks for 180 °C. Some ovens have a calibration offset in the menu; otherwise, just remember the offset. Hot spots are a separate issue — rotate trays halfway through if your back-left always browns first.