What this converter works out
Converts an oven temperature between the four scales British kitchens run into: conventional Celsius, fan Celsius, gas mark and Fahrenheit.
The one that catches people out is the fan adjustment. A fan oven circulates hot air, so it cooks faster at a lower setting, and the standard correction is 20°C down. Use the conventional figure in a fan oven and everything browns too quickly.
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Before you convert
- A recipe that gives one temperature usually means a conventional oven unless it says otherwise. British recipes normally give both, in the form "180°C / 160°C fan / gas 4".
- Fan ovens circulate hot air, so they cook faster and more evenly at a lower set temperature. The usual adjustment is 20°C down.
- Gas marks are a coarser scale. They step in intervals of 10 to 20°C, so a converted figure is the nearest mark rather than an exact match.
Moving air strips away the insulating layer of still air around food, so heat reaches it faster. Dropping the set temperature by 20°C compensates for most of that. For anything cooked quickly or browned on the surface — pastry, biscuits, roast potatoes — you may also need to shorten the time by around ten per cent. For long, slow cooking the time barely changes.
How this converter works
Fan °C = conventional °C − 20
°F = (°C × 9 ÷ 5) + 32
Gas mark = nearest step on the published scaleFahrenheit is straightforward arithmetic. The fan adjustment is a convention rather than a physical constant, and some manufacturers state 15 or 25°C for their own ovens, so check the manual if a recipe consistently comes out wrong.
Gas marks are not a formula at all. They are a fixed ladder of settings, so the converter picks the nearest one and tells you the conventional temperature that mark actually represents.
The full conversion table
| Gas mark | Conventional °C | Fan °C | °F | Typical use |
|---|---|---|---|---|
| ¼ | 110 | 90 | 225 | Meringues, drying |
| ½ | 120 | 100 | 250 | Very slow cooking |
| 1 | 140 | 120 | 275 | Slow casseroles |
| 2 | 150 | 130 | 300 | Rich fruit cake |
| 3 | 170 | 150 | 325 | Sponge cakes, slow roasts |
| 4 | 180 | 160 | 350 | The default for most baking and roasting |
| 5 | 190 | 170 | 375 | Roasting, scones |
| 6 | 200 | 180 | 400 | Roast potatoes, pastry |
| 7 | 220 | 200 | 425 | Puff pastry, initial roasting blast |
| 8 | 230 | 210 | 450 | Bread, pizza |
| 9 | 240 | 220 | 475 | Highest setting on most domestic ovens |
Worked example
An American recipe calls for 375°F and you have a fan oven:
- Celsius: (375 − 32) × 5 ÷ 9 = 190.6, so 191°C conventional
- Fan: 191 − 20 = 171°C. Published tables usually round this pair to 190 and 170, which is near enough for an oven dial.
- Nearest gas mark: gas 5
Set the fan oven to 170°C and check about ten per cent earlier than the recipe says, because fan ovens brown faster.
Common mistakes
- Using the conventional temperature in a fan oven. Twenty degrees hotter than intended, and cakes catch on top before the middle sets.
- Adjusting twice. If a recipe already gives a fan figure, use it as printed.
- Trusting the dial. Domestic ovens are commonly out by 10 to 20°C. An oven thermometer costs very little and settles the question.
- Ignoring the shelf. Even in a fan oven the top shelf usually runs hotter.
- Not preheating. Most ovens take longer to reach temperature than the indicator light suggests.
Frequently asked questions
Is a fan oven always 20 degrees lower?
It is the standard convention and works for most recipes, but it is a rule of thumb rather than a physical constant. Some manufacturers recommend 15 or 25°C for their own models. If your baking consistently comes out under or over done at the converted temperature, check the manual and adjust your own rule accordingly.
Do I need to change the cooking time as well?
Often slightly. Fan ovens cook a little faster even at the lower setting, so for anything quick or surface-browned — biscuits, pastry, roast potatoes — start checking about ten per cent earlier. For long slow braises the difference is negligible.
What is gas mark 4 in Celsius?
180°C in a conventional oven, or 160°C in a fan oven. It is the most commonly used setting in British recipes and suits most cakes, roasts and bakes.
Why do gas marks not convert exactly?
Because they are a fixed ladder of settings rather than a continuous scale. The steps are 10 to 20°C apart, so any Celsius figure lands between two marks. The converter gives the nearest, along with the temperature that mark actually represents.
My oven runs hot. What should I do?
Buy an oven thermometer, leave it in, and learn the offset. Domestic ovens commonly read 10 to 20°C away from their true temperature, and the difference is a far more common cause of failed baking than any conversion error.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
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References
These organisations publish the guidance and figures behind this page.
- Food Standards Agency — UK food safety guidance for home cooks and businesses
- GOV.UK — energy labelling and appliance standards for domestic ovens
Sources are checked at publication and can change — how I choose and check references.