How Do I Convert Oven Temperatures Between Fan, Gas and Conventional?
Fan ovens run about 20°C lower than conventional for the same result. Gas marks map to fixed temperatures. A quick table and the reasons the conversions are approximate.
In short
To convert a conventional oven temperature to fan, subtract about 20°C, so 200°C conventional becomes 180°C fan. Gas mark 4 is 180°C conventional, gas mark 6 is 200°C, and each gas mark step is roughly 14°C. Fahrenheit equals Celsius times 1.8 plus 32, so 180°C is 350°F. Fan ovens cook faster as well as hotter, so check food a little early.
Three systems describe the same thing and none of them quite agree, which is why recipes state all three and why baking still goes wrong in someone else’s kitchen.
The table
| Gas mark | Conventional °C | Fan °C | °F | Description |
|---|---|---|---|---|
| ¼ | 110 | 90 | 225 | Very slow |
| ½ | 120 | 100 | 250 | Very slow |
| 1 | 140 | 120 | 275 | Slow |
| 2 | 150 | 130 | 300 | Slow |
| 3 | 160 | 140 | 325 | Moderate |
| 4 | 180 | 160 | 350 | Moderate |
| 5 | 190 | 170 | 375 | Moderately hot |
| 6 | 200 | 180 | 400 | Hot |
| 7 | 220 | 200 | 425 | Hot |
| 8 | 230 | 210 | 450 | Very hot |
| 9 | 240 | 220 | 475 | Very hot |
Fan versus conventional
A fan oven circulates the air, which delivers heat to food faster and more evenly than the still air in a conventional oven. To get the same result, set it about 20°C lower. Some manufacturers suggest 15, some 25; 20 is the usual compromise.
Fan ovens also tend to cook a little faster, so check five to ten minutes before the recipe says. This matters most for cakes and bread, which can dry out or over-brown, and least for roasting, where a bit of extra browning is welcome.
Gas marks
The scale runs from ¼ to 9, and each whole step from mark 1 upwards is roughly 14°C, or 25°F. It was designed for the Fahrenheit scale, which is why the Fahrenheit column steps neatly and the Celsius one does not.
Fahrenheit
Multiply Celsius by 1.8 and add 32. 180°C is 356°F, which recipes round to 350. Going the other way, subtract 32 and divide by 1.8.
Why it is all approximate
Domestic ovens are not precise. Most run 10 to 20°C away from the dial, and the temperature cycles up and down around the set point as the thermostat switches the element. Two ovens set to 180 can be cooking at 165 and 195.
An oven thermometer on the middle shelf, checked once, tells you your oven’s actual offset. That single piece of information does more for consistent results than any conversion table.
When it matters most
- Baking. Cakes, biscuits and bread are sensitive to 10 or 15 degrees. Use the fan reduction and check early.
- Meringues and slow cooking. At the low end, small errors are a large proportion. If your oven runs hot, the lowest mark may be too high.
- Roasting and general cooking. Far more forgiving. If in doubt, a few degrees either way will not matter.
If a recipe gives only one figure, assume it is conventional unless it says otherwise. Most published recipes are written that way, and most British ovens sold in the last twenty years are fan, which is a mismatch worth knowing about.
Common questions
Why are fan ovens set lower?
Moving air transfers heat to food more efficiently than still air, so the same temperature cooks faster and browns more. Reducing the setting by about 20°C compensates. Some recipes also suggest shortening the time by roughly 10 percent.
Does the top or bottom shelf matter?
In a conventional oven, yes, the top is hotter. In a fan oven the circulation evens things out, which is one of its main advantages for cooking several trays at once.
Why does my oven not match the dial?
Most domestic ovens are out by 10 to 20°C, and some by more. An oven thermometer placed on the middle shelf tells you the real figure, and it is the cheapest upgrade to consistent baking there is.
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