How do I convert oven temperatures and gas marks?

Recipes come in three scales and most kitchens have an oven that disagrees with all of them. The conversions are simple; the more useful knowledge is that the dial is an instruction rather than a measurement.

Step by step

1. Learn the common gas mark conversions.Gas 1 is about 140°C, gas 2 150, gas 3 170, gas 4 180, gas 5 190, gas 6 200, gas 7 220, gas 8 230, gas 9 240. Those are conventional, not fan.
2. Reduce by about 20°C for a fan oven.A recipe stating 180°C conventional becomes roughly 160°C fan. Fan ovens move hot air, so they cook faster at the same dial setting.
3. Reduce the time a little as well.Fan ovens typically cook 10-15% faster even after the temperature reduction. Start checking earlier than the recipe says.
4. Convert Fahrenheit roughly in your head.Subtract 30 and halve. 350°F becomes about 160°C — close enough for cooking, where precision to the degree is not the point.
5. Check your own oven with a thermometer.The most useful step here. Most domestic ovens are out by 20°C or more, which swamps any conversion error.
6. Know your oven's hot spots.Almost all ovens are uneven. Rotating trays partway through matters more than getting the number exactly right.
7. Adjust for position.In a conventional oven the top is hotter than the bottom. Fan ovens are far more even, which is their main advantage.
8. Trust the food over the number.Colour, smell and a skewer tell you more than the dial. The temperature is a starting point.

Tips

• Write your oven's offset on a card inside a cupboard door. "Runs 20°C cool, hot at the back right" is all you need and you will not remember it otherwise.
• Some ovens have a fan setting that is not a true fan oven — check the manual, because the 20°C reduction may not apply.
• For anything delicate, an oven thermometer is worth more than any conversion chart.

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