Gas mark, fan, and the twenty-degree rule
· 4 min read
Recipes assume an oven. Yours is probably not that oven. Three conventions collide here, and none of them are usually stated.
Fan ovens run hotter than the dial says
A fan — convection — oven circulates air, which transfers heat to food much more efficiently than still air at the same temperature. The practical result is that food cooks faster and browns more at any given dial setting.
The usual correction is to set a fan oven about 20 °C (roughly 25–35 °F) below a conventional recipe temperature, or to keep the temperature and shorten the time. Our oven converter shows the conventional and fan figures side by side so you are not doing this in your head with floury hands.
The correction is a convention, not a law of physics — manufacturers differ, and some ovens apply it internally so the dial already reads the conventional equivalent. If yours consistently overcooks at the converted temperature, trust your oven over the rule.
Gas marks are not linear
Gas mark 1 is about 140 °C and gas mark 9 is about 240 °C, but the steps between are not even, and the low end is finer than the high end. There is no clean formula to memorise, which is why a chart beats arithmetic here.
Marks below 1 exist too — ¼ and ½ — and they matter for the slow cooking that older recipes call for.
Your oven is lying to you by about 15 degrees
Domestic ovens are rarely accurate, and rarely even. Most cycle around a set point rather than holding it, and a swing of 10–15 °C is normal. Many run consistently hot or cold from new, and drift further with age.
An oven thermometer costs a few pounds and tells you your own oven’s offset. Once you know it, every recipe you cook gets slightly better, which is a better return than any conversion chart.
Practical order of operations
- Convert the recipe temperature to the scale your dial uses.
- Apply the fan reduction only if your oven has not already applied it.
- Adjust for your own oven’s known offset.
- Then ignore all of it and judge by colour and internal temperature, because that is what actually tells you the food is done.
