Gas Mark, Celsius and Fahrenheit: Getting the Oven Right

Gas marks are a British oven scale that outlived the ovens that inspired them, so British recipes still quote them and everyone else has to look them up. The good news is that the scale is completely regular, which makes it easy to rebuild from memory if you forget the chart.

The chart, and the arithmetic behind it

Gas mark 1/4 is 225°F (110°C) and gas mark 1/2 is 250°F (120°C). Then the whole numbers run: 1 is 275°F (140°C), 2 is 300°F (150°C), 3 is 325°F (160°C), 4 is 350°F (180°C), 5 is 375°F (190°C), 6 is 400°F (200°C), 7 is 425°F (220°C), 8 is 450°F (230°C) and 9 is 475°F (240°C).

Every step is exactly 25°F, so if you remember that gas mark 4 is 350°F you can reconstruct the rest by counting. Gas mark 7 is three marks above 4, so 350 + 75 = 425°F.

Why the Celsius numbers look rounded

Because they are. Run the conversion properly and 180°C is (180 x 9/5) + 32 = 324 + 32 = 356°F, not 350°F. Going the other way, 200°C is (200 x 1.8) + 32 = 392°F, which everyone calls 400°F. The Celsius column is a set of tidy round numbers sitting near the Fahrenheit values rather than converting to them exactly, and being six or eight degrees out does not matter to a casserole. It matters more to meringue and sugar work, where you should trust a thermometer over any chart. The exact Celsius to Fahrenheit formula is worth knowing for everything outside the kitchen.

Fan ovens change everything

A fan (convection) oven moves hot air around, so it cooks faster at the same dial setting. The standard adjustment is to reduce the conventional temperature by 20°C, which is close enough to one gas mark. A recipe calling for 200°C conventional becomes 180°C fan, and both sit at gas mark 6 on a gas oven.

The other half of the adjustment is time. A fan oven usually finishes sooner as well as running hotter, so start checking about ten minutes early, especially for cakes. Recipe writers are inconsistent about which oven they mean, so if a recipe gives a single temperature with no label at all, assume conventional.

Using the converter

Enter whichever value you have and read off the other two. If your recipe says gas mark 5, type 5 into the gas mark field and you get 375°F and 190°C. If it gives 425°F, enter that and you get gas mark 7. The inputs come straight off the recipe, so the only judgement call is your own oven: the setting you dial in and the temperature inside the cavity are not the same thing. An oven thermometer sitting on the middle shelf is the cheapest fix in any kitchen, and it explains a lot of previously mysterious bakes.

When the recipe already gives a fan temperature

Look for the words fan, fan-forced or convection, or a fan symbol next to the number. If it says 160°C fan and you have a gas oven, add the 20°C back to get 180°C conventional, which is gas mark 4. Going the other way, a gas mark 6 recipe cooked in a fan oven wants 180°C rather than 200°C. Recipes that mix systems, such as an American cup measure alongside a British gas mark, are worth running through a cooking measurement converter before you start rather than halfway through.

Common questions

What is gas mark 4 in Fahrenheit and Celsius? 350°F and 180°C. It is the most common setting in British recipes and a reasonable default for roasting and baking generally.

What temperature is gas mark 6? 400°F, or 200°C conventional. In a fan oven, use 180°C instead.

Why do different charts give slightly different Celsius numbers? Because the Celsius values are rounded to convenient numbers rather than converted exactly. You will see gas mark 3 listed as both 160°C and 170°C. Either works, and the true value falls between them.

Do gas marks go below 1? Yes, most ovens have gas mark 1/2 and 1/4 below it, at 250°F and 225°F. Those are the settings for slow overnight cooking, meringues and keeping food warm.