°F, °C, Gas Mark and Fan: How to Convert Any Oven Temperature

You find a promising recipe online, it says “bake at gas mark 6,” and your oven dial only speaks Fahrenheit. Or a European cookbook calls for 180°C and you’re staring at a dial that stops at 550°F. Worse, your oven is fan-forced and the recipe was written for a conventional one, so even after you convert the number the cake comes out dry and over-browned. Oven temperature looks like it should be a single tidy conversion, but there are actually three different scales in common use and a fourth complication — fan ovens — layered on top. Get the relationships straight once and you can cook from any recipe in the world.

The Fahrenheit–Celsius formula that never changes

The two temperature scales are linked by a single exact relationship:

°C = (°F − 32) × 5/9
°F = °C × 9/5 + 32

The 32 accounts for the offset between the two zero points, and the 5/9 (or 9/5) is the ratio of the degree sizes — a Celsius degree is 1.8 times bigger than a Fahrenheit degree. So 350°F, the most common baking temperature in American recipes, works out to (350 − 32) × 5/9 ≈ 177°C, which everyone rounds to 180°C. That rounding is normal and harmless: ovens drift by ±10–15°C anyway as the thermostat cycles the element on and off, so chasing a single degree is pointless. The practical takeaway is the handful of pairs you’ll meet constantly — 325°F ≈ 160°C, 375°F ≈ 190°C, 400°F ≈ 200°C, 425°F ≈ 220°C — all of which fall out of that one formula.

Where UK gas marks come from

British gas ovens are still calibrated in gas marks, a numbering that looks arbitrary until you see the pattern. The scale anchors at the low end with two fractional marks and then climbs in even steps:

Gas ¼  110°C    Gas 4  180°C
Gas ½  120°C    Gas 5  190°C
Gas 1  140°C    Gas 6  200°C
Gas 2  150°C    Gas 7  220°C
Gas 3  160°C    Gas 8  230°C
                Gas 9  240°C

From gas mark 1 upward, each whole mark adds roughly 10°C (mark 7 to 8 and 8 to 9 each jump by a slightly larger step to keep round numbers). The two fractions below mark 1 cover the “very cool” range used for meringues and slow-drying. So the classic instruction “gas mark 4, 180°C, 350°F” is three names for the same moderate oven, and “gas mark 6” — a hot 200°C / 400°F — is the standard setting for roasting vegetables and baking bread. Because the steps aren’t perfectly linear, it’s safest to look the mark up rather than try to do mental arithmetic on it.

The fan-oven rule: subtract 20°C

This is where most converted recipes go wrong. A fan-forced (convection) oven circulates hot air with a fan, which strips away the insulating layer of cooler air that sits against the food in a conventional oven. The result is faster, more even, more aggressive heat transfer — so a fan oven set to the same number cooks hotter in practice. The standard correction is to drop the temperature by about 20°C (roughly 40°F) when a recipe was written for a conventional oven, or alternatively to keep the temperature and cut the cooking time by about 25%. Do one or the other, not both.

So a recipe that says “conventional 200°C” becomes 180°C in a fan oven. Manufacturers print this exact offset in their manuals, and it’s why fan ovens often have a separate dial marking. The reverse matters too: if you find a recipe that already specifies a fan temperature and your oven is conventional, add the 20°C back. Forgetting this single step is the most common reason a trusted recipe suddenly burns on the edges while staying raw in the middle.

Don’t confuse oven temperature with internal temperature

The oven setting controls the environment; what actually tells you food is done is the internal temperature measured with a probe thermometer, and those numbers live on a completely different scale. Beef and lamb hit medium-rare at about 57°C (135°F) and medium at 63°C (145°F); pork is USDA-safe at 63°C (145°F); poultry must reach 74°C (165°F) to be safe, though dark thigh meat tastes better pushed to around 79°C. Ground meats need 71°C (160°F). These targets don’t change with the oven scale you cook in — a chicken is safe at 74°C whether your dial reads 165°F, 74°C or gas mark 4. Always judge doneness by the core temperature, not by the clock or the dial.

Converting in practice

To move a single reading between scales you only need the two facts above: the °C/°F formula and the gas-mark table. To do it without arithmetic, the Oven Temperature Converter cross-converts any value between Fahrenheit, Celsius and gas mark, applies the fan-oven adjustment for you, and shows a full chart from “very cool” to “searing” alongside a meat-doneness table. When you’re adapting a recipe for a smaller appliance, pair it with the Oven to Air Fryer Converter, which handles the extra temperature drop and time reduction air fryers need; and if you’re changing the batch size at the same time, the Recipe Scaler keeps every ingredient in proportion. Learn the formula, the gas-mark steps and the 20°C fan rule once, and no recipe’s oven instructions will ever stop you again.