The arithmetic, and why the charts disagree with it
°C = (°F − 32) × 5/9 and °F = °C × 9/5 + 32. Run 350 through it and you get 176.67 °C. Every conversion chart prints 180. They are not wrong to; ovens are set in increments of 5 or 10 degrees, the thermostat cycles across a band of 20 or more, and 3 degrees is well inside the noise. The rounding is only worth watching at the ends: 475 °F is 246 °C, and rounding that up to 250 rather than down to 245 is a real difference on a pizza.
| °F | Exact °C | Set it to | Gas mark |
|---|---|---|---|
| 250 | 121.1 | 120 °C | 1/2 |
| 275 | 135.0 | 135 °C | 1 |
| 300 | 148.9 | 150 °C | 2 |
| 325 | 162.8 | 160 °C | 3 |
| 350 | 176.7 | 180 °C | 4 |
| 375 | 190.6 | 190 °C | 5 |
| 400 | 204.4 | 200 °C | 6 |
| 425 | 218.3 | 220 °C | 7 |
| 450 | 232.2 | 230 °C | 8 |
| 475 | 246.1 | 245 °C | 9 |
The convection adjustment is a rule of thumb
A convection oven has a fan that moves hot air across the food instead of letting it sit. Moving air strips away the cooler boundary layer at the surface, so heat transfers faster at the same air temperature. The conventional response is to set the oven 25 °F — call it 15 to 20 °C — below what the recipe says, or to leave the temperature alone and cut the time by 20 to 25%.
Neither figure is a specification. They are averages across a lot of ovens and a lot of dishes. How much faster your oven actually cooks depends on fan speed, how packed the racks are, whether the pan is dark metal or glass, and whether the food has a surface that benefits from the airflow. Roasted vegetables and anything you want browned gain the most from convection. Delicate cakes and custards gain the least and can be actively harmed, because the moving air sets the outside before the middle rises.
The failure to avoid is applying the adjustment twice. Doing both — dropping the temperature and shortening the time — leaves food raw. Doing it to a recipe that already gave a fan temperature does the same. British and Australian recipes routinely print both, as "180 °C / 160 °C fan / gas mark 4". Take the one that matches your oven and ignore the others.
Gas marks
Gas mark is a British scale where each whole mark is 25 °F, with mark 1 at 275 °F. Mark 4 is 350 °F, which is why it turns up in so many recipes; it is the British default in the same way 350 °F is the American one. Below mark 1 there are 1/2 at 250 °F and 1/4 at 225 °F for very low work. Above mark 9 there is nothing, which is fine, because there is not much above 475 °F in a domestic oven either.
Your oven is not the temperature it says
This is the part that matters more than any conversion. A domestic oven thermostat measures air at one point and cycles the heating element on and off around the setpoint, so the actual temperature swings. Being 25 °F off calibration is normal, 50 °F is common in an older oven, and the top of the oven can run 25 °F above the bottom in the same cycle. Recipes are written to conventional numbers, and the gap between the number and your oven is usually larger than the gap between 176.7 °C and 180 °C that people worry about.
Buy a standalone oven thermometer, hang it on the middle rack, preheat to 350 and read it after twenty minutes. Whatever it says is the offset you have been fighting. Once you know it, every recipe in the house gets easier at once, and none of the arithmetic above ever comes up again.
Questions people ask
What is 350 °F in Celsius?
176.7 °C exactly, and 180 °C in practice. Every conversion chart rounds it to 180 because ovens are marked in tens and the 3.3 degree difference disappears inside the thermostat swing. If you want to be closer, 175 °C is fine too — it is the same distance the other way and marginally safer for anything that browns fast. What you should not do is treat 176.7 as a meaningful number to chase; the oven cannot hold it and does not know it.
Do I lower the temperature or shorten the time on a convection oven?
One or the other. Lowering by 25 °F and keeping the recipe time is the more forgiving option because it leaves the timings you were given intact. Keeping the temperature and cutting the time by 20 to 25% browns more aggressively, which is what you want on roasted vegetables and roast potatoes and not what you want on a cake. Doing both at once is the classic mistake and produces food that looks done and is not. Whichever you pick, start checking earlier than you would have, because the first bake in a convection oven is always a calibration run.
The recipe says "160 °C fan". What do I set?
If your oven has a fan, set 160 °C. The author has already done the conversion for you. If your oven is conventional, add the adjustment back and set about 180 °C, or 350 °F. British recipes frequently print all three figures — "180 °C / 160 °C fan / gas 4" — and all three describe the same bake in different ovens. Take the one line that matches the equipment in your kitchen.
Is an air fryer the same as convection?
Mechanically yes, in a small box with a strong fan. Everything above applies, only more so: the airflow is faster and the cavity is tiny, so the food heats quicker than the same setting would suggest in an oven. Starting at 25 °F below the oven recipe and at roughly 70 to 80% of the time gets you in range, then check. The bigger difference is capacity. An air fryer only browns properly with space around the food, so a batch that fills the basket cooks slower and steams rather than crisps, regardless of what the dial says.
Does preheating actually matter?
For anything where the initial burst of heat sets the structure, yes, and there is no way round it. Bread, cookies, cakes, pastry, popovers and anything leavened all depend on hitting hot air immediately; put them into a rising oven and they spread before they set. For a casserole, a reheat, or a slow braise, the preheat is a convenience rather than a requirement and you can put the dish in cold and add time. Note that most ovens announce they are preheated when the air hits the setpoint, while the racks and walls are still catching up. Another five to ten minutes after the beep gives you an oven that actually holds the temperature when you open the door.