Short answer first, because it’s the thing everyone actually came for: yes, drop the temperature by 20°C when a recipe is written for a conventional oven and you’re baking in a fan oven. So 180°C conventional becomes 160°C fan. That rule is right often enough to trust, and wrong often enough that it’s worth knowing why.
I bake in a fairly ordinary 60cm electric oven with a fan I can switch off. I’ve had the same £9 oven thermometer clipped to the middle shelf for about four years, and the reason it lives there permanently is the answer to question six below.
What’s actually different between fan and conventional?
A conventional oven heats with elements at the top and bottom and lets the air move on its own. Hot air rises, cooler air sinks, and you end up with layers — the top of the cavity genuinely runs hotter than the bottom, often by 15–25°C in a domestic oven.
A fan oven (convection, in American recipes) has a fan in the back wall, usually with its own ring element around it, pushing air around the cavity. That does two things. It flattens out those temperature layers, and it strips away the thin cushion of cooler, moisture-laden air that sits against the surface of whatever you’re cooking. That boundary layer is a real insulator. Remove it and heat transfers into the food noticeably faster at the same air temperature.
So the air isn’t hotter. It’s just far more effective at delivering the heat it has.
Do I really have to subtract 20°C?
Most of the time, yes. Manufacturers landed on 20°C because it’s roughly the offset that makes a fan oven behave like a conventional one for typical baking, and both Delia-era British recipe books and modern oven manuals use it.
Where I’d argue with it: for anything above 220°C the gap matters less, because you’re usually chasing browning and speed anyway. Roasting potatoes at 220°C fan instead of 200°C fan will not ruin them. For anything delicate and under 170°C, I’d subtract the full 20°C and then check early.
Some newer ovens do the maths internally — you set 180°C, choose fan, and the oven targets 160°C itself. If your recipe books suddenly started underbaking when you changed ovens, that’s usually the reason. The manual will say so in the first few pages.
What are the conversions for the temperatures I actually use?
| Conventional °C | Fan °C | °F | Gas mark |
|---|---|---|---|
| 140 | 120 | 275 | 1 |
| 150 | 130 | 300 | 2 |
| 160 | 140 | 325 | 3 |
| 180 | 160 | 350 | 4 |
| 190 | 170 | 375 | 5 |
| 200 | 180 | 400 | 6 |
| 220 | 200 | 425 | 7 |
| 230 | 210 | 450 | 8 |
| 240 | 220 | 475 | 9 |
The Fahrenheit column is rounded to the numbers American ovens actually have on the dial, so 180°C shows as 350°F rather than a literal 356°F. Nobody’s oven has a 356 setting.
Does the cooking time change too, or just the temperature?
Both, and this is the half people forget. Even after dropping 20°C, a fan oven tends to finish things 10–15% faster because the airflow keeps working on the surface.
On a 25-minute tray of biscuits that’s roughly three minutes. On a 60-minute loaf it’s closer to eight, which is enough to matter. My habit: set the timer for 80% of the stated time, look then, and judge by the food rather than the clock. A skewer into the centre of a cake should come out with a few dry crumbs, not wet batter and not bone dry.
The other option is to keep the recipe temperature and just shorten the time. That works for roasting. It does not work for cakes, because the higher air temperature sets the crust before the centre has risen.
Why do my cakes dome, lean or bake lopsided in a fan oven?
Because moving air hits one side of the tin first. The side facing the fan sets sooner, the batter keeps rising on the softer side, and you get a slope.
Doming is a slightly different fault: the outside of the batter sets while the middle is still liquid, so the only direction left for the centre to expand is up. A crack down the middle is the same story taken further. Both usually mean the effective temperature was too high — either you didn’t subtract the 20°C, or your oven runs hot.
Three things fixed it for me. Rotate the tin at the two-thirds mark, use a wet strip of tea towel or a proper bake-even strip around the outside of the tin, and drop another 10°C for deep tins over 20cm. A 20cm layer cake that domed every single time at 160°C fan came out flat at 150°C fan with five minutes added.
Is my oven’s dial even telling the truth?
Probably not, and this is the single cheapest fix in home baking. An oven thermometer costs about £10. Hang it from the middle shelf, set the oven to 180°C, and wait a full 20 minutes after the preheat light goes off before you read it.
Most domestic ovens I’ve measured or heard about sit 10–20°C away from their dial, and mine reads about 12°C cool at 180°C, which I now just build into every recipe. Ovens also cycle: the element switches on and off around a set point, so the real temperature swings roughly ±10°C in a wave rather than sitting flat. That’s why the thermometer needle wanders while you watch it.
They drift with age too. Door seals go, thermostats lose calibration. If your baking times have quietly crept upward over five years, that’s worth checking before you blame the recipe.
Where are the hot spots, and how do I find them?
The bread-slice test, and it takes ten minutes. Cover a whole oven shelf with slices of cheap white bread edge to edge, set the oven to 200°C, and toast them for 6–8 minutes. Pull the shelf out and look. The dark slices mark your hot zones, the pale ones your cool ones.
Mine runs hot at the back right, which is exactly where the fan sits. Nothing wrong with the oven — that’s just where the airflow arrives. Knowing it means I rotate trays front to back rather than side to side, and I never put the one cupcake I care about in that corner.
Worth repeating the test on each shelf position you use, because the pattern changes with height. Ten minutes and a stale loaf buys you information you’ll use for years.
Which foods are genuinely better on fan?
Anything you want dry, brown and crisp on the outside. Roast potatoes, chicken skin, chips, tray-roasted vegetables. Air movement pulls surface moisture away, and dry surfaces brown. A tray of roasted vegetables at 180°C fan comes out with proper caramelised edges where the same tray on conventional steams a little in its own moisture.
Batch baking is the other clear win. Three trays of cookies or scones on three shelves will bake close to evenly on fan, where on conventional the top tray browns and the bottom one stays pale. That alone is why I leave the fan on for most weekday cooking.
Pizza and focaccia also benefit, though for a different reason — you’re usually running the oven flat out, and fan gets a domestic oven closer to the heat a bread crust wants.
Which things are better left on conventional?
Anything where you want a slow, gentle set or a fragile structure.
- Custards and baked cheesecake. Moving air dries and cracks the top. A New York cheesecake baked at 150°C conventional in a water bath has a far better chance of a smooth surface.
- Crème brûlée and other set custards. Same reason, and the water bath does the temperature control for you.
- Soufflés. The fan can push the rise sideways and collapse one edge.
- Delicate sponges and meringues. Meringue especially — you want slow drying, not forced airflow.
- Slow-rise breads that prove or finish in the oven. A fan will skin the dough surface before it’s finished expanding.
Pastry sits on the fence. A double-crust apple pie browns beautifully on fan but the top can outrun the filling, so I start it on fan at 190°C for 20 minutes and switch the fan off for the rest.
Does opening the door actually matter?
More than people think, and more in a fan oven than a conventional one. Opening the door for five seconds can dump 10–25°C, because the hot air leaves in one go and cold room air replaces it. In a fan oven the fan then blows that cooler air straight at your food.
The rule I follow: nothing gets opened in the first two-thirds of a cake’s bake. A cake that’s still rising has a foam structure held up by expanding gas, and a sudden temperature drop deflates it before the crumb has set. After the structure is firm, a quick look is harmless.
Use the light and the glass. And if your oven door glass is too fogged or filthy to see through, clean it — it’s genuinely a baking tool.
What does the fan-plus-bottom-heat setting do?
It runs the bottom element together with the fan, so you get circulating air and extra heat coming from underneath. That combination is aimed squarely at soggy bases.
Pizza, quiche, tarts, anything blind-baked. The bottom element drives heat up into the base of the tin while the fan keeps the top browning evenly. On my oven a margherita on a preheated tray at 240°C with fan-plus-bottom is the closest I get to a crisp base without a pizza stone.
The related setting worth knowing is fan-plus-grill, which pulses the grill element while the fan runs. It’s for browning the top of something already cooked — a gratin, a shepherd’s pie lid — and it will scorch a cake in about ninety seconds if you pick it by mistake. If your oven’s symbols are a mystery, the manual is almost certainly a PDF on the manufacturer’s site under the model number stamped inside the door frame.