The usual adjustment
A fan circulates the hot air, so food cooks faster than in a conventional oven set to the same number. The usual UK rule, as Which? puts it, is to set a fan oven about 20 °C lower. Smeg’s chart takes 20 °C off at every step:
| Conventional | Fan |
|---|---|
| 160 °C | 140 °C |
| 180 °C | 160 °C |
| 190 °C | 170 °C |
| 200 °C | 180 °C |
| 220 °C | 200 °C |
A 20 °C gap is 36 °F.
Makers don’t all agree
| Source | Advice |
|---|---|
| Smeg | 20 °C lower |
| AEG | 20 °C lower, and about 10 minutes less per hour |
| Hotpoint | About 25 °C lower, and about 10 minutes less per hour |
| GE (US) | 25 °F lower, about 14 °C |
GE says most of its convection ovens already lower the temperature by 25 °F for you. If yours does, don’t take it off a second time. Your own oven’s manual comes first.
Which number the recipe means
Many recipes print both, such as “180 °C (160 °C fan)”. Others explain it in the book’s notes: one Penguin Australia cookbook gives fan-forced temperatures and says to add about 25 °C for a conventional oven. With GE’s US rule, a 350 °F recipe becomes 325 °F on convection bake. If you can’t tell which oven a recipe means, check the food a little early the first time.
For gas ovens, see gas mark.