Fan vs conventional ovens: where the 20°C rule comes from
Every British recipe page carries the same footnote: “for a fan oven, reduce the temperature by 20°C.” It is quoted so often it has become folklore — but it is not folklore, it is published guidance from named sources, and those sources do not fully agree. This article shows the receipts, explains the physics that makes the rule necessary, and tells you when to break it.
Key takeaways
- −20°C is the UK default: Delia Online says reduce by 20°C or check your handbook; BBC Good Food's chart subtracts 20°C on every row.
- −15°C is the appliance-maker version: Bosch's official convection guide says ~25°F (≈15°C) plus a 10–25% shorter cook time.
- Honest range: 15–25°C. The disagreement is smaller than many ovens' calibration error.
- Never reduce twice: a recipe that already says “fan” has had the cut applied.
The rule, sourced
Two UK authorities anchor the −20°C rule. Delia Online's information centre, writing about recipes tested in conventional ovens, says: for fan ovens where the fan cannot be switched off, “we suggest you reduce the temperature by 20 degrees centigrade, or check the manufacturer's handbook” — and notes they cannot guarantee fan-oven results the way they can conventional ones (deliaonline.com). BBC Good Food goes further: its published conversion chart carries a dedicated fan column that is exactly 20°C below the electric setting on every row from gas ¼ to 9 (bbcgoodfood.com).
Then there is the manufacturer view. Bosch, which actually builds the ovens, publishes a convection guide recommending you “decrease the temperature by about 25°F (around 15°C) compared to what's recommended for conventional ovens”, because “circulating air … distributes heat more efficiently” — and adds that cooking time may shrink by 10–25% (bosch-home.com). Fifteen degrees, not twenty.
So the range of published guidance is 15–25°C. Twenty is the majority answer and the safe default; the disagreement itself tells you something useful — the exact number matters less than knowing your own oven. The temperature converter shows all three fan readings (−15, −20, −25) for any input.
What moving air actually does
A conventional oven heats by radiation from the walls and by sluggish natural convection: warm air rises, cool air sinks, and a still layer of air sits against the surface of your cake like a blanket. The thermostat reads the air near that wall — the hottest, stillest spot in the box.
A fan oven strips the blanket off. Forced convection moves air continuously across the food, so the surface exchanges heat with fresh hot air instead of its own cushioning boundary layer. The same air temperature now delivers heat to the batter faster — which means an equal bake at a lower dial setting, and a temperature gradient that is flatter front-to-back and shelf-to-shelf. That is why fan ovens also fix the “back row browns first” problem: no stratification, no hot top shelf.
The trade-offs: moving air dries surfaces faster (a plus for roast potatoes and choux, a minus for custards and cheesecakes), and it can blow the top off a wet batter before it sets. Very delicate bakes often go better on conventional top-and-bottom heat.
When to ignore the rule
- The recipe already prints a fan temperature. “200°C / 180°C fan” means the reduction is done. Setting 180°C and subtracting again gives you 160°C and a sad, pale bake — the single most common fan-oven mistake (see common temperature mistakes).
- Your oven has “true convection” with a rear element: these run fiercer than fan-assisted ovens that merely stir air around a bottom element. Bosch's gentler −15°C is closer to reality for fan-assisted boxes; −20 to −25°C suits proper convection.
- Long, low bakes: for a 3-hour fruit cake at 140°C, the fan setting matters less than blocking draughts; keep the reduction but check the thermometer.
- Anything where the clock is the recipe: macarons, meringues, choux. Convert with the tool, then trust your eyes over both dials.
The honest bottom line
Use −20°C as the default, know that 15–25°C is the published range, and spend the difference on an oven thermometer — Delia's team recommends one “at all times”, and it converts this entire debate from theory into one glance at a dial. If you bake the same chocolate cake at 160°C fan and 180°C conventional and note the times, you have calibrated the rule for your kitchen, which beats any chart including ours. Start with the oven temperature chart for the rows, and the preheating guide for the thermometer part.
Last verified: 2026-11-16 — sources: Delia Online, BBC Good Food, Bosch Home Appliances (official), all fetched live on that date.
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