Fan oven vs conventional: when to use which (and the −20°C rule)

TL;DR. A fan oven cooks faster and more evenly than a conventional oven because forced air convection transfers heat 3-5× faster than still air. Subtract 20°C (or 25°F, or one Gas Mark) when converting a conventional recipe to a fan oven.

Modern ovens come in three flavours: conventional (static heating elements top and bottom), fan-assisted (a fan circulates the heated air), and pure convection (fan + a dedicated heating element around the fan). The differences are real but small enough that most home cooks treat them interchangeably — and pay for it in dried-out chicken and soggy cookies.

The physics

Heat transfers from oven air to food through two mechanisms: radiation (infrared from the oven walls and elements) and convection(air molecules carrying heat to the food surface). In a conventional oven the air is essentially still — natural convection from the heating elements rising — and most heat transfer is radiation. In a fan oven the air is forcibly moved, accelerating convection by ~3-5×.

Net effect: at the same dial setting, food in a fan oven cooks faster than in a conventional oven, with more even browning around the food. The standard adjustment:

Subtract 20°C (or 25°F, or one Gas Mark) when converting a conventional-oven recipe for a fan oven.

When fan oven wins

• Roasts. Whole chickens, joints of beef, turkey. Fan-circulated heat browns the skin/crust more evenly and faster, locking in interior moisture better. Most professional kitchens roast in convection ovens.
• Cookies and baked goods cooked in batches. Fan ovens have more uniform temperature top-to-bottom, so trays on different racks come out evenly. In a conventional oven, the top rack browns first.
• Drying or crisping. Roasted vegetables, pavlova, meringue, granola. The moving air strips surface moisture faster, producing crisper results.
• Pizza. The convection helps crisp the base without burning the cheese (provided you preheat long enough — fan ovens can also cool faster after the door opens, so let it recover before adding the pizza).

When conventional wins

• Delicate cakes — souffles, sponges, angel food. The moving air in a fan oven can collapse a rising souffle or crack a delicate sponge top. Conventional’s still air produces the gentlest rise.
• Bread with steam.Artisan bread benefits from a humid oven during the first 10 minutes (the dough “blooms” better with steam). Fan ovens actively vent moisture; conventional ovens hold it.
• Custards and cheesecakes. Anything that can crack from rapid surface drying.
• Slow-roasting, low-and-slow cooking.At temperatures under 130°C the fan’s advantage is minimal and the air movement can dry out delicate items.

The recipe adjustment cheat sheet

Or use our oven temperature converter which (in a future update) will include a fan-oven toggle.

Time adjustment

Cooking time generally drops by 10-25% in a fan oven compared to a conventional oven at the equivalent temperature. For most recipes, start checking 10 minutes before the listed time. For delicate bakes that rely on exact timing (macarons, choux pastry), the temperature adjustment alone usually keeps the time identical.

If your recipe doesn’t specify

Modern UK and EU recipes increasingly specify both conventional and fan temperatures, e.g. “180°C / 160°C fan / Mark 4.” If a recipe lists only one temperature with no qualifier:

• UK or European recipe (1980-present): assume conventional unless said otherwise.
• US recipe: nearly always conventional (US household ovens are mostly conventional with optional convection).
• Pre-1980 recipe: definitely conventional.
• Modern professional recipe (cookbook from a working chef): often convection — check the headnote.

Numeric facts

• Convective heat transfer coefficient: still air ~5-25 W/m²·K; forced air at 2-5 m/s (typical oven fan) ~25-100 W/m²·K — a 3-5× increase at the food surface.
• Temperature reduction rule: −20°C, −25°F, or −1 Gas Mark when converting conventional to fan, per Defra/Food Standards Agency guidance.
• Time reduction: typically 10-25% shorter at the adjusted temperature; start probing 10 min before the recipe time.
• Roast turkey example: 4.5 kg turkey at 180°C conventional = ~3 h 15 min; same bird at 160°C fan = ~2 h 30 min — ~23% faster.
• Fan motor speed: typical domestic ovens run at 1,200-1,500 rpm; commercial combis run 2,500-3,500 rpm and reach >5 m/s air velocity at the food.
• Recovery time after opening the door: conventional ovens lose ~25-50°C and take 3-5 min to recover; fan ovens lose ~60-80°C but recover in 1-2 min thanks to forced circulation.
• Energy use: a 2.5 kW domestic fan oven typically uses ~15-20% less energy per cook than the conventional equivalent because cooking finishes faster and elements cycle less.
• True convection (third element) vs fan-assisted (just a fan): true convection adds another 5-10% efficiency and more uniform top-to-bottom temperature, ~±3°C vs ±10°C.

Decision matrix

Sources

• UK Food Standards Agency — Cooking your food: oven temperature conversion — food.gov.uk.
• ASHRAE Handbook — Fundamentals, Chapter 4 (Heat Transfer) — published convective coefficients for natural vs forced air.