Understanding oven temperatures
°F, °C, Gas Mark — and a fan-oven adjustment.
The three temperature systems in cookbooks, the UK Gas Mark numbers, and the fan-oven adjustment that nobody on either side of the Atlantic remembers.
The three systems.
°F (US, Liberia, a few Caribbean islands), °C (everywhere else), Gas Mark (UK gas ovens). Conversion °F → °C: (F − 32) × 5/9. The everyday rule of thumb cooks use: 350°F = 180°C = Gas Mark 4 (moderate); 400°F = 200°C = Gas Mark 6 (hot); 425°F = 220°C = Gas Mark 7 (very hot); 475°F = 240°C = Gas Mark 9 (blazing). Memorise those four pairs and you can convert most recipes without a table.
Gas Mark, the British peculiarity.
Gas Mark numbers run from ¼ to 9. Each step is 25°F (14°C). Mark 1 = 275°F / 135°C; Mark 9 = 475°F / 240°C. The scale predates digital thermostats — British gas ovens were calibrated by valve detents, and the number is the detent position. Modern gas ovens often have both Gas Mark and °C markings; recipes that originated in older British cookbooks still cite Gas Marks exclusively.
Convection / fan-oven adjustment.
A fan-assisted oven circulates hot air, which transfers heat to food faster than still air. The rule: reduce the temperature by 20°C (35°F) or one Gas Mark, OR reduce the time by 25 %. A recipe specifying 200°C conventional becomes 180°C fan. Some modern ovens do this automatically if you set a "fan" mode. Most recipes don't specify which kind of oven they were tested in (or assume conventional); fan-oven users adjust by default.
A worked conversion.
A British recipe says "Gas Mark 6 for 45 minutes". US conversion: Gas Mark 6 = 400°F = 200°C conventional. Fan oven conversion: 180°C fan (or keep 200°C and cut time to 34 minutes). Stays the same regardless of country where you cook it; the recipe's intent is "moderate-hot oven for ~45 min", and the various number systems all point at the same heat.
Gas Mark 6 → all systems
Conventional vs fan
One temp in three notations plus a fan-oven adjustment.
Gas Mark 6 ≈ 400°F ≈ 200°C ; fan = 180°C
= Four valid settings, same dish
Your oven is probably lying.
Most home ovens are 10-30°F off the dial reading. The fluctuation during a single bake (the heating element cycles on and off) can swing 50°F either side of the set point. A £10 oven thermometer left inside the oven during a bake tells you what's really happening. Bakers who follow recipes religiously and still see inconsistent results are usually fighting an inaccurate oven, not a flawed technique. Calibrate before you blame the recipe.
Air fryers are convection ovens in disguise.
An air fryer is a small convection oven with a powerful fan. The temperature numbers convert directly from oven recipes — but the small chamber and aggressive air movement mean that cook times shrink dramatically (often 25-40 % less than the oven version of the same dish). Watch the first time you cook a recipe; recipes specifically written for air fryers will have already done the conversion.