The formula
Fahrenheit to Celsius: (°F − 32) × 5 / 9. Celsius to Fahrenheit: °C × 9 / 5 + 32.
Conventional vs fan (convection)
A fan-forced oven moves hot air around, so it heats food about 20 °C / 35 °F more efficiently than a conventional oven. A common rule is to drop fan temperatures by about 20 °C: a recipe calling for 200 °C conventional becomes 180 °C fan, or roughly 350 °F.
The "moderate oven"
Older British recipes describe oven heat in words: a "very cool oven" is around 110–120 °C, a "moderate oven" is 175 °C / 350 °F (gas mark 4), and a "hot oven" is 220 °C / 425 °F (gas mark 7). The chart above lists the canonical mapping.
FAQ
Why are oven temperatures rounded? A home oven's thermostat is rarely accurate to better than ±10 °F. Rounding to nice numbers (175 °C, 200 °C, 230 °C) keeps recipes simple without sacrificing real precision.
What is gas mark 4? Gas mark 4 is roughly 350 °F / 175 °C — the most common "moderate" baking temperature.