The formula
°F = °C × 9/5 + 32 · gas mark ≈ (°C − 121) ÷ 14 + 1
Where it comes from
The gas scale is a numbering from British cookers with no direct relationship to degrees: mark 1 is about 140 °C and each mark adds around 14 °C. That is why older recipes talk about “gas mark 6” and it has to be translated.
How to work it out by hand
- Work out which scale the recipe uses
- Apply the matching conversion
- If yours is a fan oven, subtract around 20 °C from the result
What is worth knowing
The difference between conventional and fan heat matters more than it looks: the fan spreads the heat and cooks faster, so the rule is to drop around 20 °C from whatever the recipe says. And there is a detail that ruins a lot of cakes: a domestic oven thermostat drifts easily by 10 to 25 degrees. An oven thermometer that hangs from the shelf costs very little and reveals that error.