Recipes written in different countries use different temperature systems — Fahrenheit in the US, Celsius across most of the world, and gas marks on many UK and older gas ovens. This calculator converts between all three and adds a fan/convection adjustment, so a recipe from any source can be followed accurately on any oven.
Why gas mark numbers don't match the F/C formula exactly
Gas Mark ratings were standardized by UK gas oven manufacturers decades ago as a practical, round-number scale rather than a strict mathematical conversion of Fahrenheit or Celsius. That's why Gas Mark 4 is published as 350°F and 180°C, even though the exact formula conversion of 350°F is 176.7°C, not 180°C. This calculator uses the exact formula for direct Fahrenheit-to-Celsius conversions, and the standard published reference chart for anything involving a gas mark, so both numbers stay consistent with what's printed on ovens and in cookbooks.
Fan (convection) vs. conventional ovens
A fan or convection oven uses a built-in fan to circulate hot air around the food, which transfers heat faster and more evenly than a conventional oven relying on natural convection alone. Because of this, most manufacturers and recipe authors recommend reducing the set temperature by about 20°C (roughly one gas mark, or about 25°F) when using the fan setting instead of a conventional one — otherwise food set to a fan-oven-adjusted recipe temperature can overcook or brown too quickly.
When to double-check the adjustment
The −20°C fan adjustment is a widely used rule of thumb, not a universal law — some oven manufacturers publish their own recommended offset (occasionally as little as 10°C or as much as 25°C), and some recipes are already written specifically for fan ovens. If your oven's manual gives a different fan offset, or a recipe explicitly states it was tested in a fan oven, follow that guidance instead of the general 20°C rule used here.