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
Fan ovens circulate hot air, changing heat transfer to food. This calculator uses a −20°C adjustment, exactly −36°F, rather than −20°F or −25°F. A manufacturer's −25°F recommendation is a different rule of thumb. Follow the appliance manual and recipe guidance; gas-mark steps are rounded chart values, not a fixed equivalent of the fan adjustment.
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.