How to Use This Tool
Gas marks, Celsius and Fahrenheit are easy to convert. The part that actually ruins food is the fourth variable that most charts omit entirely: whether your oven has a fan. If it does, and you set it to the number printed in the recipe, you are cooking roughly 20 degrees too hot.
Why a fan oven needs a lower number
Food in a still oven sits inside a thin layer of air it has already warmed, and that layer insulates it. A fan strips that layer away continuously, so heat reaches the surface far faster at the same air temperature. The standard correction is to subtract 20 °C, or about 25 °F, or drop one gas mark. Manufacturers state this in the manual nobody reads.
The symptom is unmistakable once you know it: edges browning while the centre is still raw, biscuits that spread and burn at the rim, cakes with a domed cracked top and a wet middle, roast vegetables leathery outside and hard inside. If that is your oven's personality, it is probably not your oven — it is the missing 20 degrees.
Using the converter
Choose your oven type at the top first, because everything below depends on it. Enter the temperature the recipe gives and pick its unit. The panel shows what to actually set, in all three systems. When the fan adjustment is being applied, the note underneath says so explicitly, along with the number the recipe printed, so you can see what changed and why.
If you cannot change the temperature
Sometimes something else is already in the oven at a fixed heat. The time box estimates the compensation: roughly speaking, running 20 °C hot cuts the required time by about 15%, and running 20 °C cool adds a similar amount. This is a rough guide and works far better for roasting and traybakes than for baking, where structure sets at particular temperatures and a cake baked hot and fast is not the same cake.
Two honest caveats
The chart's two temperature columns do not convert into each other exactly. Gas 4 is quoted as 180 °C and as 350 °F, but 180 °C is really 356 °F. Both scales were rounded to convenient numbers independently, decades apart, and the six-degree gap is far smaller than any domestic oven can actually hold. The table shows the numbers printed in recipes rather than the arithmetic conversions, because those are the numbers on the dial.
Ovens lie. An oven dial can be out by 20 °C in either direction, and drift as the element ages. A cheap oven thermometer will tell you more about your baking than any conversion chart. Second, gas marks were never precisely defined in Celsius; the values here are the widely used British convention, and you will find charts that differ by five degrees at the extremes. Nobody is wrong, the scale simply was not built that way.
