Cooking and food

The cooking tools take one kitchen number that changes from oven to oven, or from country to country, and convert it with the source named. The first is the oven setting converter: enter the setting a recipe gives and it returns the same heat on every other scale.

• It reads conventional and fan temperatures in degrees Celsius, gas marks from ¼ to 9 and Fahrenheit from American recipes.
• The fan offset is an input, set to 20°C by default because that is the figure Which?, Hoover and BBC Good Food use.
• A full table under the result lists every gas mark with its conventional, fan and Fahrenheit equivalents.

The air fryer converter takes an oven recipe to an air fryer, or an air fryer recipe back to the oven. It reads conventional and fan °C, gas marks and Fahrenheit, then applies the rule Instant Brands and COSORI UK publish: 20°C lower and 20% to 25% shorter for fresh food.

Both figures can be changed, and so can the air fryer's highest setting, 200°C by default, because a recipe at gas mark 9 asks for more than that. The result also gives the halfway point, when the food is shaken or turned.

British recipes write the heat three ways because British kitchens have three kinds of oven. A cake baked at 180°C in a conventional electric oven needs about 160°C in a fan oven and gas mark 4 in a gas oven. BBC Good Food prints all three in one line.

A fan oven runs cooler for the same result because the fan keeps hot air moving over the food, so it gets the lower figure. The gas mark is older than either: a numbered dial for gas cookers, stepped by 25°F a mark, so gas mark 7 sits at 425°F.

American recipes count in Fahrenheit and assume a conventional oven, which is why a converted American bake still needs the fan reduction in a British fan oven.

A converted figure is only as good as the dial it is set on. Thermostats let an oven swing around the set temperature, the top of a gas oven runs hotter than the bottom, and makers print slightly different gas mark tables.

An oven thermometer on the middle shelf shows the real heat, and the manual shows the maker's own tested settings for each dish. Where a recipe gives a single temperature without saying which oven, Which? reads it as a fan setting, since most new ovens have a fan.

Cooking times change less than the temperatures. Hoover's advice for fan ovens allows long roasts to finish a little sooner, so the safest habit is to check the food before the time a recipe gives.