Fahrenheit, Celsius and gas mark
American recipes give oven temperatures in Fahrenheit, most of the rest of the world uses Celsius, and older British cookbooks use gas marks, a numbered scale printed on gas-oven dials. The conversion between the first two is straightforward: C = (F - 32) x 5 / 9 and F = C x 9 / 5 + 32. So 350 F is 176.7 C, which every recipe in the world rounds to 180 C; 325 F is about 165 C, 375 F is 190 C, 400 F is 200 C, 425 F is 220 C and 450 F is 230 C. Ovens are not precise enough for decimals, so this tool shows the rounded value first and the exact one in parentheses.
Gas marks step up by roughly 25 F (about 14 C) per number. Gas mark 1 is 275 F / 140 C, mark 2 is 300 F / 150 C, mark 3 is 325 F / 165 C, mark 4 is 350 F / 180 C, mark 5 is 375 F / 190 C, mark 6 is 400 F / 200 C, mark 7 is 425 F / 220 C, mark 8 is 450 F / 230 C and mark 9 is 475 F / 240 C. Below mark 1 there are marks 1/2 (250 F / 120 C) and 1/4 (225 F / 110 C) for meringues and very slow cooking. Gas mark 4, the "moderate oven," is the workhorse setting for cakes, cookies and casseroles. When you convert from Fahrenheit or Celsius, the tool picks the nearest gas mark and highlights that row in the chart.
Convection and fan ovens
A convection oven adds a fan that moves hot air around the food, which transfers heat faster and more evenly than a conventional oven. The standard adjustment is to lower the temperature by 25 F (roughly 15 C) and expect the food to be done 10-25% sooner. A recipe written for 350 F conventional becomes 325 F on convection, and 400 F becomes 375 F. Many newer ovens do this automatically in a "convection bake" mode, so check your manual before subtracting twice. Delicate items that need to rise gently, such as souffles, angel food cake and custards, are often better on the conventional setting even if you have a fan.
In the UK and Europe, recipes list the fan-oven temperature alongside the conventional one, typically 20 C lower: 180 C conventional / 160 C fan / gas mark 4. Rounded, that matches the 25 F rule used in the US.
Common baking temperatures
As a rough guide: 200-250 F (95-120 C) dries meringues and slow-roasts tough cuts; 300-325 F (150-165 C) is for cheesecakes, pound cakes and anything baked in a water bath; 350 F (180 C) covers cookies, muffins, layer cakes, brownies and most casseroles; 400-425 F (200-220 C) browns roasted vegetables, pizza, biscuits and yeast breads; and 450-500 F (230-260 C) is for artisan bread, pizza on a stone and getting a hard sear on a roast before dropping the heat.
One caution: the number on the dial is not always the number in the oven. It is common for a home oven to run 15-25 F hot or cold, and the temperature swings as the element cycles. A five-dollar oven thermometer hung from the middle rack tells you the truth, and preheating for a full 15-20 minutes before the first tray goes in matters more than most people think, especially for cookies and anything leavened with baking powder.