Four scales are in common use for oven temperature and recipes move between them without warning. The conversions are simple; what is less obvious is the fan oven adjustment, which many recipes assume you already know.
The conversions
Fahrenheit to Celsius is the familiar subtract 32, multiply by five ninths. Going the other way, multiply by nine fifths and add 32.
Gas marks are cleaner than they look: mark 1 is 275°F and every mark up is another 25°F. So mark 4 is 350, mark 6 is 400, mark 9 is 475. Below mark 1 there are quarter and half marks for slow cooking.
The formula is therefore Fahrenheit equals 250 plus 25 times the gas mark, which is worth knowing because it makes any British recipe readable instantly.
Why fan ovens need turning down
A conventional oven heats food mainly by radiation from the elements and by still hot air. A fan oven circulates that air, which strips away the insulating boundary layer at the food's surface and transfers heat much faster.
Common recipe shortcuts say to reduce a Celsius recipe by about 20°C or a Fahrenheit recipe by about 25°F. Those two drops are not mathematically equal, so this calculator applies the convention for the scale you entered and then converts the result. A recipe calling for 180°C conventional becomes 160°C fan.
Some manufacturers say 15°C, some say 25. The variation is real and depends on the fan, which is another reason to check with a thermometer.
What fan ovens are and are not better at
Fan is better for anything roasted, for multiple trays at once, and for even browning. It is better for pastry that needs a fast set.
Fan is worse for anything delicate that can be dried out or skinned over by moving air — custards, some cakes, meringues, and anything that needs to rise slowly before setting.
Bread is a mixed case: fan helps the crust but can dry it, so many bakers use conventional with steam for the first part and fan for the last.
Home ovens are not accurate
An oven thermostat measures air temperature at one point and cycles the element on and off around a setpoint. The swing is commonly 25°F either side, and the calibration itself is often out by as much.
So the difference between 350 and 375 in a recipe is smaller than the error in most ovens. This is why two people following the same recipe get different results.
An oven thermometer costs very little and stays in the oven. Once you know your oven runs 20 degrees cool, every recipe works better.
Position matters as much as temperature
Heat is not uniform in an oven. The top is hotter in most, the back is hotter than the front, and the area right above the base element is hotter still.
Middle rack is the default for a reason. Moving a tray one rack up or down changes the result more than a 25 degree setting change.
Rotating a tray halfway through is worth doing in almost every oven, and is the standard fix for bakes that brown unevenly front to back.