Oven temperature conversion:
from Celsius to Fahrenheit without guessing

The conversion formulas

A British recipe says gas mark 4, an American one says 350°F and your oven dial reads 180°C. They are all the same moderate heat, written in three scales. Converting precisely is easy; knowing how much precision a recipe expects takes a little judgement.

Baking recipes cluster around a handful of heats. Understanding the formula once lets you convert any odd figure, such as 165°C or 425°F, without a lookup.

A mental shortcut that matches the formula exactly is to double the Celsius figure, take off a tenth of that, and add 32. For 180°C: 360 − 36 + 32 = 356. It works because ×1.8 is the same as ×2 minus 10%.

Worked example: a 180°C oven

Going back, a 425°F recipe converts to (425 − 32) × 5/9 = 218.3°C, which most Celsius dials treat as 220°C. A 350°F recipe is 176.7°C, usually set at 175°C or 180°C.

A chart of common oven temperatures

Gas mark values are conventional steps rather than a formula, and they are usually quoted as above, so treat them as approximate; check your own appliance's chart if it has one.

Fan ovens and unreliable dials

Conversion fixes the number, but the oven still has to deliver it. A fan or convection oven moves air around the food and cooks faster, so a common rule is to reduce the conventional temperature by about 20°C (or 25°F). A recipe written for 180°C conventional would then be 160°C fan, which is 320°F.

• Many home ovens run 10 to 20 degrees off the dial; an oven thermometer on the middle shelf shows the truth.
• Preheating until the indicator light cycles off matters more than an extra degree of precision.
• Dark tins absorb more heat than shiny ones and can brown the base faster.
• Higher altitude and very small ovens change timing, so watch the food rather than the clock alone.

Why 350°F and 180°C are the same recipe

Fahrenheit and Celsius divide the interval between water's freezing and boiling points differently: 180 degrees on the Fahrenheit scale between 32 and 212, and 100 on the Celsius scale. That is the origin of the 9/5 factor, because each Celsius degree is 1.8 Fahrenheit degrees, and of the 32 offset, because the scales start in different places.

Recipe writers pick round numbers on their own scale. Oven dials in Fahrenheit climb in 25-degree steps, with 325, 350, 375, 400 and 425 the usual stops, while Celsius dials climb in 10 or 20-degree steps. When one is translated to the other, the sensible choice is the nearest dial position, not the mathematically exact figure. That is the reason 350°F becomes 180°C in print even though 176.7°C is the true equivalent.

Checking your own oven

An oven thermostat is a crude instrument. It switches the element on at one temperature and off at another, so the air temperature cycles above and below the setpoint by 10 to 25 degrees in a typical domestic model. A reading taken at one moment does not capture that swing.

1. 1Place an oven thermometer on the middle shelf and preheat to 180°C for at least 20 minutes.
2. 2Read it every five minutes for half an hour and note the high and low points.
3. 3Average the highest and lowest readings to see how far the dial is from the real mean temperature.
4. 4Adjust your setting by that amount whenever you convert a recipe.

If the oven consistently runs 15°C cool, a recipe converted to 180°C needs a dial setting of 195°C. Writing the correction on a sticky note inside a cupboard door saves guessing next time.

What the temperature words mean

Older recipes describe heat in words rather than numbers. Slow or cool is roughly 120 to 150°C (250 to 300°F), moderate is about 180°C (350°F), hot is around 200°C (400°F) and very hot is 230°C (450°F) and above. The exact numbers drift between cookbooks, so treat the words as a range.

The temperature also relates to what is happening in the food. Sugar begins to caramelise at about 160°C, and the browning reactions between sugars and proteins pick up speed above roughly 140°C, which is why a roast needs a hot oven to colour. Bread is often started hot at 220°C for oven spring and then lowered. Custards and cheesecakes want gentle heat below 160°C so the egg proteins set without curdling.

Knowing why a recipe asks for a given heat helps you adjust it sensibly when your oven or your conversion is a few degrees out.

When rounding is fine and when it is not

Roasting and most cakes tolerate a difference of 5°C. Sugar work, meringues, delicate custards and bread with oven spring care more, so keep the exact figure and use a thermometer. For a recipe given in °F, rounding to the nearest 5°C is normally sensible; for 375°F that gives 190°C (true value 190.6).