Three scales, one oven
An oven has exactly one temperature. The three scales printed on dials around the world are three ways of labelling it, and converting between them is the first thing you do when you cook from a book written somewhere else.
Fahrenheit is the American domestic standard. Dials are marked in 25 °F steps, which is why nearly every American recipe calls for 325, 350, 375, 400, 425 or 450 °F and almost never for 360.
Celsius is the standard everywhere else, and dials are usually marked in 10 °C steps. Recipes cluster on 150, 160, 180, 200 and 220 °C.
UK gas marks are a numbered scale used on British and Irish gas ovens. Gas 1 is 275 °F and every subsequent mark adds 25 °F, so the numbers are a compressed shorthand for the Fahrenheit scale. Below gas 1 the scale gets two fractional stops fixed by convention: gas ½ at 250 °F and gas ¼ at 225 °F.
The awkwardness is that the round numbers in one scale are not round numbers in another. 180 °C is 356 °F, not 350 °F. Gas 4 is 350 °F, which is 176.7 °C, not 180 °C. The conventional equivalences you see on the back of a baking tin round to whichever number is on the dial, and for anything you bake in an oven that tolerance is harmless.
Why you subtract 32 before you scale
Fahrenheit and Celsius disagree about two things at once: where zero sits, and how big a degree is. Celsius puts zero at the freezing point of water; Fahrenheit puts freezing at 32. And a Celsius degree is 1.8 times as large as a Fahrenheit degree, because the 100 divisions between freezing and boiling on the Celsius scale cover the 180 divisions between 32 and 212 on the Fahrenheit scale.
So the conversion has to fix the offset first, then the size:
°C = (°F − 32) × 5/9
Subtracting 32 moves the Fahrenheit reading onto a scale that starts at water's freezing point. Multiplying by 5/9 shrinks Fahrenheit-sized degrees down to Celsius-sized ones. Reverse the two operations in reverse order and you get °F = °C × 9/5 + 32. Doing them in the wrong order is the single most common error: 180 × 5/9 = 100, which is not 356 and is not anything.
Gas marks are easier because they only ever needed to describe a gas thermostat's detents. Every whole mark is 25 °F apart, and mark 1 is 275 °F, so °F = 250 + 25 × G. Reading it backwards, G = (°F − 250) / 25. Gas 4 gives 250 + 100 = 350 °F. Gas 7 gives 250 + 175 = 425 °F. The fractional marks are the one exception: applying the formula to G = ½ would give 262.5 °F, but every British chart and every oven manufacturer treats gas ½ as 250 °F and gas ¼ as 225 °F, so this calculator uses those published values rather than the extrapolation.
The −40 curiosity falls straight out of the algebra: set °C = °F in either equation and you get a single solution at −40, the one reading the two scales agree on. It is a useful sanity check that you have entered the formula the right way round.
Worked example: a British recipe says gas 6
You have a British cookbook calling for gas mark 6 and an American oven marked in Fahrenheit.
- Gas mark to Fahrenheit. °F = 250 + 25 × 6 = 250 + 150 = 400 °F. That is already a marked detent on an American dial, so you are done for practical purposes.
- Fahrenheit to Celsius, for completeness. °C = (400 − 32) × 5/9 = 368 × 5/9 = 1,840 ÷ 9 = 204.4 °C.
- Round to a dial setting. The nearest 10 °C marking is 200 °C — which is exactly what a British conversion chart prints next to gas 6.
Now run it the other way. Your American recipe says 375 °F and you have a Celsius oven:
- Subtract the offset. 375 − 32 = 343.
- Scale the degrees. 343 × 5 = 1,715; 1,715 ÷ 9 = 190.6 °C.
- Round. Set the dial to 190 °C. The gas equivalent is G = (375 − 250) / 25 = 5, so gas 5.
And the case that trips people up: a European recipe says 180 °C. °F = 180 × 9/5 + 32 = 324 + 32 = 356 °F. Set your oven to 350 °F. The 6 °F you give up is far smaller than the swing of a domestic thermostat, and 350 is where the detent is.
Full oven temperature conversion chart
| Gas mark | °F | °C exact | °C dial (traditional) | Description |
|---|---|---|---|---|
| ¼ | 225 | 107.2 | 110 | Very slow — meringues, long braises |
| ½ | 250 | 121.1 | 120 | Very slow — drying, confit |
| 1 | 275 | 135.0 | 140 | Slow — rich fruit cakes |
| 2 | 300 | 148.9 | 150 | Slow — custards, casseroles |
| 3 | 325 | 162.8 | 170 | Moderately slow — loaf cakes |
| 4 | 350 | 176.7 | 180 | Moderate — most cakes, biscuits, roasts |
| 5 | 375 | 190.6 | 190 | Moderately hot — pastry, sponges |
| 6 | 400 | 204.4 | 200 | Hot — muffins, roast potatoes |
| 7 | 425 | 218.3 | 220 | Hot — scones, choux |
| 8 | 450 | 232.2 | 230 | Very hot — bread, pizza |
| 9 | 475 | 246.1 | 240 | Very hot — searing, flatbreads |
Gas 3 is the row worth remembering: 325 °F is 162.8 °C, yet the traditional chart prints 170 °C. That is a rounding convention, not an error in your arithmetic.
How much precision actually matters
Round to the nearest dial marking and stop worrying. A domestic oven thermostat is a bimetallic switch or a thermistor with hysteresis: it lets the cavity overshoot the set point, cuts the element, lets it drift back down, and switches on again. The air temperature inside rides well above and well below the set point on every cycle, and that swing is larger than the 6 °F between 350 °F and 356 °F — which is exactly why rounding to the nearest detent costs you nothing.
What does matter is the average temperature, and that is where ovens genuinely differ. An oven can sit well off its dial, and the offset is rarely constant across the range. If your cakes brown before they set or your roast takes half again as long as the recipe says, buy an oven thermometer, hang it in the middle of the cavity, and preheat for at least 20 minutes before you read it — the walls and racks take far longer to reach temperature than the air does.
Precision starts to matter at the two ends of the range. Below about 200 °F you are in the territory of custards, meringues and slow-drying, where a 25 °F error changes the outcome from set to curdled. Above about 450 °F you are relying on the oven's peak output, and a 25 °F shortfall shows up directly as pale, under-risen bread. In the broad middle — anything between gas 3 and gas 7 — round freely.
One caveat the arithmetic cannot capture: a temperature that is correct for a conventional oven is not correct for a fan oven. Moving air strips the insulating boundary layer off the food and transfers heat faster at the same air temperature, which is why fan settings are conventionally run cooler. Use the convection oven conversion calculator once you have the temperature in your own scale.
Mistakes that ruin the conversion
- Scaling before subtracting. °C = °F × 5/9 − 32 is wrong. The offset comes off first: (°F − 32) × 5/9.
- Using the 'double it and add 30' shortcut at oven temperatures. The mental trick (°F ≈ 2 × °C + 30) is fine for weather. At 180 °C it predicts 390 °F against a true 356 °F — a 34 °F error, more than a full gas mark.
- Treating a fan temperature as a conventional one. European recipes often quote both, written as "180 °C (160 °C fan)". Convert the one that matches your oven, not whichever appears first.
- Applying the 25 °F-per-mark rule below gas 1. It predicts 262.5 °F for gas ½; the published value is 250 °F.
- Converting a probe temperature with an oven chart. Doneness temperatures for meat are conversions of the same two scales, but the safe minimums are fixed numbers you should take from the USDA chart rather than round to a dial.
- Trusting the dial over a thermometer. A converted number is only as good as the thermostat carrying it out.
Key terms
- Gas mark
- The numbered scale on British and Irish gas ovens, running ¼, ½, then 1 to 9. Whole marks are 25 °F apart with gas 1 at 275 °F.
- Thermostat swing
- The band a cycling oven's air temperature moves through either side of the set point. It is normally far wider than any rounding you apply when converting.
- Fan or convection setting
- An oven mode with a fan circulating cavity air. It cooks faster at the same dial temperature, so recipes written for conventional ovens need adjusting.
- Moderate oven
- The old recipe shorthand for roughly gas 4 / 350 °F / 180 °C. "Slow" is around gas 2, "hot" around gas 7.
Where temperature conversion sits among the other recipe adjustments
Changing the number on the dial is the smallest of the adjustments a travelling recipe usually needs. Three others frequently arrive at the same time.
Yield. If you are also cooking for a different number of people, convert the temperature first and then run the quantities through the recipe scaling calculator. Temperature does not scale with batch size; bake time does, though not linearly.
Pan geometry. A recipe written for a 9-inch round tin behaves differently in a 20 cm square one, and the fix is a depth and area calculation rather than a temperature change — see the baking pan size conversion calculator.
Altitude. Above about 3,000 ft, lower air pressure changes how leavening gases expand and how fast water evaporates, and the standard remedy actually does include raising the oven temperature by 15–25 °F. Convert to your own scale first, then apply the elevation adjustment with the high altitude baking adjustment calculator.
For roasting, the number that decides doneness is the internal temperature of the food, not the oven. The USDA publishes safe minimum internal temperatures — 165 °F (74 °C) for poultry, 145 °F (63 °C) with a three-minute rest for whole cuts of beef, pork, veal and lamb — and those are conversions of the same two scales, so the arithmetic above applies unchanged. A timing tool such as the turkey cooking time calculator gives you the schedule; the probe gives you the verdict.
Bakers working from weight rather than volume rarely need to convert temperature at all, because professional formulas are usually written in Celsius. If you are converting a recipe's ingredients as well as its oven setting, the baker's percentage calculator handles the ratios that survive any change of scale.