Oven temperature conversion

Two of the four numbers on this page are physics and two are furniture. Celsius and Fahrenheit are locked to each other by definition — 180 °C is 356 °F and always will be — so that conversion is exact and nobody publishes a table of it. Gas marks and fan offsets are the opposite: they are settings on appliances, agreed by custom, and the published charts genuinely differ. Which?, the UK consumer association, puts gas mark 4 at 180 °C and 350 °F. Smeg, which builds ovens, puts gas mark 4 at 180 °C and 356 °F. Both are right about the gas mark and only one of them can be right about the Fahrenheit, because 350 °F is 176.7 °C. Which? is quoting the round American oven numbers, Smeg is converting its own Celsius column. So this tool asks whose chart you want, prints the exact conversion beside what that chart actually says, and does not average the two.

How it is calculated

°F = °C × 1.8 + 32, and everything else is a lookup

The Celsius to Fahrenheit line is the whole of the arithmetic, and NIST states it as t/°C = (t/°F − 32)/1.8. Gas mark and fan setting are not computed at all — they are read off the row of the chart you chose, and when your temperature is not one of that chart's rows the tool says so instead of rounding you onto the nearest one. The convection line is a third convention again: GE Appliances tells American cooks to "reduce the temperature by 25 degrees Fahrenheit when convection baking", which is 13.9 °C — not the 20 °C the British charts take off for a fan.

Source: NIST SP 811 (2008), Appendix B.8 — degree Fahrenheit (temperature) (°F) to degree Celsius (°C): t/°C = (t/°F − 32)/1.8

Questions people ask

What is gas mark 4?

180 °C on both charts here, and that much is not controversial. The Fahrenheit is: Which? prints 350 °F on the same row, Smeg prints 356 °F. The gap exists because 350, 375 and 400 °F are the numbers on an American oven dial and 180, 190 and 200 °C are the numbers on a European one, and the two ladders were never the same ladder. Nobody converted anything wrongly; they simply anchored the row to different columns. Six degrees Fahrenheit will not spoil a cake, which is why the discrepancy has survived this long unremarked.

Why does the chart matter for the fan setting?

It does not, much, and that is worth saying plainly: both charts here take exactly 20 °C off for a fan oven, on every row. Which? explains it as a rule rather than a table — "fan oven cooking temperatures are typically around 20°C less than conventional ovens" — and Smeg's column is the same subtraction carried out. Where the world does disagree is across the Atlantic. GE Appliances tells you to drop 25 °F for convection, and 25 °F is 13.9 °C, so an American convection instruction is a gentler correction than a British fan one by about six degrees Celsius. Both figures are on this page, each with its own name attached.

Why will it not give me a gas mark for 175 °C?

Because there is not one. A gas mark is a detent on a dial, not a reading, and neither chart has a row between 170 and 180 °C. Snapping 175 °C to gas mark 4 would be inventing a step that the appliance does not have, and snapping it down to 3 would be a different invention. What you actually do at the oven is pick the nearer mark and watch the bake, and that is a decision, not a conversion.

Which chart runs further?

Smeg's. It starts at 120 °C with gas mark 1/4 and includes half steps — 1/2 at 130 °C, 3.5 at 170 °C, 6.5 at 210 °C — where Which? runs from gas mark 1 at 140 °C to gas mark 9 at 240 °C in whole marks only. If your recipe calls for gas mark 1/2 or your oven dial has half positions, Smeg is the chart that covers you. Above 240 °C neither publishes anything at all.

Is my oven actually at the temperature I set?

Very possibly not, and it is the largest error on this page by a wide margin. Which? reports testing ovens that "soar to 220°C when set at 180°C", and others that struggle to get there — one measured at 150 °C in part of the cavity when set to 180. Against that, the six degrees Fahrenheit two charts disagree by is noise. An oven thermometer costs very little and settles the question for your own oven, which is the only one that matters.

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