Oven Temperature Converter (°C, °F, Gas, Fan)

Convert between °C, °F, gas mark and a fan oven setting in any direction, with the exact gas-mark definition, the 20 °C fan convention in both directions, the descriptive names older recipes use, and an honest account of how far published Celsius tables sit from the exact conversion.

Last updated 24 Sep 2026

Oven temperature

°C ⇄ °F ⇄ gas mark ⇄ fan

What have you got?

Conventional oven (°C)

Temperature (°F)

Gas mark

Fan / convection setting (°C)

Fan offset (°C)How much lower you set a fan oven. Which? give about 20 °C, which is the convention recipes use. Set it to 0 to turn the adjustment off.

The oven seen from the front, not a circuit: the cavity, the shelf, the bottom element and the fan that makes a fan oven a fan oven. The four figures on the right are the same setting written four ways, computed from what you entered — the conventional dial in °C and °F, the nearest gas mark, and the number to set on a fan oven. The drawing is illustrative; no oven holds one temperature everywhere inside it, which is why the page keeps saying to buy a thermometer.

180°CExample

A recipe that says 180 °C in a conventional oven

The three relations, and which of them is exact

°F = 250 + 25k (exact, k = −1, 0, 1 … 10) · °C = (°F − 32) × 5/9 · fan °C = conventional °C − 20

k: the gas-mark index: −1 for the mark called a quarter, 0 for the half, then 1 to 10. Each step is 25 °F
°F = 250 + 25k: exact, and the definition of the scale. Gas mark 1 is 275 °F
°C: the exact conversion of that Fahrenheit figure. Published tables round it to a multiple of ten
− 20 °C: the fan convention (Which?). Adjustable above, because ovens differ and some are already calibrated for it

Worked example

A recipe that says 180 °C in a conventional oven
180 °C × 9/5 + 32 = 356 °F
Gas marks are 25 °F apart from 275 °F: (356 − 250) / 25 = 4.24, so the nearest mark is gas mark 4
Gas mark 4 is exactly 350 °F = 176.7 °C, so 180 °C is 3.3 °C above the mark — the tables' 180 °C is a rounding
A fan oven wants 20 °C less: 160 °C fan, which is 320 °F
That is the “180 °C / 160 °C fan / gas 4” line printed in British recipes, derived rather than looked up

The gas mark scale

The Fahrenheit column is exact and it is a straight line: °F = 250 + 25k, where the index k runs −1, 0, 1, 2 … 10 for marks ¼, ½, 1, 2 … 10. That is the only part of this table with a formula behind it, and it is what this page implements. The Celsius columns are a convention: every published figure is its own Fahrenheit value converted and then rounded to a multiple of ten, and two of them (marks 3 and 9) are more than 5 °C from the exact conversion. Two sources used here disagree outright at mark 3 — Which? print 160 °C, the usual table 170 °C, and the exact conversion of 325 °F is 162.8 °C. Note also that ¼ and ½ are names, not fractions: the three lowest marks are 25 °F apart like all the others, which is why the index and not the mark is what the arithmetic uses.
Gas mark	°F (exact)	°C (exact)	°C as usually printed	°C per Which?	Older recipes call it
¼	225	107.2	110	—	Very cool
½	250	121.1	120	—	Very cool
1	275	135.0	140	140	Cool
2	300	148.9	150	150	Cool
3	325	162.8	170	160	Moderate
4	350	176.7	180	180	Moderate
5	375	190.6	190	190	Moderately hot
6	400	204.4	200	200	Moderately hot
7	425	218.3	220	220	Hot
8	450	232.2	230	230	Hot
9	475	246.1	240	240	Very hot
10	500	260.0	260	—	Extremely hot

Fan oven, both directions

Which?’s own table, which is where the 20 °C convention on this page comes from: fan temperatures are around 20 °C lower than conventional ones for the same result. Read it the other way for the other direction — a fan oven set to 180 °C is doing the work of a conventional oven at 200 °C, gas mark 6. The offset is a convention and not a physical constant: it stands for the fact that moving air carries heat to the food faster than still air, and how much faster depends on the oven, the shelf, the tin and how full the oven is. The input above lets you change it or switch it off.
The recipe says	Conventional oven	Fan oven	Gas mark
140 °C	140 °C	120 °C	1
150 °C	150 °C	130 °C	2
160 °C	160 °C	140 °C	3
180 °C	180 °C	160 °C	4
190 °C	190 °C	170 °C	5
200 °C	200 °C	180 °C	6
220 °C	220 °C	200 °C	7
230 °C	230 °C	210 °C	8
240 °C	240 °C	220 °C	9

Gas marks, fan ovens and the parts Google cannot answer

The arithmetic is not the point. °C to °F is one multiplication and an addition, and a search engine will do it above every result on the page. Two things about oven temperature are not arithmetic at all, and they are what this page is for: the gas mark, which is a British lookup with no metric formula behind it, and the fan offset, which is a convention about how ovens behave rather than a conversion between units.

Gas marks. The scale comes from the Regulo thermostat that Radiation Ltd fitted to gas cookers in the 1920s, and it is defined in Fahrenheit: gas mark 1 is 275 °F and each mark up is 25 °F more, so gas mark 4 is 350 °F and gas mark 9 is 475 °F. Below 1 the marks are named a half and a quarter but they are not halves of anything — they are 250 °F and 225 °F, the same 25 °F apart as the rest of the scale. That is why this page works in an index k rather than in the mark number: k is −1 for a quarter, 0 for a half and then 1 to 10, and °F = 250 + 25k holds for all of them. Convert that line to Celsius and you get the figures every chart prints — after rounding. 350 °F really is 176.7 °C, not 180 °C, and 325 °F is 162.8 °C, which is why one chart says 160 and the next says 170.

The fan. A fan oven at 160 °C cooks roughly like a conventional oven at 180 °C, so a recipe that wants a conventional 180 °C prints “180 °C / 160 °C fan”. The reason is not that the fan oven is lying about its temperature: it is that moving air carries heat into food far faster than still air, so the food heats and browns faster at the same air temperature. Which? put the usual difference at about 20 °C and that is the figure this page defaults to, but it is a convention. The real offset depends on the oven, how full it is, the shelf and the tin, and the input above lets you change it. Some manufacturers quote 15 °C, some 25 °C, and a few fan ovens are calibrated so that the dial already reads the conventional-equivalent number, in which case you subtract nothing — set the offset to 0.

The descriptive names. A recipe written before dials were trusted says “a moderate oven” or “a hot oven”, and those map onto the scale: cool is gas mark 1 to 2, moderate 3 to 4, moderately hot 5 to 6, hot 7 to 8 and very hot 9. They are deliberately vague, and the vagueness was appropriate.

Only a thermometer settles it. Everything above is a conversion between numbers written on dials. What matters is the air temperature around the food, and domestic ovens are poor at delivering the number on the dial: Which?’s testing found ovens 40 °C out. Put a thermometer on the middle shelf, find out what your oven actually does at the setting you use most, and adjust the dial rather than the recipe. For the other half of a foreign recipe, the cups to grams converter turns its cups into grams, the US, UK and metric measures converter sorts out its pints, and the recipe scaling calculator deals with a different tin and what that does to the time.

Frequently asked questions

What is gas mark 4 in °C?

180 °C by convention, 176.7 °C exactly. Gas mark 4 is defined as 350 °F, and 350 °F converts to 176.7 °C; every published table rounds that to 180 °C. On a fan oven, set 160 °C.

Why does a recipe say 180 °C / 160 °C fan?

Because a fan moves hot air over the food, and moving air carries heat into it faster than still air does. The same browning and the same cooking rate are reached at a lower air temperature, and the convention is about 20 °C lower. The 160 °C is not a different temperature for the food, it is the dial setting that produces the same result.

Is there a formula for gas marks?

For Fahrenheit, yes and it is exact: °F = 250 + 25k, with the index k being 1 to 10 for marks 1 to 10 and 0 and −1 for the marks called a half and a quarter. Gas mark 1 is 275 °F and every mark is 25 °F above the last. For Celsius there is no exact formula, because the Celsius figures in print are that line rounded to the nearest ten — sometimes to the nearest ten the wrong way.

Why do two conversion charts give different °C for the same gas mark?

Because both are rounding, and rounding a number like 162.8 to a multiple of ten can honestly go either way. Gas mark 3 is 325 °F, which is 162.8 °C; Which? print 160 °C and most other charts print 170 °C. Neither is wrong and the 10 °C between them matters less than the accuracy of your oven does.

How accurate is my oven?

Probably not very. Which? report testing ovens that reached 220 °C when set to 180 °C, and others that managed only 150 °C in places, with the worst models out by 40 °C. A cheap oven thermometer left on the middle shelf is the only way to know, and it will tell you more than any conversion table can.

What does a moderate oven mean in an old recipe?

Around gas mark 3 to 4, 160 to 180 °C. The old descriptive names are in the table above: cool is gas mark 1 to 2, moderate 3 to 4, moderately hot 5 to 6, hot 7 to 8 and very hot 9. They were how recipes were written before dials were reliable, and they are wider than a number — which is honest, because the ovens were too.

Related calculators

References

Which?. Oven temperature conversion table: gas mark against °C, fan °C and °F, the descriptive names, the statement that fan temperatures are around 20 °C lower than conventional ones, and the oven-accuracy testing quoted here (models reaching 220 °C when set to 180 °C, and up to 40 °C out).
Gas mark (encyclopaedia article), and the sources it cites: the Oxford English Dictionary for the term’s first use in 1958, an advertisement in the Sunday Pictorial of 13 May 1923 for the Regulo-fitted New World cooker, BBC Good Food’s conversion guides and The Guardian’s cooking conversion charts of 24 November 2007. Used for the full ¼-to-10 table and the “×14 + 121” Celsius rule, both of which this page checks against the exact Fahrenheit line rather than repeating.
National Institute of Standards and Technology. NIST Handbook 44 (2026 edition), Appendix C, General Tables of Units of Measurement. The US liquid gallon is 231 cubic inches exactly and the British imperial gallon is 4.546 09 L exactly; every volume on this page is derived from those two definitions and the exact inch.
The Fahrenheit-to-Celsius relation itself is the definition of the two scales: °C = (°F − 32) × 5/9, with the degree Celsius fixed by the kelvin in the SI.