180°C Is Gas Mark 4 — and the Chart That Says So Is Not a Conversion

Gas mark 4. That is the answer the standard British chart gives, and it is the one to use.

It is also, strictly, not a conversion. Gas mark 4 is defined as 350 °F, and 180 °C is 356 °F — six Fahrenheit degrees apart. The chart pairs them anyway, because the Celsius column beside the gas marks was never a translation of them. It is a list of tidy dial settings that somebody chose.

The standard chart

Gas mark ¼

110 °C — 225 °F

Gas mark ½

120 °C — 250 °F

Gas mark 1

140 °C — 275 °F

Gas mark 2

150 °C — 300 °F

Gas mark 3

170 °C — 325 °F

Gas mark 4

180 °C — 350 °F

Gas mark 5

190 °C — 375 °F

Gas mark 6

200 °C — 400 °F

Gas mark 7

220 °C — 425 °F

Gas mark 8

230 °C — 450 °F

Gas mark 9

240 °C — 475 °F

The Fahrenheit column is the scale itself. The Celsius column is the conventional dial setting printed alongside it — and it is a convention rather than a standard, so a manufacturer handbook or a particular cookbook may print a degree or two differently.

Look at the Fahrenheit column on its own and the pattern is immediate: 275, 300, 325, 350, and on in steps of exactly 25 all the way to 475. Every gas mark is 25 Fahrenheit degrees above the one below it.

That is why the scale runs to nine and stops. Nine marks at 25 degrees each spans 275 to 475 °F, which is the working range of a domestic gas oven, and the numbers are round in Fahrenheit because Fahrenheit is the scale the marks were laid out on.

So where does the Celsius column come from?

If the Celsius numbers were simply the Fahrenheit ones converted, they would follow. Convert each mark exactly and compare.

Gas mark 1 — 275 °F

135.0 °C exact · chart says 140

Gas mark 2 — 300 °F

148.9 °C exact · chart says 150

Gas mark 3 — 325 °F

162.8 °C exact · chart says 170

Gas mark 4 — 350 °F

176.7 °C exact · chart says 180

Gas mark 5 — 375 °F

190.6 °C exact · chart says 190

Gas mark 6 — 400 °F

204.4 °C exact · chart says 200

Gas mark 7 — 425 °F

218.3 °C exact · chart says 220

Gas mark 8 — 450 °F

232.2 °C exact · chart says 230

Gas mark 9 — 475 °F

246.1 °C exact · chart says 240

The chart runs up to 7.2 °C above the arithmetic at gas mark 3 and 6.1 °C below it at gas mark 9. The gap changes sign three times across the scale.

A rounding error does not behave like that. Round nine numbers to the nearest five and every gap is under 2.5 degrees. These gaps reach 7.2, and they go both ways, which means the Celsius column was not produced by rounding the Fahrenheit one.

Round to the nearest ten and it very nearly works. Seven of the nine marks fall out correctly. Two do not.

Gas mark 3 — 162.8 °C

nearest ten is 160, chart prints 170

Gas mark 9 — 246.1 °C

nearest ten is 250, chart prints 240

The other seven marks

nearest ten matches exactly

The chart is nearest-ten rounding

at seven marks out of nine

And the two departures are not random. Look at the Celsius column as a list — 140, 150, 170, 180, 190, 200, 220, 230, 240 — and it skips 160 and 210, two entirely ordinary oven settings. The column was built to give nine tidy numbers for nine marks, not to convert anything.

None of which makes the chart wrong. It makes it a different kind of object: a table of conventional settings rather than a conversion. Use gas mark 4 for 180 °C with confidence, and know that the reason it is not exactly 180 is that it was never trying to be.

Where our own converter disagrees with your cookbook

This is worth saying plainly, because you can catch us at it. Put 350 °F into our converter and it returns 175 °C, while the British chart prints 180 for the same recipe.

Both are right, and the difference is the whole point of this page. 350 °F is 176.7 °C, and our converter rounds to the nearest five degrees because that is the resolution an oven dial offers, which gives 175. The gas mark chart rounds to a tidy dial number in a country that cooks in Celsius, which gives 180.

There is a second consequence of that rounding worth knowing, because it looks like a bug and is not. Convert 350 °F to Celsius and back and you land on 345, not 350.

325 °F

→ 165 °C → 330 °F

350 °F

→ 175 °C → 345 °F

375 °F

→ 190 °C → 375 °F — returns

400 °F

→ 205 °C → 400 °F — returns

425 °F

→ 220 °C → 430 °F

450 °F

→ 230 °C → 445 °F

Four of the six common settings shift by 5 °F on the return trip. A five-degree step in Celsius is nine degrees in Fahrenheit, so the Celsius grid is the coarser of the two and a round trip through it can lose up to 4.5 °F.

Any converter that rounds to what a dial can be set to has this property; the alternative is printing 176.7 °C, which no oven can be set to and no recipe would ask for. Convert once, in the direction you need, and do not convert back.

Fan ovens: two conventions, and they are not the same cut

Almost every domestic oven sold in Britain now has a fan, and every fan oven needs the temperature dropped. The rule you have heard depends on which side of the Atlantic you heard it from.

These are usually quoted as if they were the same instruction in two currencies. They are not. Twenty-five Fahrenheit degrees is thirteen and nine tenths Celsius degrees, so the British rule takes off substantially more heat.

British rule

180 − 20 = 160 °C

The same oven in Fahrenheit

356 °F

American rule

356 − 25 = 331 °F

Which is

166 °C

The two rules land

6 °C apart

Six degrees is small enough that either will bake the cake, and our converter deliberately applies each rule on its own scale rather than converting one into the other — so the Fahrenheit output follows the American convention and the Celsius output follows the British one.

What a bare "180 °C" means in a British recipe

Here is the ambiguity the charts never address, and it matters more than any of the arithmetic above.

Modern British recipes usually print both numbers — "180 °C (160 °C fan)" — and in that format the first number is the conventional setting and the parenthetical is what to dial on a fan oven. That much is unambiguous.

The problem is the recipe that gives one number and no label. If it says 180 °C and your oven is a fan oven, you have to decide whether the author meant a conventional 180 that you should reduce to 160, or a fan 180 that you should set as written. Get it wrong in one direction and the cake is underdone; get it wrong in the other and the edges are dark before the middle sets.

The number on the dial is not the number in the oven

Everything above argues over differences of five and seven degrees, so it is only fair to say how large the error already sitting in your kitchen is. Which? tests more than fifty built-in ovens a year and records how closely each holds the temperature it claims.

“Best Buys stay very close to the set temperatures, but the least accurate ovens can overheat by more than 30°C. This is enough to result in burnt food.”

Thirty degrees is four gas marks. It dwarfs every discrepancy on this page — the 7 °C the chart drifts at gas mark 3, the 6 °C between the two fan conventions, the 5 °C between our converter and your cookbook. All of that is noise beside an oven that simply runs hot.

Which is the practical conclusion, and it is not a conversion at all: if temperature genuinely matters to your baking, an oven thermometer tells you more than any chart can. It is the only way to find out which of these numbers your particular oven is actually delivering.

Preheating varies about as widely. Which? found the best ovens reach 180 °C in under five minutes and the poorest take nearly three times as long, so the indicator light going out is a statement by the appliance's own thermostat rather than independent proof the cavity has arrived.

The marks below 1, and whether there is a 10

Gas mark ¼ and gas mark ½ exist and they do not follow the 25-degree step. The scale below mark 1 was extended downwards for slow cooking rather than continued arithmetically: ¼ is 110 °C and ½ is 120 °C, against the 256 and 263 °F the formula would give.

That is a second sign that the low end of the chart is convention rather than calculation. Those two marks are where meringues dry out and a very slow braise sits, and 110 and 120 are the numbers a Celsius dial can actually be set to.

At the other end, there is no gas mark 10 on a standard domestic oven. Nine, at 475 °F or 240 °C, is the top of the scale, and a recipe asking for more than that is asking for something a domestic gas oven was not built to do.

The short version

Gas mark 4 is 180 °C, and it is exactly 350 °F. The Fahrenheit column is the scale — 250 plus 25 per mark — and the Celsius column is a set of round dial numbers that agrees with the arithmetic at seven marks out of nine and quietly skips 160 and 210. Because those numbers are a convention rather than a standard, the chart in a manufacturer handbook may not match the one in a cookbook to the degree.

So if a recipe gives you a gas mark, use the Celsius the chart gives you. If it gives you Fahrenheit, convert it. Do not expect the two routes to land on the same number, because they were never the same journey.