The gas mark is a formula, and it is exact
Degrees Fahrenheit equals 250 plus 25 times the mark. Mark 1 is 275, mark 4 is 350, mark 9 is 475, and every one of them lands on a round 25 degrees.
That regularity is the whole reason the scale exists. Early domestic gas ovens had no thermometer and no meaningful temperature marking, so the tap was numbered in even steps and the numbers were spaced rather than measured. Once thermostats arrived the marks were pinned to Fahrenheit values, and the spacing they had already been given made 25 degrees the obvious interval.
And then it stops working at the bottom
Feed a quarter into the formula and you get 256°F. Every British cookbook prints 225. Mark a half comes out at 262 and is printed as 250. The two low marks were added to the scale after the fact, for slow cooking and for meringues, and the values that stuck were round numbers somebody chose rather than the ones the arithmetic produced.
So the table on this page follows the printed convention below mark 1 and the formula above it, and puts an asterisk on the two rows where those differ. Presenting the whole scale as one clean rule would be tidier and would be wrong at exactly the settings people reach for when they are doing something slow and expensive.
Why this page will not tell you 200C is 392F
It is, arithmetically. It is also a number that appears on no dial, in no cookbook and in no kitchen. The pairing everybody uses is 200°C, 400°F, gas mark 6, and an American cooking a British recipe sets the oven to 400.
Those printed pairings are conventions settled over a century of two scales sharing a kitchen, and they are rounded in both directions: 350°F is really 176.7°C and prints as 180. A converter that hands back 392 and 176.7 has answered a maths question instead of a cooking one. This tool uses the pairing where the table has an opinion, falls back to arithmetic where it does not, and says so on the result when the two disagree.
There are two fan rules and they disagree by ten degrees
A fan pushes the hot air onto the food rather than letting it drift up, so heat arrives faster at the same dial setting and the temperature comes down to compensate. Everybody agrees on that. Nobody agrees on the number.
Britain publishes the reduction as 20°C. North America publishes it as25°F. Those look like the same instruction in different units and they are not: 20°C is 36°F, so the British rule takes nearly half as much off again.
Work it through from a conventional 180°C. The British rule gives 160°C. The American rule takes 25 off 350°F, giving 325°F, which is 170°C. Ten degrees apart, for the same oven doing the same job, and it holds at 190 and 200 as well.
Neither is measured and neither is wrong. They are two kitchens' rules of thumb that grew up separately, and a converter that quietly applies one of them everywhere hands somebody a number their own oven manual contradicts. This page computes both, labels which country publishes which, and leads with the one your market uses.
There is one temperature where they agree exactly, and it is an accident of the printed table rather than a meeting of minds. Gas mark 6 to gas mark 7 is 200°C to 220°C, a step of 20, while every Fahrenheit step on the scale is 25. So from 220°C both rules land on 200°C, and only from 220°C.
What matters more than either number is not mixing fan and conventional up in the first place. 180°C fan is a conventional 200°C, which is gas mark 6. Setting a fan oven to a conventional 180 leaves you 20 degrees hot for the whole bake, and on a cake that is the difference between risen and cracked.
Three countries, three words for the same fan
Britain says fan. Australia says fan-forced. America and Canada say convection. It is the same appliance and the same deduction, and the switch on this page changes the word as well as the scale, because a recipe that calls it something you have never seen written is a recipe you hesitate over.
The bigger difference is the gas mark, which is British and essentially nowhere else. Britain carries three notations at once because it went metric in the 1970s with a lot of gas cookers installed and a lot of older cookbooks in circulation, so recipes print Celsius, Fahrenheit and a mark on one line. An Australian or Canadian recipe prints Celsius. An American one prints Fahrenheit. Nobody outside Britain has to think about this at all, which is why an American hitting "gas mark 4" in a recipe has nothing on the cooker to compare it with.
The three pound thermometer beats all of it
Everything above assumes your oven does what its dial says, and most do not. Ten to twenty degrees out is normal, more at the extremes, and the error is not consistent across the cavity: the back is hotter than the front and the top shelf is hotter than the bottom.
An oven thermometer sitting on the middle shelf costs about three pounds and tells you what your own oven actually does at each setting. Once you know it runs 15 degrees cool you can correct for it permanently, and no conversion table on any website can give you that, including this one.
Common questions
What is gas mark 4 in Celsius and Fahrenheit?
180°C, 350°F, or 160°C in a fan oven. It is the most common setting in British cooking and the one most cakes are baked at. The Fahrenheit figure is exact rather than rounded: the gas scale is defined as 250 plus 25 times the mark, so mark 4 is 250 plus 100, which is 350 on the nose.
How does the gas mark scale work?
Degrees Fahrenheit equals 250 plus 25 times the mark. Mark 1 is 275°F, mark 5 is 375°F, mark 9 is 475°F, and every single one lands on a round 25 degrees. That is not a coincidence, it is the point: the scale was a way of marking a gas tap in even steps at a time when domestic ovens had no thermometer, so the numbers were chosen to be spaced rather than to mean anything on their own.
Why does the formula not work for gas mark 1/4 and 1/2?
Because the bottom of the scale was settled by printing rather than by arithmetic. The formula puts mark 1/4 at 256°F and mark 1/2 at 262°F. Every British cookbook prints 225°F and 250°F, and has done for decades. Neither set is wrong exactly, but the printed ones are what ovens and recipes actually use, so this tool follows the table below mark 1 and the formula above it, and marks which is which with an asterisk.
Do I reduce the temperature for a fan oven?
Yes, and how much depends on which country wrote the rule you learned. Britain publishes 20°C off. North America publishes 25°F off for convection. Those are not the same instruction, because 20°C is 36°F, so the British reduction is nearly half as much again. From a conventional 180°C the British rule gives 160°C and the American rule gives 325°F, which is 170°C. Both are conventions rather than measurements, so use the one your own oven manual gives and start checking ten minutes early either way.
Is a convection oven reduction 25F or 20C?
Whichever your market publishes, and they are ten degrees apart in practice rather than being two ways of saying the same thing. 25°F is a reduction of about 14°C; 20°C is a reduction of 36°F. If you are cooking an American recipe in a British fan oven you have two conversions stacked on each other and it is worth doing them in order: convert the conventional temperature first, then apply the fan rule, never the other way round. This tool shows both rules side by side for exactly that reason.
Is 180C fan the same as 180C?
No, and this is the most expensive misreading in home baking. 180°C fan is the equivalent of a conventional 200°C, which is gas mark 6. If a recipe says 180°C and you set a fan oven to 180°C, you are running about 20 degrees hot for the whole bake, and on a cake that is the difference between risen and cracked. If the recipe does not say which it means, assume conventional: that is the default a recipe writer omits.
Why do American recipes never give a gas mark?
Because gas marks are British and almost nowhere else has ever used them. American ovens are marked in Fahrenheit, Australian and Canadian ones in Celsius, and none of the three carries a gas scale. Britain ended up with all three at once for a specific reason: it went metric in the 1970s while a large share of the housing stock kept gas cookers, and the cookbooks in everyone kitchens predated the change. So a British recipe often prints Celsius, Fahrenheit and a gas mark on one line, and a recipe from anywhere else prints one number.
What is 200C in Fahrenheit?
400°F, in every kitchen, on every dial and in every cookbook. The exact arithmetic gives 392, and if a converter hands you 392 it has answered a maths question rather than a cooking one. The printed pairings are 200/400/gas 6, 180/350/gas 4, 220/425/gas 7, and they have been settled by usage for a century. This tool gives you the pairing where there is one and the arithmetic where there is not, and it tells you when the two disagree.
What does fan-forced mean on an Australian recipe?
The same thing a British recipe means by fan and an American or Canadian one means by convection: an oven with a fan in the back wall circulating the air. Three words, one appliance, and the 20 degree deduction applies to all of them. Australian ovens are marked in Celsius like British ones, so an Australian reading a British recipe has no conversion to do at all beyond the fan question.
How accurate is my oven dial?
Less than you would like, and it is the reason none of the numbers above are worth arguing over. Domestic ovens routinely run 10 to 20 degrees off their own dial, they are hotter at the back and hotter at the top, and they cycle around the set point rather than holding it. A three pound oven thermometer left on the middle shelf tells you what your oven actually does, which is more useful than any conversion table including this one.