UK Gas Marks Explained: From Mark ¼ to Mark 9

What a gas mark is

A gas mark is a number on the temperature dial of a gas oven. It is used in the United Kingdom, Ireland and some other Commonwealth countries, and it still turns up in many British and Irish recipes, even though most newer ovens are labeled in degrees Celsius.

The scale is not a measurement of temperature in the way Celsius or Fahrenheit is. It is a set of numbered dial positions that each correspond to a target oven temperature. Because it is just a label for a setting, there is no formula that turns a gas mark into degrees; you look it up in a chart.

Where the scale comes from

Early gas ovens had no thermostat. Cooks adjusted a manual valve and judged the heat by experience. In the 1920s a manufacturer introduced an automatic controller called the Regulo, whose dial was marked with numbers rather than degrees. The design was widely copied, and “Regulo” gradually became a generic name for the numbered scale, which is now simply called gas mark.

This history explains why the scale looks a little odd. The numbers were chosen for a dial, not for a thermometer, so they do not line up neatly with round Celsius values.

How the numbers are spaced

The pattern is easiest to see in Fahrenheit. Gas mark 1 is 275 °F, and every whole mark above it adds 25 °F. Below mark 1 the dial has two fractional settings: ¼ is 225 °F, ½ is 250 °F, and then 1 is 275 °F. Each of those steps is also 25 °F, even though the labels look as if they are halving.

Gas mark 9, the top of the usual scale, is therefore 275 + 8 × 25 = 475 °F. Counting from ¼ to 9 gives eleven settings in total.

A handy check follows from this: if you know that gas mark 4 is 350 °F, then mark 5 is 375 °F and mark 3 is 325 °F. Remembering one anchor point and the 25 °F step lets you rebuild most of the chart from memory, although the Celsius equivalents still need the rounded values below.

The full chart

Celsius values are rounded to convenient figures, which is why the steps look uneven in metric. This is the chart used by this site’s oven temperature converter, and other charts vary slightly:

• 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

Why the Celsius figures are rounded

Take gas mark 4. Converting 350 °F exactly gives (350 − 32) × 5 ÷ 9 = 176.7 °C, but recipes and ovens use 180 °C. Mark 6 works out at 204.4 °C and is printed as 200 °C. Mark 7, at 425 °F, is 218.3 °C and appears as 220 °C.

These small differences are deliberate rounding to a figure that is easy to set on a dial. For most baking they do not matter, but they do mean a conversion that goes from Celsius to gas mark and back may not land on exactly the same number. A tool that shows the nearest gas mark for a given temperature, as the oven tab of a cooking converter such as this one does, is picking the closest listed setting rather than calculating an exact value.

Reading a recipe that uses gas marks

Most traditional British recipes give the gas mark alongside a Celsius and sometimes a Fahrenheit figure, for example “180 °C / 350 °F / gas mark 4”. When all three are given, use whichever your oven displays. If only a gas mark is given and your oven shows degrees, read across the chart above and use the matching Celsius value.

Common reference points are worth remembering: gas mark 4 for a moderate oven used for many cakes and bakes, gas mark 6 for a hot oven, and gas mark 7 or higher for roasting vegetables or baking bread at a high heat. Low settings from ¼ to 2 are used for slow cooking, drying and meringues.

A caution for fan ovens

Gas mark charts assume a conventional oven without a fan. Fan-assisted electric ovens are normally set about 20 °C lower for the same recipe, so a recipe written for gas mark 6 (200 °C) would usually be set to around 180 °C on a fan oven. This is a separate adjustment that you apply by hand after converting. If the recipe or the oven manual gives its own guidance, follow that instead, and always check your oven’s manual for how its settings behave.

Finally, ovens vary. Many run hotter or cooler than their dial suggests, so an oven thermometer is a worthwhile low-cost check if results are inconsistent. Use gas marks as a starting point and judge doneness by the look and texture described in the recipe.