Gas Mark 1 — all four oven scales
Gas marks are the scale on British and Irish gas ovens: gas mark 1 is 275°F / 140°C and each whole mark up adds 25°F. Gas mark 1 lands at 275°F / 140°C, which the traditional cookbook descriptors call a slow oven. If your recipe is older or British and your oven is not, these two numbers are the whole translation.
In a fan (convection) oven, use 120°C — the standard adjustment is conventional minus 20°C, because moving air transfers heat faster than still air. Make the temperature adjustment or shorten the bake time by about a quarter, never both. Bake times are otherwise unaffected by converting between scales: 140°C and 275°F are the same heat wearing different units.
Where gas mark 1 sits on the chart
| °F | °C | Fan °C | Gas mark | Descriptor |
|---|---|---|---|---|
| 225°F | 110°C | 90°C | ¼ | Very slow |
| 250°F | 120°C | 100°C | ½ | Very slow |
| 275°F | 140°C | 120°C | 1 | Slow |
| 300°F | 150°C | 130°C | 2 | Slow |
| 325°F | 160°C | 140°C | 3 | Moderately slow |
See the full chart on the oven temperature converter →
What bakes at this temperature
275°F / 140°C is the territory of rich fruit cakes and other long, slow bakes. Remember that most home ovens swing ±25°F around their set point and many run permanently hot or cold — a standalone oven thermometer on the middle rack tells you more than any conversion chart can.