The gas mark system explained: a Fahrenheit scale in disguise
Gas marks look like their own mysterious unit — until you see the pattern. Gas mark 1 is 275°F, and every step up adds exactly 25°F. That is the whole system. Once you know the two anchors, you can rebuild any gas mark chart on a napkin, and every conversion becomes arithmetic instead of table-lookup.
Key takeaways
- The formula: gas mark G = (275 + 25 × (G−1))°F. Gas 4 = 350°F. One mark = 25°F ≈ 14°C.
- Quarter marks extend below 1: ¼ = 225°F ≈ 110°C, ½ = 250°F ≈ 120°C (BBC Good Food rows).
- The °C pairings are rounded: gas 3 = 325°F = 163°C exactly, published as 160°C or 170°C depending on the chart.
- Gas + fan: convert to °C first, then −20°C (gas 4 → 180°C → 160°C fan).
The pattern, from two anchors
Pull up Delia Online's conversion table and BBC Good Food's conversion guide side by side and the Fahrenheit columns line up perfectly: 275, 300, 325, 350, 375, 400, 425, 450, 475. That is an arithmetic sequence — start at 275, step by 25. In other words:
°F = 275 + 25 × (gas mark − 1)
The quarter marks continue the sequence downwards: half a step below gas 1 is 262.5°F, rounded to 250°F and published as gas ½; a quarter step is 225°F, gas ¼. (Delia's table starts at gas 1; the ¼ and ½ rows here follow BBC Good Food, the UK reference that publishes them.)
Where the Celsius column comes from (and wobbles)
Here is the honest part. Convert the spine exactly and you get values that end in odd numbers — 163°C for gas 3, 246°C for gas 9 — because 25°F steps are 13.9°C steps. Charts publish round Celsius instead, and rounding choices differ:
| Gas | °F | °C exact | Delia publishes | BBC Good Food publishes |
|---|---|---|---|---|
| 1 | 275 | 135 | 140 | 140 |
| 3 | 325 | 163 | 170 | 160 |
| 4 | 350 | 177 | 180 | 180 |
| 9 | 475 | 246 | 240 | 240 |
Agreement everywhere except gas 3 — and both are defensible roundings of 163. This is why OvenMath's converter stores 165°C as the gas-3 anchor (the midpoint): both published pairings then resolve to gas 3, and the full temperature chart shows the disagreement instead of picking a winner silently.
Reading a gas oven like a local
Three habits make gas marks painless:
- Memorise one row, derive the rest. Gas 4 = 350°F = 180°C. Need gas 6? Two steps up = +50°F = 400°F ≈ 200°C. The formula does the rest.
- Gas ovens cycle harder than electric. The burner lights, the cavity overshoots, it cools, it relights — which is exactly why Delia's team “strongly recommend[s] you use an oven thermometer at all times”. More on that in the preheating guide.
- Gas + fan is its own setting. Many UK gas ovens are fan-assisted; the −20°C rule applies after converting the mark to °C. Modern recipes that print “180°C / gas 4 / 160°C fan” are doing this arithmetic for you.
Why the system persists
Gas marks survive because they solve a real interface problem: a gas thermostat marking “163°C” would be a lie the dial cannot hold. A number scale that steps by roughly 14°C matches the precision of the hardware — no false accuracy. Recipes, cookbooks and two generations of home bakers standardised on it, and as long as UK kitchens have gas ovens (and electric ovens that print the marks for compatibility), “gas mark 4” remains the fastest way to say “the moderate setting”.
Last verified: 2026-11-16 — sources: Delia Online, BBC Good Food, fetched live on that date.