Oven Temperature Converter — Fahrenheit, Celsius, Gas Mark & Fan

Two very different kinds of number sit inside an oven conversion, and most charts blur them together. Separating them is what this tool does.

The arithmetic is exact. Fahrenheit and Celsius are related by a definition, not a measurement:

• °C = (°F − 32) × 5/9
• °F = °C × 9/5 + 32
• K = °C + 273.15

The 32 is where water freezes on the Fahrenheit scale; the 5/9 is because 180 Fahrenheit degrees cover the same span as 100 Celsius degrees. Since the 2019 SI redefinition fixed the Boltzmann constant, the kelvin and the degree Celsius are exact quantities, so no rounding enters anywhere. The converter runs this conversion twice by two different routes — the direct formula and a detour through kelvin — and reports whether they agree, which is a live check that nothing in the code has drifted.

The gas mark scale is a ladder. From mark 1 upwards it obeys a closed form: °F = 250 + 25 × G. Mark 1 is 275 °F, and every mark adds another 25 °F, so mark 4 is 350 °F and mark 9 is 475 °F. This page re-derives all 10 whole rungs from that formula every time it renders and compares them against the published chart, so a typo in the table cannot reach you unnoticed. The two sub-marks are the exception: ¼ and ½ are fixed at 225 °F and 250 °F by convention, and the formula would put ¼ at 256.25 °F, so they are read from the table rather than calculated.

The Celsius column of every chart is a convention. Gas mark 4 is exactly 350 °F, but its printed Celsius is 180 °C where the arithmetic gives 176.7 °C. That is not sloppiness — Celsius oven dials are marked every 10 °C, so a chart has to give you a number you can select. Auditing the chart against plain nearest-10 rounding shows 11 of 12 rows follow it exactly. The single exception is gas mark 9, printed as 240 °C where 475 °F converts to 246.1 °C — a gap of 6.1 °C that the chart has simply carried for decades.

The fan adjustment is three conventions, not one. A fan strips away the still layer of air that insulates food, so the same dial reading browns faster. The UK, Ireland and Australia subtract 20 °C. US convection guidance subtracts 25 °F, which is only about 14 °C. A third rule cuts 10% off the Celsius figure. The converter shows all three with the spread between them rather than picking one silently, and defaults to the 20 °C rule because that is the one printed on recipe charts. Fan ovens also finish sooner, so start checking about 10% before the stated time.

Finally, every result carries a dial figure: the conversion rounded to the nearest 10 °C or 25 °F, because those are the increments domestic oven dials are marked in. Knowing that 180 °C is 356 °F is arithmetic; knowing to set the dial to 350 °F is the answer you came for.

The full gas-mark ladder as UK recipe charts print it. The final column is the exact conversion of the Fahrenheit value, which is what the printed Celsius figure is rounded from.

Source: BBC Good Food oven temperature conversion guide. Every rung from gas 1 up satisfies °F = 250 + 25 × G; the ¼ and ½ sub-marks are fixed by the chart. Last verified 2026-08-13.

Starting from the Fahrenheit figures a US recipe prints. Every column here is computed from the °F value by the converter above, so the two can never disagree.

“Set °C” is the exact conversion rounded to the nearest 10 °C, since that is how Celsius dials are marked. Fan °C applies the UK 20 °C reduction to it. Most domestic ovens stop near 300 °C.

• BBC Good Food — Oven temperature conversion guide (gas mark, °C, fan °C and °F chart)
• NIST Special Publication 811 — Guide for the Use of the SI, Appendix B.8 (temperature conversions)
• BIPM — SI Brochure, 9th edition (the kelvin and the degree Celsius as exact quantities)
• Which? — Oven reviews and buying guide (fan-oven behaviour and thermostat accuracy)

The °F/°C/K arithmetic is fixed by definition and cannot drift. The gas-mark rungs and the printed Celsius column are conventions, last re-read against the sources above on 2026-08-13 and reviewed yearly. Every whole gas rung on this page is re-derived from the 25 °F step formula at render time rather than trusted from the table.