Hong Life CalculatorsUnderstanding the oven temperature converter — concept and principle
This converter translates a conventional oven temperature in degrees Celsius into degrees Fahrenheit, the usual setting for a fan (convection) oven, and the British gas mark. It is useful when a recipe comes from another country or was written for a different type of oven.
How it is calculated
- °F = °C × 9/5 + 32.
- Fan oven (°C) = conventional °C − 20.
- Gas mark = the nearest mark in the standard UK table, from ¼ = 225 °F and ½ = 250 °F through 1 = 275 °F to 9 = 475 °F, in steps of 25 °F.
Enter the temperature the recipe gives for a conventional (top-and-bottom heat) oven.
The principle
The Celsius and Fahrenheit scales are both linear: water freezes at 0 °C = 32 °F and boils at 100 °C = 212 °F at standard pressure, so 100 Celsius degrees span 180 Fahrenheit degrees. That gives the factor 9/5 and the offset 32. Gas marks were defined in Fahrenheit: gas mark 1 is 275 °F and each mark adds 25 °F, so mark n ≈ 250 + 25n °F. In Celsius that is about 121 + 13.9n °C, so each mark is roughly 14 °C. A fan oven blows hot air over the food, which removes the cool layer of air around it and transfers heat faster, so a lower setting gives a similar result.
Worked example
- Input: 180 °C, conventional oven.
- °F = 180 × 9/5 + 32 = 324 + 32 = 356 °F (recipes usually round this to 350 °F).
- Fan oven = 180 − 20 = 160 °C.
- Gas mark: 356 °F is nearest to the 350 °F entry, so 4.
| Temperature (°C, conventional) | °F |
|---|---|
| 45 | 113 |
| 90 | 194 |
| 135 | 275 |
| 180 | 356 |
| 225 | 437 |
| 270 | 518 |
| 360 | 680 |
How to read the result
About 150–160 °C is a low oven for slow cakes and custards, 175–190 °C is moderate for most cakes and casseroles, and 200–230 °C is hot for roasting and bread. Many recipes list rounded pairs such as 180 °C / 350 °F / gas 4, which differ slightly from an exact conversion; that is normal. Home ovens often differ from the dial by 10 °C or more, so an oven thermometer is more useful than precise arithmetic.
Common mistakes
- Reducing the temperature for a fan oven when the recipe already gives a fan temperature.
- Converting a temperature difference with the +32 offset; a change of 10 °C is a change of 18 °F, not 50 °F.
- Expecting gas marks to be exact; each mark covers a range of about 14 °C.
- Ignoring that dark tins and glass dishes brown food faster.
Frequently asked questions
What is 200 °C in Fahrenheit?
200 × 9/5 + 32 = 392 °F, usually written as 400 °F or gas mark 6.
Should I also shorten the baking time in a fan oven?
Often slightly, especially for large batches; check about 10 minutes earlier than the recipe says.
What does a "moderate oven" mean?
In older recipes it usually means about 180 °C, 350 °F or gas mark 4.
Sources: NIST Special Publication 811, Guide for the Use of the International System of Units (SI), temperature conversion