How do I convert a recipe to a different tin size?

Recipes are written for one tin and everyone owns a different one. The conversion is straightforward once you know it is an area calculation, and the reason people get it wrong is that they scale by the number printed on the tin.

Step by step

1. Scale by area, not by diameter.Area grows with the square of the radius, so a small increase in width is a large increase in volume. This is the whole thing, and it is why a 'slightly bigger' tin gives a disappointingly thin cake.
2. Calculate the ratio for round tins.Divide the square of the new radius by the square of the old. Going from 20cm to 23cm: 11.5² ÷ 10² = 1.32, so multiply every ingredient by about 1.3.
3. Use length times width for square and rectangular tins.A 20cm square tin is 400cm². A 20cm round tin is about 314cm². A square tin of the same nominal size holds roughly a quarter more.
4. Convert between round and square with the same arithmetic.Work out both areas and divide. As a rough rule, a square tin takes the recipe for a round tin about 2cm larger.
5. Round the result sensibly.Multiplying by 1.32 gives awkward egg quantities. Round to a whole number of eggs and adjust the other ingredients to match — a small deviation matters far less than the overall ratio.
6. Keep the temperature the same.Temperature is set by the type of cake, not the tin. Changing it as well as the quantity introduces a second variable and makes the result impossible to learn from.
7. Adjust the time by depth, not by volume.A wider, shallower cake bakes faster; a deeper one takes longer. If the depth of the mixture is roughly the same, the time is roughly the same regardless of how much bigger the cake is.
8. Start checking early.Check 10 minutes before you expect it, then every 5. A skewer into the exact centre, not the edge.
9. Fill tins to about two-thirds.If your scaled mixture fills the tin more than that, use a bigger tin or make cupcakes with the excess. Overfilled tins dome, crack and overflow.
10. Take extra care with rich fruit cakes.These do not scale as freely — deep, dense mixtures need a lower temperature and a much longer bake as they grow. Most fruit cake recipes publish a table for each tin size, and it is worth using.
11. Write the conversion on the recipe.Once you have done the arithmetic for the tin you actually own, note it. You will not want to do it again.

Tips

• Measure your tins rather than trusting the size printed on them, and measure across the base — sloped-sided tins are smaller than they look.
• Scaling down works the same way and is often more useful. Halving a recipe by area rather than by eye gives a much better result.
• Baking by weight makes scaling trivial, which is another reason to weigh rather than use cups.

Common mistakes

• Scaling by the tin's width — Volume grows with the square of the radius. Increasing a 20cm tin to 23cm needs about 30% more mixture, not 15%.
• Raising the temperature for a bigger cake — It burns the outside before the centre cooks. Keep the temperature and adjust the time based on how deep the mixture is.

If it doesn't work

Questions people ask