Why size matters so much
A cake bakes from the outside in, and how long that takes depends on how far the heat has to travel — which is a matter of depth, not of overall volume.
Put a 20cm quantity of batter into a 23cm tin and the depth drops by about a third. It bakes considerably faster, the edges dry before the middle is done, and the cake is flatter and drier. Go the other way and the middle is raw when the outside is done.
The area of a round tin goes up with the square of the radius, so the jump from 20cm to 23cm is about a 30% increase in area — much more than the numbers suggest. That is why "it is only 3cm bigger" is misleading.
Rough conversions
By area. Adjust the baking time and check early.
- 18cm round
- About 75% of a 20cm.
- 20cm round
- The reference size for most recipes.
- 23cm round
- About 130% of a 20cm — increase quantities by a third.
- 20cm square
- Roughly equivalent to a 23cm round.
- 900g loaf tin
- Roughly equivalent to a 20cm round, but much deeper, so it needs a lower oven and considerably longer.
- Deeper tin, same width
- Lower the temperature by 10–20°C and extend the time substantially. Depth is what causes burnt edges and raw middles.
Material and finish
- Light anodised aluminium
- The best all-round choice. Conducts well, does not over-brown, holds its shape.
- Dark non-stick steel
- Browns the crust much faster. Reduce the temperature by about 10°C, or expect dark edges.
- Loose-bottomed
- For anything hard to turn out. Not watertight, so wrap it for a water bath.
- Springform
- For cheesecakes and anything delicate. Check the seal — cheap ones leak.
- Silicone
- Flexible and easy to release, and it insulates, so cakes brown poorly and can stay pale and damp at the sides.
- Glass and ceramic
- Slow to heat, so bases stay pale. Good for puddings, poor for cakes and pastry.
Lining a round tin properly
- Grease the tin lightly with butter or oil, including the sides.
- Cut a disc of baking parchment for the base by drawing round the tin and cutting just inside the line.
- Cut a strip for the sides, a couple of centimetres taller than the tin and long enough to overlap.
- Fold a 1cm cuff along one long edge of the strip and snip into it at intervals, so it can splay flat around the base.
- Press the strip round the inside with the snipped cuff on the bottom, then lay the disc over it.
- For a rich fruit cake, use a double layer and wrap brown paper round the outside as well — it insulates during the long bake.
Common questions
How full should I fill a tin?
Around two-thirds for most cakes. Fuller and it domes and overflows.
Can I use a loaf tin instead of a round one?
Yes, if the volumes are close, but expect a much longer bake and cover the top with foil once it is brown.
Do I need to line the sides?
For a sponge, greasing and flouring is usually enough. For anything sticky, fruited or long-baked, line them.
Why did my cake stick?
Under-greased, or turned out too soon. Let it settle in the tin for ten minutes first — but not until cold, or it will steam and stick.
Try this next
- Victoria sponge the reference cake
- Baking oven behaviour
- Dense cake when it goes wrong
- Cheesecake where a springform matters