Cake tin sizes, and how to convert any recipe

Every recipe here is written for a 20 cm round springform tin. If yours is 23 cm, the cake will be noticeably thinner and will bake faster, and the reason is that tins scale by area while we tend to think about them by width.

That mismatch is where failed conversions come from. Three centimetres sounds like a rounding error. In a tin it is a third more cake, or a third less.

Two questions hide inside any conversion: how much batter the tin wants, which is area, and how long it needs in the oven, which is depth.

The arithmetic

The area of a round tin is π × r². Going from a 20 cm tin to a 23 cm tin sounds like a 15% increase. It is actually 32%:

Tin	Radius	Area	Relative to 20 cm
15 cm round	7.5 cm	177 cm²	0.56×
18 cm round	9 cm	254 cm²	0.81×
20 cm round	10 cm	314 cm²	1.00×
23 cm round	11.5 cm	415 cm²	1.32×
25 cm round	12.5 cm	491 cm²	1.56×
20 cm square	n/a	400 cm²	1.27×
23 cm square	n/a	529 cm²	1.68×
900 g loaf	n/a	~215 cm²	0.68×

A square tin of the same stated size always holds more than a round one, about 27% more, because you are keeping the corners.

Spelled out: a 20 cm tin has a radius of 10 cm, so its area is π × 10 × 10, which is 314 cm². A 23 cm tin has a radius of 11.5 cm, so its area is π × 11.5 × 11.5, which is 415 cm². Divide 415 by 314 and you get 1.32.

You never need π, because it cancels. Take the ratio of the diameters and square it: 23 ÷ 20 = 1.15, and 1.15 × 1.15 = 1.32. Going down to 18 cm, 18 ÷ 20 = 0.9 and 0.9 × 0.9 = 0.81. Squaring is what makes the figures feel wrong: widen a tin by a tenth and it holds a fifth more.

Using the table

To scale the recipe: multiply every ingredient by the relative figure. Moving a 20 cm recipe into a 23 cm tin means ×1.32 on everything. In practice, round to something you can weigh: 200 g of flour becomes 265 g, three eggs become four.

To keep the recipe as written: accept a thinner cake and cut the baking time. A cake in a larger tin is shallower, so the heat reaches the middle sooner.

Round every weight to the nearest 5 g, and all the same way, or the ratio drifts. Then check the raising agent against the flour it now has to lift: one to one and a half teaspoons of baking powder per 150 g.

Scaling the eggs

Multipliers rarely land on a whole egg. Three eggs by 1.32 is 3.96; by 0.81 it is 2.43. Rounding either to the nearest egg is where a careful conversion goes wrong, because egg changes the structure and the moisture together.

Weigh them instead. A large egg out of its shell is 50 g, the figure every recipe here is built on: about 17 g of yolk and 33 g of white.

So three eggs is 150 g. Scaled up by 1.32 that is 198 g, near enough to four eggs at 200 g. Scaled down by 0.81 it is 121.5 g, which is no whole number of anything: beat three eggs together in a jug and weigh out 120 g.

Depth is what sets the baking time

Volume tells you how much cake you get. Depth tells you how long it takes, because heat travels inwards and the distance it has to cover is the depth of the batter rather than its width.

A 20 cm recipe here makes about a litre of batter. Over 314 cm² that is 3.2 cm raw, rising to near 5 cm baked. The same litre is 2.4 cm deep in a 23 cm tin, 2.0 cm in a 25 cm tin, and 3.9 cm in an 18 cm tin.

As a rule of thumb at 175°C, past about 3 cm of raw batter every extra centimetre adds 8 to 10 minutes, and every centimetre less takes about as much off, up to roughly 5 cm. Beyond that, stop adding time and drop the oven 10°C instead, because more minutes at the same heat brown the outside long before the centre sets. Deep tins pushed past that point are the usual reason cakes come out raw in the middle.

Never fill a tin more than two-thirds up, measured raw. A 20 cm tin 7 cm deep holds about 2.2 litres to the brim, so two-thirds is roughly 1.5 litres.

Adjusting the time

Depth drives time far more than volume does. As a working rule:

• Shallower than the original (same batter, bigger tin): start checking at 70% of the stated time.
• Deeper than the original (same batter, smaller tin): expect 15 to 25% longer, and drop the oven 10°C so the outside does not set hard before the middle cooks.
• Two tins instead of one: roughly half the time, but check at 40%. A 45-minute single-layer cake is usually 22 to 25 minutes as two layers.

Never trust a converted time. Trust the skewer.

Square tins, and why the corners cook first

A 20 cm square holds 400 cm², which is 1.27 times a 20 cm round, so swapping one for the other is a scaling job: multiply by 1.27, or keep the recipe and expect a cake a fifth shallower, done 8 to 10 minutes early.

Corners are the real difference, and the reason most cake recipes are round. Heat enters a corner from two walls at once, so that batter sets well before the middle, and on a 45-minute cake the corners can be ten minutes ahead.

Line the base and sides with paper rather than only greasing, because paper is a poor conductor and evens the edges out. Drop the oven 10°C on a sugary batter, and test in the geometric centre, never halfway to a corner.

Springform, solid and loose-bottomed

Every recipe here says springform because these cakes are soft when warm, and a ring that unclips is the only way to release one whole. The cost is a seam, and seams leak: a thin batter will find its way out, so sit the tin on a baking tray.

A solid tin cannot leak, but the only way out is to invert it, which suits a firm sponge and destroys a fudgy one. A loose-bottomed tin sits between the two: stand it on a tin of beans and let the ring fall away, with the same seam and the same leak.

The metal matters as much as the mechanism. Dark, thin tins brown the crust faster, so drop the oven 10°C. Heavy pale tins are slower and more even, and silicone conducts poorly and wants 5 to 10 minutes longer.

What changes for a bundt tin

Bundt tins are sold by volume rather than diameter, so the table does not apply. Find out what yours holds by filling it with water from a jug: a 10-cup bundt is about 2.4 litres, a 12-cup about 2.8.

Fill one no more than two-thirds. Since a 20 cm recipe here makes about a litre, multiply everything by 1.5 for a 10-cup tin and by 1.75 for a 12-cup, both of which land a little under two-thirds full.

The funnel is a heated surface, so no part of the batter is far from metal, and a 10-cup bundt usually wants 45 to 55 minutes at the recipe’s own temperature. Grease every crevice with melted butter and a brush, then flour it: a non-stick coating alone is not enough.

This is the one place the house cooling rule changes. A bundt comes out at 15 minutes rather than 20, because the sugar against the wall sets like glue as it cools.

A worked example, both ways

Take the blueberry and lemon cake here. For a 20 cm tin: 200 g butter, 200 g caster sugar, 3 eggs, 260 g plain flour, 2 tsp baking powder, half a teaspoon of salt, 120 g Greek yoghurt, 250 g blueberries, the zest of 3 lemons and the juice of 1. It serves 10 and bakes 45 to 50 minutes at 175°C.

Up to a 23 cm tin, ×1.32. 265 g butter, 265 g sugar, 345 g flour, 160 g yoghurt, 330 g blueberries, 2.5 tsp baking powder, two-thirds of a teaspoon of salt, the zest of 4 lemons. The eggs go from 150 g to 198 g, so use 4. It serves 13.

Down to an 18 cm tin, ×0.81. 160 g butter, 160 g sugar, 210 g flour, 95 g yoghurt, 200 g blueberries, 1.5 tsp baking powder, a scant half teaspoon of salt, the zest of 2 lemons and 30 ml of juice. Beat 3 eggs together and weigh out 120 g. It serves 8.

Check both. The baking powder lands at about 1.1 teaspoons per 150 g of flour either way, inside the rule. And the depth does not move: 1.32 litres over 415 cm², 810 ml over 254 cm² and the original litre over 314 cm² all come to 3.2 cm. Same temperature, same time, in all three tins.

The three conversions worth memorising

20 cm round → 900 g loaf tin: use two-thirds of the recipe, bake 10 minutes longer at 10°C lower. A loaf is deep, so it needs gentler, longer heat.

20 cm round → 12-hole muffin tin: use the full recipe, fill two-thirds up, bake at the same temperature for 18 to 22 minutes.

20 cm round → 23 cm round, keeping the recipe: bake at the same temperature and start checking 10 minutes early. You get a cake about 2 cm shallower, which suits anything you plan to pour a sauce over.

What not to convert

Anything relying on a specific depth for its texture. A tall, fudgy chocolate cake made in a wide, shallow tin becomes a brownie: pleasant, but not the recipe. And a batter that rises tall and cracks decoratively in a 20 cm tin will not crack at all in a 25 cm one, because it never gets deep enough to build pressure.

If a recipe makes a point of the tin, it means the tin.