Any Tin Will Do: A Practical Decoder for Cake Pan Sizes
Photo: assorted round loaf and square cake tins on kitchen counter, via a57.foxnews.com
Most cake recipes arrive with a confident instruction: use a 20cm round tin. Your cupboard, meanwhile, contains an 18cm sandwich tin, a 2lb loaf pan and a square brownie tin that has seen things. The good news is that cake recipes are more adaptable than they look. Once you understand a few simple ideas about area, depth and heat, you can turn almost any sponge into the cake you actually need.
First, measure what you own
Measure the inside of each tin across the top, from edge to edge, and note the depth. A tin that says 18cm on the base may be wider at the top. For round tins, measure the radius and use the formula pi times radius squared. For square and loaf tins, multiply length by width. You do not need to be precise to the millimetre, but you do need a rough number.
A few common UK tins are worth knowing. An 18cm round sandwich tin has an area of roughly 254 square centimetres. A 20cm round tin is about 314 square centimetres. A 23cm round tin is about 415 square centimetres. A 2lb loaf tin is often around 21cm by 11cm, giving about 231 square centimetres. A 20cm square brownie tin is about 400 square centimetres. Write these on a note and stick it inside a cupboard door. Future you will be grateful.
The batter-depth rule
Most sandwich sponges want a batter depth of about 3cm to 4cm per layer. A deeper cake, such as a madeira or a fruit cake, may want 5cm to 7cm. If your batter is much shallower than that, the cake will bake quickly and may dry out. If it is much deeper, the outside will set before the middle has caught up, giving you a domed top and a slightly raw centre.
This is why tin size matters more than tin shape. A recipe written for two 20cm tins spread across two 18cm tins will give a deeper batter. The cake may still work, but it will need a lower oven temperature and a longer bake. A recipe written for a 20cm round tin and baked in a 23cm tin will give a shallower cake, so check it early. The same volume of batter behaves differently depending on the area it covers.
Scaling maths without tears
To keep the same batter depth, scale the recipe by the ratio of the tin areas. Divide the area of your tin by the area of the tin in the recipe. If the recipe uses two tins and you use two tins, the number of tins stays the same. If the recipe uses two tins and you use one, you need a different calculation: multiply the recipe by the area of your single tin divided by the combined area of the recipe tins.
For example, a recipe for two 20cm round tins has a combined area of about 628 square centimetres. If you want to bake it in one 23cm round tin, with an area of about 415 square centimetres, multiply the recipe by 415 divided by 628, which is roughly 0.66. In other words, make about two-thirds of the batter. Use the spare batter for a few cupcakes, or scale the recipe down properly.
If maths makes your eyes glaze over, use a simpler rule of thumb. Going from a 20cm round tin to an 18cm round tin increases the depth by around a quarter. Reduce the batter by about 20 per cent or expect a slightly longer bake. Going from 20cm to 23cm reduces the depth by about a quarter. Increase the batter by around 30 per cent, or accept a shallower cake and check it sooner.
A cheat sheet for common swaps
| Recipe tin | Your tin | What to expect | What to do |
|---|---|---|---|
| Two 20cm round | Two 18cm round | Deeper batter | Lower oven by 10C, add 5 to 15 minutes, check with a skewer |
| Two 20cm round | One 23cm round | Shallower batter | Use about two-thirds of the batter or make cupcakes with the rest |
| 20cm round | 2lb loaf tin | Deeper, narrower batter | Use about three-quarters of the batter and bake longer at a lower temperature |
| 20cm square | 23cm square | Shallower batter | Increase batter by about a third or check early |
| 12-hole cupcake tray | 18cm round tin | Much deeper batter | Do not fill more than two-thirds; bake lower and longer |
Adjusting baking time and temperature
When the batter is deeper than the recipe expects, lower the oven by about 10C and add 5 to 15 minutes. When the batter is shallower, keep the temperature the same or reduce it slightly, and start checking 5 to 10 minutes early. Always trust the skewer and the feel of the cake rather than the clock. A cake is done when it pulls away from the sides slightly and a skewer inserted into the centre comes out clean or with a few moist crumbs.
If you are baking a large deep cake, consider using a baking strip or a folded newspaper collar around the outside of the tin. This slows down the setting of the outer edge and helps the middle catch up. Do not open the oven door repeatedly in the first half of baking, especially for sponges, as a draught can cause a collapse.
Common pitfalls and how to avoid them
Domed tops usually mean the oven is too hot, the batter is too deep or the recipe contains too much raising agent. Try lowering the temperature by 10C and baking for longer. If the dome is stubborn, trim it off once cool and call it a feature. Overflow happens when the tin is too small or too full. For sandwich sponges, fill the tin no more than two-thirds full. For loaf tins, two-thirds is also a sensible maximum. If you have excess batter, make cupcakes rather than risking a lava flow of cake.
Uneven layers are often caused by an uneven oven or a warped tin. Rotate the tin halfway through baking if your oven runs hot at the back. If you are baking two tins on one shelf, leave a gap between them for air to circulate. If your tins are different sizes, do not assume they will bake at the same speed.
The final rule: keep notes
Bake a simple vanilla sponge in each of your tins and write down what happened. Note the batter weight, the oven temperature, the baking time and the depth of the finished cake. After two or three test bakes, you will have a personal cheat sheet that is far more useful than any generic conversion chart. Your tins are not a limitation. They are simply the shape of your next cake.
