Cake Tin Converter

Half the recipes in a British cookbook seem written for a tin size you don't own. This calculator solves the problem by working out the area of both tins and giving you the exact scaling factor to multiply your ingredients by, so a recipe designed for a 20 cm round becomes a 23 cm round or a 20 cm square without guesswork. It works across round, square and rectangular tins, and gives you a sensible baking-time and temperature note based on how much the size is changing.

It's for anyone converting a favourite family recipe to a different tin, scaling a celebration cake up for a bigger gathering, or halving a large recipe to fit a smaller kitchen. Get the tin conversion right and the cake bakes the way the original writer intended; get it wrong and you'll end up with either an overflow disaster or a wafer-thin sponge.

How the calculation works

We work out the base area of both tins and divide the new by the original. For round tins the area is π × (diameter ÷ 2)²; for square tins it's side × side; for rectangles it's length × width. The scaling factor is new area ÷ original area, and every ingredient in the recipe (flour, sugar, butter, eggs, liquid) is multiplied by that number.

The method assumes the two tins are similar in depth — most standard sandwich, sponge and celebration tins in UK kitchens are 4–8 cm deep, so this assumption holds for the majority of cases. Where depths vary significantly (say, converting a shallow Swiss roll tin to a deep speciality tin), scaling by area alone will give you a very different bake and you may want to divide the mixture across two tins instead.

A worked example

Your favourite Victoria sponge is written for two 20 cm round tins and serves eight. You want to make it in a 23 cm round tin for a bigger family gathering. The original area is π × 10² ≈ 314 cm² per tin; the new area is π × 11.5² ≈ 415 cm². The scaling factor is 415 ÷ 314 = 1.32.

That means 200 g flour becomes 264 g (round to 265), 200 g sugar becomes 264 g, and 4 eggs becomes 5.3 — so use 5 large eggs and add a splash of extra milk. Bake at the original temperature but start checking 5 minutes earlier because the batter is only slightly deeper. Expect about 10 servings instead of 8.

Why this matters

Cake batter isn't just a solution — it's a leavened structure that has to set at the right rate. Too much batter in a tin and the outside sets before the middle can rise properly, giving you a domed, cracked cake with a wet centre. Too little and the cake will be dry, over-baked and disappointingly shallow. Scaling by area keeps the depth (and therefore the physics) close to what the recipe was designed for.

Limitations to be aware of

Scaling factors work best in the 0.5×–2× range. Beyond that, the physics of larger cakes changes — big fruit cakes need low, slow bakes with lined tins; small individual cakes need very quick, hot bakes. Recipes that rely on precise whisking (Genoise, angel cake, chiffon) also don't scale as cleanly, because the incorporated air doesn't automatically scale with the ingredients.

Common mistakes

• Scaling by diameter, not area — a 25 cm tin isn't 25% bigger than a 20 cm tin, it's 56% bigger by area.
• Overfilling the tin — most cake batters should fill no more than two-thirds of the tin's depth to leave room to rise.
• Doubling the baking time — bigger cakes need lower heat, not necessarily twice the time. Use a skewer to check.
• Ignoring the shape change — a square tin holds more batter than a round of the same diameter (about 27% more). The calculator handles this automatically.
• Not scaling eggs — some people scale flour and sugar but leave the eggs alone. Every ingredient needs the same factor.
• Trusting the clock over the cake — visual signs (colour, spring-back, clean skewer) are more reliable than any suggested time.

Related tools: Recipe Scaler · Baking Time Adjuster · Oven Temperature · Egg Size Converter