Cake Tin Size Converter

Doubling the diameter of a round tin quadruples its area (since area = pi x radius squared). So a 20cm recipe in a 30cm tin isn't 1.5x — it's 2.25x. Scaling by area ensures the batter depth stays consistent.

If the batter ends up shallower (larger tin), reduce baking time by 5-10 minutes. If deeper (smaller tin), increase time and consider reducing temperature by 10°C to avoid burning the outside before the centre is done.

A round tin and a square tin of the same stated size (e.g. both 20cm) have different areas. The square is larger. Use this calculator to get the exact ratio, or as a rule of thumb, a round tin is equivalent to a square tin about 2cm smaller.

This calculator scales by the base area, which is correct when both tins have the same depth. If the new tin is shallower, you may need to reduce the quantity slightly or use two tins. If deeper, the recipe should still work but may take longer to bake.

A shallower tin holds less batter for the same base area, so filling it to the same level uses only part of the scaled-up mixture. Either pour to no more than two-thirds full and bake any excess as cupcakes, or split the batter across two tins. Overfilling a shallow tin risks it spilling over and the centre staying raw. As a rough guide, fill a standard sponge tin about half to two-thirds deep with raw batter.