Cake Tin Conversion Calculator
Swap a recipe from one cake tin to another — round or square, any size — with the batter adjustment and a sensible change to the baking time.
What matters is the area of the tin, not its width, because batter spreads across the base at roughly a constant depth.
How the cake tin conversion calculator works
What matters is the area of the tin, not its width, because batter spreads across the base at roughly a constant depth. A 20 cm round tin has an area of 314 cm²; a 25 cm round has 491, so it needs 1.56 times the batter, not 1.25. A square tin of the same nominal size holds noticeably more than a round one — a 20 cm square is 400 cm², about 27% more than the 20 cm round, which is the usual reason a recipe comes out thin.
Baking time changes with depth rather than area. Keep the depth similar and the time barely moves; make the cake deeper and it needs longer at a slightly lower temperature so the outside does not overcook before the middle sets.
Formula: factor = new tin area ÷ old tin area; round area = πr², square area = side²
Worked examples
| Inputs | Multiply the recipe by | Note |
|---|---|---|
| 20 cm round recipe into a 23 cm round tin | 1.322 | ×1.32 — scale up or accept a thinner cake |
| 20 cm round into a 20 cm square | 1.273 | ×1.27 — the square holds more than it looks |
| 23 cm round down to 18 cm | 0.612 | ×0.61 — or a much deeper cake |
FAQFrequently asked questions
How do I convert between cake tin sizes?
Compare the areas, not the widths. A 23 cm round tin has 32% more area than a 20 cm one, so it needs 32% more batter for the same depth.
Is a 20 cm square the same as a 20 cm round?
No — the square is about 27% larger, which is why a recipe moved straight across comes out disappointingly thin.
Does the baking time change?
It follows depth rather than area. Same depth, similar time. Deeper cake, longer time and often 10°C lower so the edges do not overbake.
What if my tin is deeper?
Depth changes how long, not how much. Fill to the same depth as the original and scale the batter by the area factor.
How full should a cake tin be?
About two-thirds. Fuller than that and it domes heavily or spills.
Where these figures come from
- US FDA — Safe Minimum Internal Temperatures — the internal temperatures the roasting calculator targets
- NIST SP 811 — Guide for the Use of the International System of Units — the exact weight and volume conversions used throughout
- UK Food Standards Agency — food safety and labelling standards
Last checked: September 2026. Baking ratios follow standard baker's percentage practice; roasting guidance follows published safe internal temperatures.