Cake Calculator: Tin Size Converter
This cake calculator scales a recipe by comparing the base area of the two tins. Going from a 20 cm round tin to a 23 cm round tin means multiplying every ingredient by 1.32, because the bigger tin has 32% more area. Enter your tins below to get the multiplier, the new quantities, a baking time guide and portion counts.
- 20 cm → 23 cm round× 1.32
- 20 cm round → 20 cm square× 1.27
- Batter depthno more than half the tin
Updated November 2026 · Based on tin base area
Scale your cake recipe
Your result
Fill in the details, then press Calculate to see your result with the working shown.
You changed your details. Press Calculate to update the result.
How to scale a cake recipe to a different tin
Cake batter fills a tin by area, so you scale a recipe by comparing the base areas of the two tins. Work out each area, divide the new area by the old one, and multiply every ingredient by the answer.
- Round tin: area = π × diameter² ÷ 4. A 20 cm tin is 3.1416 × 400 ÷ 4 = 314 cm².
- Square tin: area = side × side. A 20 cm square tin is 400 cm².
- Example: 20 cm round to 23 cm round is 415 ÷ 314 = 1.32, so 200 g butter becomes about 265 g and 4 eggs become about 5.
Cake tin size conversion chart
| From | To | Multiply recipe by |
|---|---|---|
| 15 cm round | 20 cm round | 1.78 |
| 18 cm round | 20 cm round | 1.23 |
| 20 cm round | 23 cm round | 1.32 |
| 20 cm round | 25 cm round | 1.56 |
| 20 cm round | 20 cm square | 1.27 |
| 20 cm square | 23 cm square | 1.32 |
| 23 cm round | 20 cm round | 0.76 |
A square tin holds about 27% more than a round tin of the same size, so a 20 cm round recipe fits an 18 cm square tin almost exactly (324 cm² against 314 cm²).
Adjusting the baking time
When the batter depth stays the same, baking time changes far less than the quantity of mixture. A smaller tin usually bakes a little faster, so start testing 5 to 10 minutes before the recipe time. A bigger tin takes a little longer, and very large tins (25 cm and up) can brown at the edges before the middle sets: lowering the oven by about 10 °C and baking for longer helps. The calculator uses a gentle rule of thumb, scaling time by the fourth root of the area ratio, which gives 27 minutes instead of 25 for the 20 to 23 cm example. Always test with a skewer: it should come out clean.
How many portions does a cake make?
Portion counts depend on how you cut. Party slices of about 5 × 2.5 cm suit a sponge served with tea; 2.5 cm square fingers are the usual size for rich fruit wedding cake. The calculator divides the tin area by the portion area, so treat the numbers as a guide.
| Tin | Party slices | Fingers |
|---|---|---|
| 15 cm round | 14 | 28 |
| 20 cm round | 25 | 50 |
| 23 cm round | 33 | 66 |
| 20 cm square | 32 | 64 |
| 25 cm square | 50 | 100 |
Frequently asked questions
How do I convert a recipe from an 8 inch to a 9 inch tin?
An 8 inch (20 cm) round tin has about 314 cm² of base and a 9 inch (23 cm) tin about 415 cm², so multiply the recipe by about 1.3.
How do I convert a round cake tin to a square one?
A square tin of the same size holds about 27% more, so multiply by 1.27. Or use a square tin about 2 cm smaller than the round one, for example an 18 cm square for a 20 cm round recipe.
What size square tin equals a 20 cm round tin?
An 18 cm square tin has a base of 324 cm², very close to the 314 cm² of a 20 cm round tin.
Do you change the baking time for a bigger cake tin?
Yes, slightly. If the batter is the same depth, add only a few minutes and test with a skewer. For deeper batter, lower the oven by about 10 °C and bake for longer.
How many slices does an 8 inch round cake make?
About 25 party slices of 5 × 2.5 cm, or about 50 finger portions of 2.5 cm square.
Sources and method
We use official UK guidance and recognised formulas, and test each calculator against worked examples. Results are estimates to help you plan, not professional advice. See how we work.