Cake Tin Size Converter & Recipe Scaler
The recipe says a 20cm round tin, but yours is 23cm — or square. Rather than a sunken middle or an overflowing oven, scale the mixture properly: tell the converter both tins and it gives you the exact multiplier for every ingredient, based on what each tin actually holds.
Your tin is bigger, so the mixture sits in a thinner layer: expect it to bake a little faster — start checking about 5 minutes earlier than the recipe says.
Example: a sponge written with 200g flour and 4 eggs becomes 265g flour and about 5.5 eggs (match the butter and sugar to the flour weight, as usual).
Cake tin capacities and scaling factors
Every factor below is measured against the most common recipe tin of all, the 20cm (8") round. Capacities are worked out from base area and a standard cake depth — the same figures the converter uses.
| Tin | Capacity | Factor vs 20cm round |
|---|---|---|
| Round 15cm (6") | 1.3 litres | × 0.56 |
| Round 18cm (7") | 1.9 litres | × 0.81 |
| Round 20cm (8") | 2.4 litres | × 1.00 |
| Round 23cm (9") | 3.1 litres | × 1.32 |
| Round 25cm (10") | 3.7 litres | × 1.56 |
| Round 28cm (11") | 4.6 litres | × 1.96 |
| Round 30cm (12") | 5.3 litres | × 2.25 |
| Square 15cm (6") | 1.7 litres | × 0.72 |
| Square 18cm (7") | 2.4 litres | × 1.03 |
| Square 20cm (8") | 3.0 litres | × 1.27 |
| Square 23cm (9") | 4.0 litres | × 1.68 |
| Square 25cm (10") | 4.7 litres | × 1.99 |
| Loaf tin 1lb (450g) | 0.9 litres | × 0.38 |
| Loaf tin 2lb (900g) | 1.8 litres | × 0.76 |
| Rectangular 20 × 30cm (traybake) | 3.0 litres | × 1.27 |
Why capacity, not diameter, does the maths
A tin that is only 3cm wider sounds like a small change — but area grows with the square of the width, so a 23cm tin holds about a third more mixture than a 20cm one. Pour a 20cm recipe into it unscaled and you get a thin, dry sponge; pour it the other way and the middle never cooks through. Scaling by capacity keeps the depth of mixture — and therefore the bake — exactly as the recipe writer intended.
Two practical rules cover most swaps. First, the 10% rule: if the converter gives a factor between 0.9 and 1.1, change nothing. Second, the shape rule: square tins hold about a quarter more than round tins of the same stated size, so never swap those like for like. And if you are choosing between loaf tins, our 1lb vs 2lb loaf tin size guide has the exact dimensions and the water test for identifying a tin you already own.
Frequently asked questions
How do I convert a recipe from a 20cm tin to a 23cm tin?
Multiply every ingredient by about 1.3. A 23cm round tin holds roughly a third more mixture than a 20cm one, so a 200g-flour sponge becomes about 265g flour, with butter, sugar and eggs scaled to match. The converter above gives the exact figure for any pair of tins.
Can I use a square tin instead of a round one?
Yes — but a square tin holds more than a round tin of the same width (roughly 27% more for the same measurement, because of the corners). A 20cm square is closer to a 23cm round. Use the converter rather than swapping like for like.
Do I need to change the baking time when I change tins?
Usually a little. A bigger tin means a thinner layer of mixture, which bakes faster — start checking about 5 minutes early. A smaller, deeper tin needs longer, roughly 10 extra minutes, and may need the top covered with foil near the end.
What if the factor is only slightly more or less than 1?
Within about 10% either way, do nothing: use the recipe as written. Cake mixture is forgiving at that level, and the difference disappears in the baking. The converter says so when your two tins are that close.
How do I scale a 2lb loaf recipe down to a 1lb tin?
Halve everything. A 1lb loaf tin holds about 900ml and a 2lb about 1,800ml, so the factor is exactly 0.5. Alternatively, make the full mixture and bake the spare half as a few fairy cakes — nothing wasted.
Put it to work
Three bakes that regularly meet the wrong tin: