What this calculator works out
Compares the tin a recipe asks for with the tin you actually own, and tells you how much to scale the ingredients by.
The trap is assuming that the same number means the same capacity. A 20 cm square tin holds about 27 per cent more than a 20 cm round one, because of the corners. Bake a round-tin recipe in a square tin of the same width and you get a noticeably shallower cake.
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Before you calculate
- Measure across the inside of the tin at the base. Sloping sides mean the top measurement is larger.
- This compares tins of the same depth. If your tin is deeper or shallower, the batter depth changes, and so does the baking time.
- A square tin holds noticeably more than a round tin of the same width — about 27 per cent more.
A 20 cm round tin has an area of π × 10², which is 314 cm². A 20 cm square tin is 20 × 20, which is 400 cm². The corners are the difference. It is why recipes often say a 20 cm round tin or an 18 cm square one — those two are much closer in capacity than the same number in both shapes.
How this calculator works
Round area = π × (diameter ÷ 2)²
Square or rectangular area = length × width
Ingredient factor = new area ÷ original areaComparing base areas rather than volumes works because the batter depth stays roughly the same when the recipe is scaled to fill the tin. That is what keeps the baking time close to the original.
If instead you pour the original quantity into a larger tin, the mixture is shallower and bakes faster; into a smaller tin, deeper and slower. The calculator flags both cases.
Common tin equivalents
| Tin | Base area | Roughly equivalent to |
|---|---|---|
| 15 cm round | 177 cm² | 13 cm square |
| 18 cm round | 254 cm² | 16 cm square |
| 20 cm round | 314 cm² | 18 cm square |
| 23 cm round | 415 cm² | 20 cm square |
| 25 cm round | 491 cm² | 22 cm square |
| 20 cm square | 400 cm² | 23 cm round |
| 23 cm square | 529 cm² | 26 cm round |
| 2 lb loaf tin | approx. 21 × 11 cm | 18 cm round |
Worked example
A recipe calls for a 20 cm round tin and you own a 20 cm square one:
- Round: π × 10² = 314 cm²
- Square: 20 × 20 = 400 cm²
- Factor: 400 ÷ 314 = 1.27
Two options. Scale the recipe up by 1.27 to fill the square tin at the same depth, keeping the baking time about the same. Or bake the original quantity in the square tin, accepting a shallower cake that will be done perhaps ten minutes sooner.
Common mistakes
- Assuming a 20 cm round and 20 cm square are interchangeable. The square holds over a quarter more.
- Ignoring depth. A deeper cake needs a lower temperature and a longer bake.
- Measuring the outside. Measure the inside of the base.
- Filling a tin too full. Two thirds is the usual maximum for a rising batter.
- Keeping the same time after changing tin. Check earlier for a shallower cake, later for a deeper one.
- Forgetting loose-bottomed and springform tins differ. A springform of the same nominal size is often slightly larger inside.
Frequently asked questions
Can I use a square tin instead of a round one?
Yes, allowing for the difference in capacity. A square tin holds about 27 per cent more than a round tin of the same width, so either scale the recipe up by roughly a quarter, or accept a shallower cake and check it earlier. As a rule of thumb, a round tin is roughly equivalent to a square tin 2 cm smaller.
What if my tin is deeper than the recipe assumes?
Depth changes the bake more than most people expect. A deeper mixture needs longer for the heat to reach the centre, and the outside can dry out before it does. Drop the oven temperature by 10 to 20°C and extend the time, checking with a skewer. Shallower mixtures cook faster and need checking early.
How full should I fill a cake tin?
About two thirds for a batter that rises. Fuller than that and it can dome heavily or overflow. If the calculator suggests scaling up and your tin is not deep enough to take it, use two tins or a larger one rather than overfilling.
Does this work for loaf tins?
Use the rectangular option with the internal length and width. Loaf tins are sold by capacity rather than dimensions, so a 2 lb tin is roughly 21 by 11 cm and a 1 lb tin roughly 16 by 10 cm, though brands vary. Measure yours.
Why not compare volumes instead of areas?
Because when you scale a recipe to a different tin you normally keep the depth of the batter about the same, which is what keeps the baking time close to the original. Comparing base areas captures that directly. Volume would only be the right comparison if you intended to change the depth as well.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
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References
These organisations publish the guidance and figures behind this page.
- Food Standards Agency — UK food safety guidance for home cooks and businesses
- NHS — guidance on eggs, and on reheating and storing cooked food safely
- British Standards Institution — British Standards for metric measurement and the conversion factors used between millimetres and inches
Tin sizes vary between manufacturers. Measure your own rather than trusting the label.