Tin and pan size converter
Work out how much batter a different tin needs, from base area — and why that does not give you a new baking time.
What it answers
I have a different tin from the one the recipe specifies. How much batter do I need?
What it assumes
- Quantity scales with the BASE AREA of the tin, for a batter poured to a similar depth.
- Both tins are filled to a comparable depth. Where they are not, the bake changes as well as the quantity.
- An inch is exactly 25.4 mm, so the unit conversion itself carries no uncertainty.
- Tins sold as the same nominal size vary between manufacturers. Measure yours if the answer matters.
Your tins
Common tin sizes, if you know yours by name
- 15 cm / 6" round
- 18 cm / 7" round
- 20 cm / 8" round
- 23 cm / 9" round
- 25 cm / 10" round
- 20 cm / 8" square
- 23 cm / 9" square
- 900 g / 2 lb loaf tin (approx. 21 × 11 cm)
- 450 g / 1 lb loaf tin (approx. 16 × 10 cm)
- Traybake / 20 × 30 cm
Tins sold as the same size vary between manufacturers. Measure yours if the answer matters.
The answer
Scaling factor: 1.32
Your tin is larger. Multiply every ingredient by 1.32 — or accept a shallower bake, which will cook faster and brown more.
Scale a quantity
Enter one ingredient weight at a time. The same factor applies to all of them.
The working
How this was calculated
Area of the round tin the recipe specifies
- Formula
A = π × (d ÷ 2)²- With your numbers
A = π × (20 ÷ 2)² = 314.2 cm²- Result
- 314.2 cm²
Area of the round tin you have
- Formula
A = π × (d ÷ 2)²- With your numbers
A = π × (23 ÷ 2)² = 415.5 cm²- Result
- 415.5 cm²
Scaling factor
- Formula
F = A(new) ÷ A(original)- With your numbers
F = 415.5 ÷ 314.2- Result
- 1.322
What that means for the quantity
- Formula
new quantity = original quantity × F- With your numbers
every ingredient × 1.322- Result
- 132% of the original
What the symbols mean
- A — base area of the tin, in square centimetres
- d — diameter of a round tin
- s — side length of a square tin
- l, w — length and width of a rectangular tin
- F — scaling factor — how much the quantity changes by
Where multiplying stops being enough
Eggs do not divide neatly. Scaling a three-egg cake by 1.4 asks for 4.2 eggs, and the usual answers — round, or beat an egg and weigh part of it — are not equivalent. Weighing is the accurate one.
Chemical leavening does not scale linearly at the extremes. A larger, deeper batter takes longer to set, so the same proportion of raising agent has longer to act and can over-expand before the structure fixes.
Salt and yeast are already percentages of flour weight, so they scale correctly by arithmetic — but fermentation time does not. A larger dough mass holds its own heat and ferments faster than a small one at the same room temperature.
Crust is a surface property and volume is not. A scaled-up bake has proportionally less surface, so it browns less for its size and keeps differently.
Why there is no new baking time here
This is the number everyone wants and the one that cannot be derived from area. Time depends on how deep the batter sits, on the temperature at which that particular formulation sets, on what the tin is made of and on how your oven actually behaves — none of which a tin measurement contains.
What is reliable: a shallower bake finishes sooner and browns more, a deeper one takes longer and can set at the edges before the centre is done. So start checking earlier than the recipe says if your tin is wider, and expect to add time if it is narrower — and judge by the bake rather than by the clock.
What it cannot tell you
IT WORKS IN CENTIMETRES OR INCHES, and the unit conversion carries no uncertainty — an inch is exactly 25.4 mm. What carries uncertainty is the tin: two tins sold as the same nominal size differ between manufacturers, and a tin measured across the top rim is wider than the same tin measured at its base. Measure yours where the batter will actually sit.
IT WILL NOT GIVE YOU A NEW BAKING TIME. Time depends on batter depth, on the temperature at which that formulation sets, on the tin material and on your oven — none of which a tin measurement contains. Every conversion chart that offers one is guessing.
Equal area does not mean equal bake. A 20 cm round and an 18 cm square have almost the same area and do not behave identically, because heat reaches the centre over a different distance.
Scaling is linear and baking is not. Eggs do not divide neatly, chemical leavening behaves differently in a deeper batter, and crust is a surface property while volume is not.
Depth is used only to warn you. It never changes the quantity, because there is no defensible arithmetic linking depth to a corrected weight.
It knows nothing about tin material. A dark tin, a glass dish and a loose-bottomed tin of identical dimensions bake differently.