Cake tin & baking pan size converter

Swap a round cake tin for a square one, or a rectangular traybake pan for a round tin, by matching baking area — and see how much to scale the recipe when the tin you own is bigger or smaller.

Round tin diameter (cm)

Result

Equivalent square tin side

Baking area

How to use this tool

Round → square: enter the round tin's diameter to get the square tin with the same area.
Rectangle → round: switch direction and enter the pan's length and width.
Changing tin size instead? Compare the two baking areas; the ratio is the multiplier to enter in the recipe scaler.

Assumptions

Matches baking area only, not depth — use a tin of similar depth, or adjust baking time if depths differ noticeably.
Measurements are internal; the outside of a tin is a few millimetres bigger.

Technical details: what the fields mean

Round tin diameter roundDiameter: Diameter of the round tin, measured across the inside top edge. 8 inches ≈ 20cm, 9 inches ≈ 23cm.

Methodology

Round area = π × (diameter ÷ 2)². Equivalent square side = √area. Rectangle area = length × width, and equivalent round diameter = 2 × √(area ÷ π). Matching area keeps the batter at about the same depth, so baking time stays similar.

Worked example

Round → square: a 20cm round tin

Inputs: Round tin diameter: 20 cm

Result: 17.7cm square (same ~314cm² area). A 20cm square tin would be 27% bigger and give a thinner cake.

Rectangle → round: a 20 × 20cm brownie pan

Inputs: Pan length: 20 cm, Pan width: 20 cm

Result: 400cm², the same as a 22.6cm round tin.

Rectangle → round: a 33 × 23cm traybake pan

Inputs: Pan length: 33 cm, Pan width: 23 cm

Result: 759cm², equivalent to a 31.1cm round tin — so a traybake recipe roughly matches two 22cm round tins.

Recipe for a 20cm round tin, but you only have 23cm

Inputs: Round tin diameter: 20 → 314cm²; Round tin diameter: 23 → 415cm²

Result: 415 ÷ 314 ≈ 1.32. Multiply every ingredient by 1.32 in the recipe scaler to keep the same depth.

Practical guidance

A cake in a different tin of the same area usually needs a similar baking time — check for doneness a few minutes early, since square corners cook faster than curves.
If you can't match area, keep the batter depth similar and expect a deeper cake to need longer at the same or a slightly lower temperature.
Fill tins to about two-thirds; don't scale a recipe up so far that batter reaches the rim.

Common mistakes

Matching a round tin's diameter directly to a square tin's side — a 20cm square holds about 27% more than a 20cm round.
Scaling a recipe by the change in diameter instead of area: going from 20cm to 23cm is 15% wider but 32% more batter.

FAQs

What square tin is equivalent to a 20cm round tin?

About 17.7cm × 17.7cm, matching the same baking area.

What round tin is equivalent to a 20cm × 20cm square pan?

About 22.6cm in diameter, matching the same 400cm² area.

How do I adapt a recipe to a bigger tin?

Divide the new tin's area by the old tin's area and multiply every ingredient by that number — for 20cm to 23cm round tins it's about 1.32.

Last reviewed: 2026-09-25

Matches area only — depth, tin material and corner heat distribution mean baking time may still need a practical check.

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