What is the conversion factor for round to square tins?

Multiply the round tin diameter by 0.886 to get the equivalent square tin side length. This gives both tins the same base area, so the same recipe fills them to the same depth.

Why is the square tin smaller than the round tin?

A square has more area per unit of side length than a circle has per unit of diameter. A 20cm square tin has 400 cm² of area, while a 20cm round tin has only 314 cm². So you need a smaller square to match a round tin's area.

Can I use the nearest standard tin size?

Yes. If the calculation gives you 17.7cm but your tin is 18cm, that's close enough. The batter will just be very slightly shallower, which might shave a minute or two off baking time.

Does this work for deep tins and sandwich tins equally?

Yes, as long as both tins have the same depth. The calculation is based on base area only. If the depths differ, you'll need to account for volume differences too.

What standard tin sizes are available in the UK?

Common UK round tin sizes are 15cm (6in), 18cm (7in), 20cm (8in), 23cm (9in), 25cm (10in), and 30cm (12in). Square tins are available in similar sizes. Knowing the standard sizes helps you choose the closest match when converting between shapes.

BakeCalcs

Round to Square Tin Calculator

Find the equivalent square tin for a round tin (or vice versa) based on equal baking area.

Conversion Direction

Round Diameter

How We Calculate This

This calculator uses the equal-area principle to convert between round and square tins. Two tins with the same base area will hold the same amount of batter to the same depth.

The Maths

Round area: π × (d/2)² = π × r²
Square area: s²
Setting them equal: s² = π × r², so s = d × √(π/4) ≈ d × 0.886
Conversely: d = s ÷ 0.886 ≈ s × 1.129

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Last updated: February 2026

All calculations are based on standard ratios and conversions. Results may vary with specific ingredients, equipment, and conditions.