A fellow blogger The Ordinary Cook recently asked about square to round cake tin conversations so I thought it would be nice to share the information with everybody.
Square cake tin to round cake tin conversions
As a general rule of thumb, a square tin holds approximately 25% more than a round tin of the same size; therefore reduce the round cake tin size between 2cm and 2.5cm (±1 inch) to arrive at the equivalent size of a square tin.
Approximate conversions
Round Tin Square Tin
15cm or 150mm (6 inch) 13cm or 130mm (5 inch)
18cm or 180mm (7 inch) 12.5cm or 125mm (5 inch)
20cm or 200mm (8 inch) 18cm or 180mm (7 inch)
23cm or 230mm (9 inch) 20cm or 200mm (8 inch)
25cm or 250mm (11 inch) 23cm or 230mm (9 inch)
28cm or 280mm (11 inch) 25.5cm or 255mm (10 inch)
30cm or 300mm (12 inch) 28cm or 280mm (11 inch)
Also, If you are using a square tin for a round tin recipe, keep the oven temperature the same, and turn the cake halfway through baking, as the corners tend to cook faster than the middle.
For additional conversions, have a look at:
The Complete Book 
Thank you Mandy, this is a brilliantly useful post.
Kath
Thanks Kath, glad it is so useful 🙂
I want to convert the ingredients for a 12″ round cake tin to a 10″ square tin. How do I calculate it?
Hi Charles, thanks for stopping by. It depends on the ingredients, if there are even or odd amounts of eggs etc. Some recipes (for whatever reason) don’t work well with being halved or doubled and changing a recipes ingredients quantities also changes the baking time. I would rather suggest making up the mixture as per the recipe and bake the extra portion in a separate smaller tin or rather muffin tins. Hope this helps. Have a super day. 🙂
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Thanks so much for the linky love. Have a super day. 🙂 xo