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.
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: