As a rule of thumb, a square tin holds about 25% more than a round tin of the same size. If you're using a square tin for a round tin recipe, keep the temperature the same, and turn the cake during baking, as the corners tend to cook faster than the middle.
Round tin Square tin
6 inch (15cm) 5 inch (13cm)
8 inch (20cm) 7 inch (18cm)
9 inch (23cm) 8 inch (20cm)
11 inch (28cm) 10 inch (25.5cm)