Is there a rule on how to adjust sponge cake recipes to fit different shape / size tins?

[fedupofpeppa]

I have a sponge recipe I like to use but it's for a circular tin. I want to use the mix but cook a big square cake instead which i ideally would like to be a bit thinner than normal so I can build a birthday cake design.

Is there a rule on how long to cook cakes for and on what temperature for different size tins?

Oh I'm curious about this. I never have the right size tin for the recipe!

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.
Approximate conversions

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)

Thank you. So same temperature bit turn it. That's helpful. As it's a bit shallower I will just keep an eye on it as will probably cook quicker.

PreAdvent's rule of thumb is about right.

Area of a square cake = length of side squared.
Area of a circle = TT (pie) r squared.

With larger cakes 10" (+) it may be best to double line and cover the top to prevent the extremities cooking too quickly and give the centre of the cake a chance of catching up.