HELP!! What size sponge mix for a 12" x 9" square tin?

[UmBongo]

I have to make my son a football pitch cake for his birthday tomorrow and need to know how big a mix to put in it, and roughly how long to cook it? I defer to the power of mumsnet!

Have a look on a search engine for cake tin converter. I use one by the Cakeulator (don't download app though, that won't be helpful for what you need). Good luck!

Thank you - I have been playing with my cake tins and water to work out volume. I think I need 1.5 times the usual traybake - but I'll have to guess at how long to cook the damn thing for!

Bake it on a lower heat, maybe 130 fan and if it looks like it's going a bit too dark too quickly around the sides puts some parchment on the top and around the edges after 2 thirds of the baking time.
Hope it goes well!

2 eggs is enough for a 7"x10" tin, so I would have thought that 3 eggs would be enough.

Bit late for you, but you can convert any recipe by working out the volume difference (what you are doing with water, but with maths.;))
So you have a recipe that makes an 8" round cake and uses 4 eggs, say. Assume your cake is 3" deep when baked. Volume of a cylinder is pi (3.14) x radius x radius x depth. So 3.14 x 4 x 4 x 3 = 150.72
The volume you want is 12 x 9 x 3 (to get a 3" deep cake) = 324.
You can see already it's roughly double, but if you divide one by the other you get 2.14. If you multiply that by your eggs you get 8.6 - just round it up to 9.
I know it seems like a faff, but once you know how to do it you can convert any size to any other size and shape.