Take your average 8 in cake tin and fill it with water to the level you normally fill it with cake batter. Measure the amount of water you have and multipy by two (if your recipe normally does two eight inch layers). You now have the number of cups in your recipe. Now, again using water) measure the number of cups you need to fill your new cake tin to the proper level with cake batter. Divide your new tin # of cups by your old tin number of cups and you will have an approximate multiple of your recipe for your new tin.
For example: if your recipe normally makes 8 cups of batter and your new tin wants 12 cups to fill it to the right level, 12 divided by 8 will tell you that you need approximately 1.5 recipes to fill your new tin.
Thanks. I'll try this
I've just sat down to do this... My old cake tin takes 4 cups and my new one takes 3 so do I divide 3 by 4? This would mean 3/4 of my recipe.
I also think my new cake pan will only do one layer rather than a thick cake that I cut in half so I need to do 4.
Shall I just make 3 times my recipe?
Put a collar of parchment around the inside of your pan so your batter won't run out the pan as it rises and use your four cup recipe. That will give you a hint as to wether you can get away with just baking two layers. Just remember that a deeper layer will cook longer and probably at a bit lower temperature.