The number of portions cut from a cake depends on the how the caterer cuts it, but as a rule allow 1in square portions for buffet slices (usual for fruit cake) and 2in x 1in for dessert sized portions (usual for sponge). My standard cake tiers are 4 inches deep, so this will give you an idea of the finished size of portion.

If you want larger portions, say for a birthday cake cut in wedges, allow extra cake.

Guide to servings for celebration and wedding cakes
Size Buffet Portions Dessert Portions
Round Square
3" 9 4
4" 3" 12 6
5" 4" 16 8
6" 5" 24 12
7" 6" 34 17
8" 7" 46 24
9" 8" 58 28
10" 9" 70 35
11" 10" 95 47
12" 11" 115 57
13" 12" 137 68
14" 13" 150 75

To discuss ideas, please contact me on: lisa@gorgeous-cakes.co.uk, or phone 07925 109 717.