This calculator will help you figure out how many servings you might get from a round cake based on the cake's size and the portion size you will be serving your guests.
The formula to calculate this is to divide the cubic inches of serving size into the cubic inches of cake. Let's say our cake is 8 inches in diameter and 4 inches in height. Half of the cake's diameter is it's radius (8/2=4). Square the radius (4*4=16). Multiply that by it's height (16*4=64). Multiply that by pi (64*3.1416=201). So we know our cake is 201 cubic inches.
Now we figure out the serving size of our cake slices. For party cakes, servings are usually 2"x2". For weddings, portions are usually 1"x2". So using the wedding portion size, we multiply the width by length by height to get the cubic inches of each serving (1*2*4=8). Our wedding guests will receive cake slices that are each 8 cubic inches.
By dividing the cake by the slices (201/8=25), we should get approximately 25 servings from our 8 inch cake that is 4 inches high.