Assuming your layers are 1 3/4" thick, the total square inches of side to be covered is:

- 12" layer = Pi*d = 3.14159*12=37.699 use 37.7 sq in/in of height, for 1 3/4" = 65.97, use 66 sq in
- 10" layer = Pi*d = 3.14159*10=31.4159 use 31.4sq in/in of height, for 1 3/4" = 54.97 use 55 sq in
- 8" layer = Pi*8 = 3.14159*8 = 25.132, use 25.1 sq in/in of height, for 1 3/4: = 43.98 use 44 sq in

That totals 165 sq in. of sides to be covered

For the tops, the formula is A = Pi* r squared

- 12 " layer = Pi*6*6= 3.14159*6*6 = 113.097, use 113.1 sq in
- 10" layer = Pi*5*5 = 3.14159*5*5 = 78.539, use 78.5 sq in
- 8" layer = Pi*4*4 = 3.14159*4*4 = 50.265, use 50.3 sq in

That totals 241.8 sq in of top to be covered.

A combined total of 407 sq in

So now we decide on thickness, if we use 1/4", we need (165 + 242)* 1/4 = 407*1/4 = 407/4 = 101.75, use 102 cubic inches. There are 14.4375 cubic inches in 1 cup, so we will need 107/14.4375 = 7.065 cups of frosting. Obviously, if the frosting is only 1/8" thick, we'll need half as much and if it is 1/2" thick, twice as much.

So, the same numbers for a 9" two layer cake are:

- Top = Pi*r*r = 3.14159*4.5*4.5 = 63.617
- Middle = same as top = 63.617
- Side = Pi*d*h = 3.14159*9*3.5 = 98.960

Those total 226.194, use 226.2 or at 1/4" thick, about 56.55 cubic inches or just shy of 4 cups of frosting; 2 cups if 1/8" thick.

In other words, you will need 407/226.2 = 1.799, say 1.8 times as much frosting as for the two layer 9" cake. I'd say twice as much to be safe.

I hope that helps and **I REALLY NEED SOME BAKERS TO JUMP IN ANYTIME NOW!**