I have been asked to make a 5 tier cake with alternating circles and squares, starting at the top with a 6" circle.  As I was putting the pans together to figure the dimensions, it looks like I will end at the bottom with an 18" round (which I don't have a pan for).  Also, the bride doesn't need this much cake, so we were planning on a styrofoam bottom layer.  My cake store doesn't carry that size.  Soooo... my question for ya'll is, does anyone have a tried and true formula for the stacking of circles with squares and if I do indeed need an 18" bottom, where can I order a styrofoam round that size?  Thanks, Lisa

try to talk her into a smaller top tier like a 4" or at least a 5" -- and you can get great foam pieces at http://www.dallas-foam.com/

however -- you could do a 6"r  x 6"sq x 9"r x 9"sq x 13"r -- that's 5 tiers alternating round/square 170 servings

but a prettier graduation/silhouette would be start with a 5" round then a 6" square a 9" round a 10" square then a 14" round

Thanks so much!!

A 10" square would slightly hang off a 14" round.

Here's the table for Right Angle Isosceles Triangles:

4" 5.66 
5" 7.07 
6" 8.49 
7" 9.9 
8" 11.31 
9" 12.73 
10" 14.14 
11" 15.56 
12" 16.97 
13" 18.38 
14" 19.8 
15" 21.21 
16" 22.63 
17" 24.04 
18" 25.46

thank you cwr -- it fit my pans which of course are off a bit -- i'm glad you posted this