My math is strictly for "Dummies", but ... fill each pot, measuring as you go. If a 4" square pot takes 2 1/2 cups of mix and the round takes 2 cups you have a 5:4 ratio (this may all be wrong but it always works for me). So if you increase the size of the pot to 12 inch, you increase it 3 times, and 3x5 = 15 cups of soil. 3x4 = 12 cups. Or not. I use this little formula for everything and it mostly works with an adjustment here and there.
