Recipe scaling

Cynthia, I believe you're thinking of 2(pie)r

All Recipes has a nice conversion table:
http://allrecipes.com/howto/cake-pan-size-conversions/

aj, Pi r squared
that's exactly what i've always done as well!

I use online volume calculators. A round cake pan is basically a cylinder, and a square one a cube:

http://www.online-calculators.co.uk/volumetric/cylindervolume.php
http://www.online-calculators.co.uk/volumetric/cubevolume.php

If you know how much batter will fill 2 6" pans, given the volume of the pan you want to use as well as that of the 6" round pans, you can easily calculate the percentage increase to fill the square pan.

Or, if you don't know the volume by cups and the pans are approximately the same height, you can calculate surface area using basic geometry: for the round pans, multiply the radius (1/2 the diameter) by itself and then multiply that by 3.14159. Compare that to the area of the square pan using simple algebra. That's how I've done it for years. Works every time. ;o)

One way to do this is by starting with the cup measure for each pan. a 6"x2" round cake pan holds 3.75 cups. An 8" square (x2") holds 8 cups. Your recipe needs pans that will hold 11.25 cups. So to put it into 2 8" square pans you might want increase your recipe by half