Outside Cross-sectional area Wall
dimensions --------------------- thickness
(inches) Inside Outside
----------- ---------- -------- ----------
Square Square
inches feet Inches
4½ by 8½ 23.6 0.26 5/8
4½ by 13 38.2 .41 5/8
7½ by 7½ 39.1 .39 5/8
8½ by 8½ 52.6 .50 5/8
8½ by 13 80.5 .78 3/4
8½ by 18 109.7 1.10 7/8
13 by 13 126.6 1.20 7/8
13 by 18 182.8 1.70 7/8
18 by 18 248.1 2.30 1-1/8
20 by 20 297.6 2.60 1-3/8