Length times width times thickness divided by 144 inches. all the calculations are done in inches. example 4.5 inches wide by 77 inches long = 346.50 inches times 4/4 thickness 1 inch = 346.50 again divide this by 144 = 2.40 board feet. This example is for one of my bed boards. I figure about 27 board feet for my 53 1/2 ton.
Thickness for the last mutiplication are as follows=
5/4 wood multiply by 1.25, 6/4 wood multiply by 1.5, 8/4 multiply by 2, 4/4 multiply by 1

6.5 width x 77" length = 500.50 x 1 [4/4] =500.50 divided by 144= 3.48 board feet
6.5 x 77=500.50 x 1.5 = 750.75 divide by 144= 5.21 board feet

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