The amount of lumber is expressed in terms of Board Feet.  A Board Foot is a piece of lumber 1 foot (12 inches) long, 1 foot wide and 1 inch thick which is equal to 144 cubic inches.  If all dimensions are expressed in inches, the amount in board feet is equal to:
(l x w x t)/ 144.

Measuring lumber in board feet, L x W x thickness.

In the Philippines, it is customary to express width and thickness in inches while we use feet to express length.  So a 2 x 3 x 10 means 2 inches thick by 3 inches wide and 10 feet long piece of lumber  computed as (t x w x l)/12.

Example 1a:     How many board feet is 2” x 3” x 10’ ?

          ( 2 x 3 x 10) / 12 = 60/12 = 5 bd. ft.

To simplify calculations, factors making up 12 should be cancelled as early as possible instead of multiplying out the thickness by the width and then by the length before dividing it by 12

Example 1b:      How many board feet is 2” x 3” x 10’?

2 x 3 x 2 x 5= 5 bd. ft.

Example 2:     2” x 6” x 8’  =  2” x 6” x 8’  =  8 bd. ft.

Example 3:      2” x 5” x 12’  =  2” x 5” x 12’  =  10 bd. ft.

Example 4:      1 ½” x 2” x 8  =  1 ½” x 2” x 4 x 2  =  2 bd. ft.

Example 5:      1 ½” x 2” x 10  =  1 ½” x 2” x 2 x 5  =  5/2 bd. ft.

1 ½” x 2” x 2 is only 6 so we still have to divide by 2

Example 6:      2” x 2” x 10‘  =  2” x 2” x 10‘  =  10/3  =  3.33 bd. ft.

2 x 2 is only 4 so we still have to divide by 3

We can include the number of pieces in the calculations.

Example 7:      3 pcs – 2” x 2” x 14’  =  3 pcs – 2” x 2” x 14’  =  14 bd. ft.

Example 8.  4 pcs – 1 ½” x 2” x 10= 4 pcs – 1 ½” x 2” x 10 = 10 bd. ft.

With this method you won’t ever have to use a calculator in computing lumber.