How much does lumber weigh?

It depends upon the species as weights can very greatly, even among softwoods, such as pine, fir, or especially cedar or cypress. Additionally, moisture content and treatments can impact the finished weight. Southern Yellow pine, fresh cut varys from 36 lbs/cubic foot (20% moisture) to 53 lbs/cubic foot for fresh cut (green). Pressure treating also adds additional moisture, thus weight.

If the lumber is from a lumber yard, not treated, kiln dried, dimensional pine lumber would typically weigh 37 lbs / cubic foot, so a 16' 2 x 4, actually 16' 1.5" x 3.5" would be:

1.5 X 3.5 X 192 = 1008 cubic inches

1728 cubic inches in a cubic foot so:

1008 / 1728 = 0. 583 cubic feet

weight of pine = 37 lbs/cubic foot X 0.583 = 21.58 lbs for a 16' 2" x 4".

or

1 1/2" = 0.125 cubic feet

3 1/2" = 0.292 cubic feet

.125 X .292 X 16 = 0.583