Knowing how to figure out lumber weight comes in handy for a number of applications. If you need to haul a stack of two-by-sixes in your own trailer or figure out how much you might save by letting a load of green wood dry out before shipping, you'll want to make the acquaintance of the formula you need. Your calculations can also help you find out the weight of the structure of a floor or deck and how much it can support. You can use online calculators to arrive at the weight -- but it’s handy to learn how to calculate the figures from scratch.
Gather the three variables you’ll need to make your calculations: the specific gravity of the type of wood you want the weight of, whether it is green wood or kiln-dried, and its moisture content.
Consult a table such as the USDA's "Specific Gravity and Other Properties of Wood and Bark for 156 Tree Species Found in North America" to obtain the wood’s specific gravity. Test the moisture content with a moisture meter. Look for “KD” in green or black ink on the lumber’s markings to determine if it's dried or green wood. For this example, say you want to know the weight of Southern yellow pine, with a specific gravity of 0.59 for kiln-dried lumber, and with a moisture content of 12 percent.
Plug the moisture content and specific gravity into this formula, where G is specific gravity and m.c. is moisture content: Pounds per cubic foot = 62.4 x [G / (1 + G x 0.009) x (m.c.)] x [1 + m.c./100]. In this case, the formula would read: 2.4 x [0.59 / (1 + 0.59 x 0.009) x (.12)] x [1 + .12 /100], which yields a result of 34.4 pounds per cubic foot.
Figure out the dimensions of one of the units you wish to weigh -- say, a nominal 2-by-12 beam measuring 16 feet long, remembering that a 2-by-12 is actually 1 1/2 inches by 11 1/4 inches. The beam measures 3,240 cubic inches, based on 1.5 x 11.25 x 16 x 12. Southern pine weighs 34.4 pounds per 1 cubic foot, and 1 cubic foot is 1,728 cubic inches. Thus, the beam in this example measures 1.8 cubic feet, times 34.4 pounds per cubic foot, or 64.5 pounds.
If you have a spreadsheet, you can plug this formula into it to calculate the weight: =62.4_(0.55/(1+0.55_0.009_0.12)_(1+(0.12/100))), substituting the specific gravity of your wood for the "0.55" figure.
If you don't have a spreadsheet, you can paste this formula into the URL bar of a search engine. The search engine will automatically produce the same calculation a spreadsheet would.