Hardwood lumber is typically sold per board foot when purchased in bulk. The board footage is calculated before the lumber is surfaced, planed or sanded.
Based on Volume
Board foot is a calculation of volume, allowing for numerous boards to be tallied up at the same time, even though they are various widths.
In the Rough
Calculations of board foot are typically based on rough, nondimensioned lumber. Rough lumber is exactly what the name implies; it appears fuzzy, with loose fibers, and is typically not consistent in width or thickness.
Thickness and Length
Typical thicknesses for rough lumber are 1 or 2 inches, also commonly referred to as 4/4, or four-quarter, and 8/4, or eight-quarter. Length is somewhat consistent for dimensioned or rough lumber, even though each piece may be a few inches longer or shorter than the stated size. The length is rounded to the nearest foot.
You can purchase dimensioned lumber -- lumber that has been surfaced or sanded -- using board-foot calculations, by rounding the thickness up to the nearest inch. It's a common practice and widely accepted by woodworkers. Prices for dimensioned lumber calculated in board footage typically reflect the price of sanding, planing or milling.
A single board foot of lumber measures 1 inch thick, 12 inches wide and 12 inches long. The formula for calculating board feet is first to multiply thickness by width by length -- in inches. Then divide the total by 144 for the total board footage of a given board.
Suppose you have a board that measures 1 inch by 12 inches by 96 inches. Multiply the three dimensions together to get 1,152. Divide 1,152 by 144 to get the actual board feet, which is 8.
Measure the width of individual boards, and write each measurement down on a piece of paper. Add the numbers together.
Measure the length of one board only. There might be slight differences in the length of individual boards; that's fine. Round it to the nearest 12-inch mark. For example, write 96 inches for an 8-foot board.
Multiply the numbers for total width, length and thickness. Divide that number by 144 to get the board footage of all the boards combined.
Calculate by Layer
Woodworkers may calculate board feet by layer. For example, the back of a pickup truck is about 48 inches wide. Using the formula, a single layer of hardwood -- 96 inches in length -- in the back of the truck equals approximately 32 board feet. If you stack four layers on the truck -- 4 x 32 -- the total board footage is 128.
It's not often that you purchase rough lumber thicker than 1 inch, and rough lumber doesn't typically graduate in fractions. Occasionally, however, if you want to purchase 8/4 rough lumber, which is 2 inches thick, calculate for a 1-inch thickness and then double the board footage.
Sometimes lumber is priced per MBF, or 1,000 board feet. If you want to buy 250 board feet of lumber, for example, and it costs $500 per MBF, divide 250 by 1,000 to get 0.25 MBF, then multiply that by $500 to get a price of $125.