If the material’s width in inches and linear length in feet are both known, convert the width to a fraction of a foot, then multiply the width by the length to determine the square footage. For instance, W (.5 ft) x L (6 ft) = 3 sq. ft. represents the square footage of a board that is 6 inches broad and 6 feet long.
Since linear feet are a measurement of length and not area, the width is required to determine square footage. Typically, the timber industry sells boards by linear feet and disregards the width, thus it must be added to the equation. The goal is to maintain unit consistency throughout the calculation.
This can be challenging with lumber, as the breadth is not always perfect. For instance, a common 24 measures 1 1/2 inches by 3 1/2 inches, not 2 inches by 4 inches, so to compute square feet, the 3 1/2-inch width must be converted to a percentage of 1 foot (in this case, divide 3.5 by 12 for a result of .29 feet). As there are modest variances in lumber sizes, rounding up is a prudent approach.
You can also repeat the calculations in inches and divide the result by 144 (12 by 12) to determine square footage. For the 24 example, use the 3.5-inch measurement, then convert linear feet to inches. Multiply width by length and divide by 144 to obtain the total square footage. This is a handy approach for estimating the amount of lumber needed for a floor or wall when measuring a room.