If the width of the material in inches and the linear length in feet are both known, convert the width to a fraction of a foot, then multiply width times length to find square footage. For example, the square footage of a board 6 inches wide and 6 feet long is represented by the formula W (.5 ft) x L (6 ft) = 3 sq. ft.
Linear feet are a measure of length, not area, so a key piece of information -- the width -- is needed to calculate square footage. The lumber industry typically sells boards by linear feet and doesn't take the width into account, so it must be added to the equation. The key is to ensure consistent units are used throughout the calculation.
With lumber this can be tricky, since the width is not always exact. For example, a typical 2x4 actually measures 1 1/2 inches by 3 1/2 inches, not 2 inches by 4 inches, so to calculate square feet, you must convert the 3 1/2-inch width into a percentage of 1 foot (in this case, divide 3.5 by 12 for a result of .29 feet). Rounding up is a good practice since there are slight variations in lumber sizes.
Alternatively, you can perform the calculations in inches, then divide the result by 144 (12 by 12) to determine square footage. For the 2x4 example, use the 3.5 inches, then convert the linear feet to inches. Multiply width times length, and divide that result by 144 to get the total square footage. This can be a useful method when measuring a room to estimate the lumber required for a floor or wall.