If you know how wide the material is in inches and how long it is in feet, you can turn the width into a fraction of a foot, then multiply the width by the length to get the square footage. For example, the formula W (.5 ft) x L (6 ft) = 3 sq. ft. represents the square footage of a board that is 6 inches wide and 6 feet long.
Linear feet are a way to measure length, not area, so to figure out square footage, you need to know the width. In the lumber business, boards are usually sold by the linear foot. The width is not taken into account, so it needs to be added to the equation. The important thing is to use the same units throughout the calculation.
This can be hard to do with lumber because the width isn’t always the same. For example, a typical 24 is actually 1 1/2 inches by 3 1/2 inches, not 2 inches by 4 inches. To figure out square feet, you need to figure out what percentage of 1 foot the 3 1/2-inch width is (in this case, divide 3.5 by 12 for a result of .29 feet). Rounding up is a good idea because lumber sizes vary just a little bit.
You can also do the math in inches and divide the result by 144 (12 by 12) to figure out the square footage. For the 24 example, use 3.5 inches and then change the linear feet to inches. To find the total square footage, multiply the length by the width and divide that number by 144. This can help you figure out how much wood you need for a floor or wall when you measure a room.