Use a lumber measuring chart by locating the nominal size of the lumber you need and comparing it to the actual size of the lumber. For example, locating the very common nominally sized 2-by-4 piece of lumber on a measuring chart shows that its actual size is 1.5 by 3.5 inches. A measuring chart only gives information about a piece of lumber's height and width, as boards are available in many different lengths.
Nominal and actual dimensional lumber sizes vary because of advances in the timber processing industry during the 20th century. In the past, a 2-by-4 was actually 2 inches by 4 inches when it was purchased. At the time, however, sawmills usually produced lumber that had warping, surface irregularities and other imperfections that required builders to plane and mill the boards for a more finished look. After milling, a board took on dimensions closer to those of a modern 2-by-4.
Some lumber measuring charts also list the weight per foot of each size of dimensional lumber. Using these figures requires caution, however, as lumber, being a natural product, can vary considerably in weight per foot between boards. Weight-per-foot measurements typically only apply to softwoods like pine, hemlock and other species commonly used in framing and building applications. Hardwoods like maple, mahogany and ash are much heavier than softwoods.