Why is a 2×4 not a 2×4?

Ever wondered why a 2×4 isn’t actually two inches thick and four inches wide? A 2×4 should be called a 1½ x 3½, but that doesn’t exactly roll off the tongue! The route to the current dimensions of the 2×4 is circuitous so there’s not one single reason for the difference, but a number of factors that culminated over time.

Before industrialized lumber production, trees were felled close to the mill and sized according to the needs of the local carpenters. As the demand for lumber grew it led to a reliance on the hinterland for lumber (almost exclusively white pine – white pine would only apply to limited geographic areas) and trees were floated down rivers and across lakes to the major centers for use mills for processing. This meant the lumbermen were no longer cutting to the carpenter’s specifications. 

When the railroad made transportation of lumber easier, new species of wood were introduced on the market. “Competition occurred among regions, or more fundamentally, among species,” Oliver Curtis tells the Harvard Design Magazine. “For example, southern pine forests called “pineries”—consisting of shortleaf pine, slash pine, longleaf pine, loblolly pine, and other less common species (collectively known as southern yellow pine, SYP)—were marketed against western Douglas fir, western hemlock, and spruce forests. Buyers needed convincing to purchase lumber from unfamiliar species, and hence marketing focused on differentiating attributes such as ease of use, workability, strength, load capacity, and grain movement.”

It was the purchaser who was responsible for the lumber transport costs, so smaller, kiln-dried lumber began to increase in popularity. Lumber classification varied from manufacturer to manufacturer. A move in the mid 19th century from timber-braced to stick and balloon framing techniques further increased the popularity of small dimensional lumber like 2x4s. It was in April 1919, at the first American Lumber Congress, that members called for a uniformity of lumber sizes so that costs could be compared. It wasn’t until 1964 that the congress set “size standards, maximum moisture content, and nomenclature.” By then wood shortages and a need to compete with wood alternatives had seen a reduction in the size of a 2×4 to 1½ x 3½.