Nominal vs. Actual Lumber Sizes Explained
One of the first surprises in woodworking and building is that a '2×4' does not measure two inches by four. The names are nominal — they describe the rough size before drying and planing — and the finished board is always smaller. Knowing the real numbers is essential for accurate cut lists and joinery.
Why the names don't match the wood
Softwood is named by its green, rough-sawn dimensions. After it is dried and surfaced smooth on four sides (S4S), it shrinks and loses material to the planer, so the finished piece ends up smaller than its label. The reductions are standardised across the industry, which is why a 2×4 is the same actual size wherever you buy it.
A nominal one-inch board finishes at ¾ inch thick. Nominal two-inch framing finishes at 1½ inch. Widths lose more as they grow: a nominal 6 is 5½, a nominal 10 is 9¼, and a nominal 12 is 11¼.
Common sizes at a glance
A 1×6 is actually ¾ × 5½ inches. A 2×4 is 1½ × 3½. A 2×6 is 1½ × 5½. A 2×10 is 1½ × 9¼. The thickness of nominal two-inch stock stays 1½ inch regardless of width, while the width follows the pattern of losing roughly ½ to ¾ inch from the nominal figure.
Sheet goods behave differently again. A sheet labelled ¾ inch may really measure about 23/32, and plywood is often a hair under nominal — which matters when you cut grooves and rabbets to receive it.
Designing around real sizes
The fix is simple: always work from actual dimensions, never nominal ones. Three 2×4s laid flat are 4½ inches thick, not six. A stud wall's real cavity is 3½ inches deep. Stacking nominal numbers is how a cut list ends up wrong and a part fails to fit.
Our nominal vs. actual reference converts the labels to true sizes instantly, and the cut list generator works from real dimensions so your layout is honest from the start.
Frequently asked questions
Why is a 2×4 actually 1½ × 3½ inches?
The name is the rough green size; drying and planing to a smooth finish reduce it to the standard 1½ × 3½ inch actual dimension.
Does plywood match its labelled thickness?
Often not exactly — a ¾-inch sheet may measure around 23/32 inch. Measure the actual thickness before cutting joinery to fit it.