This nominal versus actual size reference converts the lumber dimensions printed on the label into the real measurements you will get. A '2×4' is not two inches by four — it is 1½ by 3½ once dried and surfaced. Knowing the true sizes is essential for accurate layout and joinery.
Why nominal and actual differ
Softwood is named by its rough, green dimensions before it is dried and planed smooth on four sides (S4S). Drying shrinks it and surfacing removes material, so the finished board is smaller than its name. The differences are standardised: a nominal 1-inch board finishes at ¾ inch, and nominal 2-inch framing finishes at 1½ inch. Widths lose more as they get wider — a nominal 6 finishes at 5½, a nominal 12 at 11¼.
Sheet goods behave differently again: a sheet sold as ¾ inch may actually measure 23/32, and plywood thicknesses are often a hair under nominal, which matters when cutting dadoes and rabbets to fit them.
Designing around real sizes
Stacking actual dimensions, not nominal ones, is what keeps a design honest. Three '2×4s' laid flat are 4½ inches thick, not six. A stud wall's real cavity depth is 3½ inches. Working from actual sizes from the start prevents the small surprises that throw out a cut list or leave a part that does not fit.
You are designing a workbench top from laminated 2×4s laid on edge.
- A nominal 2×4 actually measures 1½ in × 3½ in.
- Laminating eight of them face to face: 8 × 1½ in = 12 in wide.
- The finished top is 3½ in thick (the actual width of a 2×4 on edge).
Eight 2×4s give a 12 in wide, 3½ in thick top — plan to real, not nominal, sizes.
