Lumber size converter
Find the actual size of a nominal board dimension (a 2×4 is really 1½ × 3½ in), or the nominal size for a measured board.
3 1/2in actual
That's about the same as a 4×4 post.
What it means
How big is that board really?
Dry, surfaced softwood dimensions (PS 20). Convert each dimension separately: a 2×6 is 1½ by 5½ in. Hardwoods and rough-sawn lumber differ.
Every unit
That dimension in every unit
Learn
Three things worth knowing
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Minus ½, then minus ¾
2 → 1½, 4 → 3½, 6 → 5½; 8 → 7¼, 10 → 9¼, 12 → 11¼.
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Shrinking boards
In the early 1900s a 2×4 was nearly 2 by 4 inches; standards in 1964 fixed today’s dried, planed sizes.
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Buying by nominal
Three 2×4s stacked flat are 4½ in thick, not 6. Plan with actual sizes.
Shortcuts
Popular board size conversions
Background
About nominal lumber
A 2×4 is not 2 by 4 inches. Lumber is named by its rough-sawn size; drying and planing take it down to 1½ × 3½ in (38 × 89 mm). Up to 6 in, subtract ½ in; from 8 in, subtract ¾ in.