Dimensional Lumber Sizes: Nominal vs Actual

A nominal size names a piece as it leaves the saw. Planing, which the standard calls dressing, takes the rough faces off: rough lumber must stay at least 1/8 inch over the dressed minimum, and the dressed minimum is what the tables on this page give.

Drying takes the rest. Dry lumber holds 19% moisture or less; anything wetter is green, and green pieces are planed fuller so they shrink to roughly the dry size, which the USDA Wood Handbook places near 15% moisture. A nominal 2 in thickness is 1-1/2 in dry and 1-9/16 in green.

The trade kept the old names. The Wood Handbook notes that lengths are sold at their real measure while thickness and width keep nominal names, and the standard counts board feet from the nominal size, so a 2x4 tallies as 2 by 4 whatever it measures.

Inches are the official figures in the standard; its millimeters are 25.4 times the inch size, given for comparison. The board foot calculator works from the same nominal sizes.

Table 5 of PS 20-25 gives redwood and two cedars, western red and northern white, their own green widths. From nominal 5 in up they come out smaller than Table 3's green widths for other species: a green cedar 2x6 is 5-9/16 in wide where a green pine one is 5-5/8 in.

At nominal 3 and 4 in the two tables agree, and dry sizes and thicknesses are the same for every species. The converter above applies the exception when you pick green and the cedar group.

A 4x4 post is 3-1/2 in square dry and 3-9/16 in green; the Western Red Cedar Lumber Association prints its dressed 4x4 cedar post at 3-9/16 in, the width the fence post concrete calculator takes out of the hole. Master Halco lists its pine fence boards at actual widths of 3-1/2 and 5-1/2 in, the dry widths of nominal 4 and 6; the fence picket calculator counts boards by the width you enter.

A 5/4x6 deck board, nominal 1-1/4 in thick, dresses to 1 by 5-1/2 in. Our data holds no weights for sawn lumber, so this page lists none.