6x6, 20 ft — actually 139.7 × 139.7 mm

Same length, other sizes

The amount removed is fixed in absolute terms, so thin stock loses more in proportion. A 2×2 loses half an inch on both sides and keeps just 56 % of its nominal cross-section, while a 6×6 loses the same half inch and keeps 84 %. When comparing prices, compare actual cross-sections rather than board feet.

Three bands of shrinkage

Up to a nominal inch the loss is a quarter inch; from two to seven inches it is a half; from eight inches up it is three quarters. So a 2×4 is 1.5 × 3.5 inches and a 2×10 is 1.5 × 9.25. Thicker stock loses more because it shrinks more as it dries; thin stock has little to spare, so less is taken.

Board feet are counted on the nominal size

Board feet are nominal thickness × nominal width × length in feet ÷ 12. Because the count uses the nominal size, you get less wood than the figure you paid for: a 2×4 is only 66 % of nominal, and a 2×2 falls to 56 % — the thinner the stock, the larger the share planed away.

Nearby lengths

Same size, other lengths

Why the name never changed

At first the name matched the wood. Freshly sawn green lumber really was two by four inches, and only the finished size shrank as drying and planing became standard. The name was already embedded in yards, drawings and job sites, so it stayed — and the standard fixed the actual dimensions instead, so any supplier ships the same thing.

How to read this

• Subtract 1/4 inch up to one nominal inch, 1/2 from two to seven, 3/4 from eight up.
• So a 2×4 is 1.5 × 3.5 inches, that is 38 × 89 mm.
• Board feet are counted on the nominal size, not the actual one.
• To compare stock, compare actual cross-sections rather than board feet.