100 lumber cells — a two-by-four is neither two nor four inches
It was that size when it came off the saw, but drying shrinks it and planing takes more, leaving 38 × 89 mm. The amount removed follows a rule, so every meeting of 10 sizes and 10 lengths gets its real dimensions, cross-section and board feet.
Actual size
- 1x40.75 × 3.5 in · 19 × 88.9 mm · 65.6%
- 1x60.75 × 5.5 in · 19 × 139.7 mm · 68.8%
- 2x21.5 × 1.5 in · 38.1 × 38.1 mm · 56.3%
- 2x41.5 × 3.5 in · 38.1 × 88.9 mm · 65.6%
- 2x61.5 × 5.5 in · 38.1 × 139.7 mm · 68.8%
- 2x81.5 × 7.25 in · 38.1 × 184.1 mm · 68%
- 2x101.5 × 9.25 in · 38.1 × 235 mm · 69.4%
- 2x121.5 × 11.25 in · 38.1 × 285.8 mm · 70.3%
- 4x43.5 × 3.5 in · 88.9 × 88.9 mm · 76.6%
- 6x65.5 × 5.5 in · 139.7 × 139.7 mm · 84%
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.
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.
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.
The thinner the stock, the worse the deal
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.
Standards differ by country
These figures follow the North American standard, PS 20. Europe and Japan set their sizes in millimetres from the start, printing 38 × 89 mm directly, and often have no notion of a nominal size at all. Even within one 2×4, tolerances differ by moisture content — green versus dry — and by grade.
Find it by size and length
Actual size
1x4Nominal size 1 × 4 in
1x6Nominal size 1 × 6 in
2x2Nominal size 2 × 2 in
2x4Nominal size 2 × 4 in
2x6Nominal size 2 × 6 in
2x8Nominal size 2 × 8 in
2x10Nominal size 2 × 10 in
2x12Nominal size 2 × 12 in
4x4Nominal size 4 × 4 in
6x6Nominal size 6 × 6 in
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.
Frequently asked questions
Q. How big is a 2×4 really?
1.5 × 3.5 inches, or 38 × 89 mm — half an inch off each nominal dimension.
Q. Why does the name not match?
Green lumber straight off the saw was that size; drying shrank it and planing took more. The name had already stuck.
Q. Why does a 2×10 lose 3/4 rather than 1/2?
From eight nominal inches up the standard calls for three quarters, because thicker stock shrinks more as it dries.
Q. Are board feet counted on the actual size?
No, on the nominal one. A 2×4 gives you 66 % of the wood you counted, a 2×2 only 56 %.
Q. Is it the same in Europe or Japan?
No. They set sizes in millimetres from the start and simply print 38 × 89 mm.