Lumber Dimensions Chart: Nominal vs Actual Sizes
A 2×4 is 1-1/2 × 3-1/2 in (38.1 × 88.9 mm), not 2 by 4 inches. The nominal size is only the name the board is sold under; the board is planed smaller. The tables below give the actual size of every common board, dimension lumber size and post, in inches and millimetres. To count the studs a wall needs, use the stud calculator.
Important: these are the minimum dressed sizes the American Softwood Lumber Standard sets for dry softwood lumber. A piece can come in slightly larger, and lumber moves as it takes on or loses moisture, so measure the boards you bought before you cut anything to a tight fit.
Nominal vs actual lumber sizes
Each table lists thickness × width. The last column shows how far the board falls under its nominal size, so you can work backwards from a plan that uses nominal names.
| Nominal size | Actual size (in) | Actual size (mm) | Under nominal (in) |
|---|---|---|---|
| 1×2 | 3/4 × 1-1/2 in | 19.1 × 38.1 mm | 1/4 × 1/2 in |
| 1×3 | 3/4 × 2-1/2 in | 19.1 × 63.5 mm | 1/4 × 1/2 in |
| 1×4 | 3/4 × 3-1/2 in | 19.1 × 88.9 mm | 1/4 × 1/2 in |
| 1×6 | 3/4 × 5-1/2 in | 19.1 × 139.7 mm | 1/4 × 1/2 in |
| 1×8 | 3/4 × 7-1/4 in | 19.1 × 184.2 mm | 1/4 × 3/4 in |
| 1×10 | 3/4 × 9-1/4 in | 19.1 × 235.0 mm | 1/4 × 3/4 in |
| 1×12 | 3/4 × 11-1/4 in | 19.1 × 285.8 mm | 1/4 × 3/4 in |
| Nominal size | Actual size (in) | Actual size (mm) | Under nominal (in) |
|---|---|---|---|
| 2×2 | 1-1/2 × 1-1/2 in | 38.1 × 38.1 mm | 1/2 × 1/2 in |
| 2×3 | 1-1/2 × 2-1/2 in | 38.1 × 63.5 mm | 1/2 × 1/2 in |
| 2×4 | 1-1/2 × 3-1/2 in | 38.1 × 88.9 mm | 1/2 × 1/2 in |
| 2×6 | 1-1/2 × 5-1/2 in | 38.1 × 139.7 mm | 1/2 × 1/2 in |
| 2×8 | 1-1/2 × 7-1/4 in | 38.1 × 184.2 mm | 1/2 × 3/4 in |
| 2×10 | 1-1/2 × 9-1/4 in | 38.1 × 235.0 mm | 1/2 × 3/4 in |
| 2×12 | 1-1/2 × 11-1/4 in | 38.1 × 285.8 mm | 1/2 × 3/4 in |
| Nominal size | Actual size (in) | Actual size (mm) | Under nominal (in) |
|---|---|---|---|
| 4×4 | 3-1/2 × 3-1/2 in | 88.9 × 88.9 mm | 1/2 × 1/2 in |
| 6×6 | 5-1/2 × 5-1/2 in | 139.7 × 139.7 mm | 1/2 × 1/2 in |
NIST Voluntary Product Standard PS 20-25, American Softwood Lumber Standard, Table 3, "Nominal and minimum-dressed sizes of boards, dimension, and timbers", dry sizes, checked 2026-10-30. PS 20 treats the inch sizes as the units of record and prints the millimetre sizes for information only. A 4×4 counts as dimension lumber and a 6×6 as a timber. Southern Pine Inspection Bureau and ProWood list the same actual sizes: SPIB, Nominal vs. Actual Lumber Sizes and ProWood, Lumber Dimensions.
Deck boards
| Nominal size | Actual size (in) | Actual size (mm) |
|---|---|---|
| 5/4×6 | 1 × 5-1/2 in | 25.4 × 139.7 mm |
PS 20 does not list decking, so this size comes from ProWood's Lumber Dimensions chart, checked 2026-10-30. A 5/4 board is 1 in thick, not 1-1/4 in. To count the boards for a deck, use the deck board calculator.
Why is a 2×4 1-1/2 × 3-1/2 in?
PS 20 defines the nominal size as a label that does not reflect the dressed size: the nominal size is always greater than the dressed size. A mill saws the board rough, then planes (dresses) the faces smooth, and the planing takes material off. PS 20 sets the minimum size the board must keep afterwards, and for a dry 2×4 that is 1-1/2 × 3-1/2 in.
"Dry" has a number. PS 20 calls lumber under nominal 5 in thick dry when it has been seasoned to a moisture content of 19 percent or less, and green above that. Green lumber is planed larger, because it shrinks as it dries: the PS 20 minimum for a green 2×4 is 1-9/16 × 3-9/16 in (39.7 × 90.5 mm). PS 20 builds the green sizes on average shrinkage factors, so a green board ends up near the dry size once it has dried. The tables above use the dry column.
Standard lumber lengths
Dimension lumber is sold in standard lengths of 6 to 24 ft, in 2 ft steps: 6, 8, 10, 12, 14, 16, 18, 20, 22, 24 ft. PS 20 allows 1 ft steps as well, and which lengths a yard stocks, especially past 16 ft, varies by store.
PS 20-25, section 5.3.1, standard lengths, checked 2026-10-30; SPIB gives the same 6 to 24 ft range for dimension lumber.
Worked example: a bench top from 5 boards
A bench top is 5 2×6s laid flat, side by side. Counted by nominal size, that is 5 × 6 = 30 in wide. At the actual width of 5-1/2 in each, it is 5 × 5-1/2 = 27-1/2 in, which is 2-1/2 in narrower than the nominal figure, before any gaps between the boards. Plan the frame and the legs around 27-1/2 in, not 30. The gap grows with every board you add.
FAQ
What is the actual size of a 2×4?
A 2×4 is 1-1/2 × 3-1/2 in (38.1 × 88.9 mm). Every 2× board is the same 1-1/2 in thick, from a 2×2 up to a 2×12.
How thick is a 1× board?
A 1× board is 3/4 in thick (19.1 mm) at every width. A 1×6 is 3/4 × 5-1/2 in.
Is a 4×4 really 3-1/2 inches?
Yes. A 4×4 is 3-1/2 × 3-1/2 in (88.9 × 88.9 mm), and a 6×6 is 5-1/2 × 5-1/2 in (139.7 × 139.7 mm), because PS 20 takes 1/2 in off a timber that is 5 or 6 in nominal.
Is lumber sold wet a different size?
Yes. PS 20 lets a mill plane lumber while it is still green, over 19 percent moisture, to a larger minimum size: a green 2×4 is 1-9/16 × 3-9/16 in (39.7 × 90.5 mm), against 1-1/2 × 3-1/2 in dry. The extra size allows for the shrinkage as it dries.
What lengths does dimension lumber come in?
Standard lengths run from 6 to 24 ft in 2 ft steps. Which lengths a yard keeps in stock, especially past 16 ft, varies by store, so check before you plan around a long board.
How thick is 5/4 decking?
A 5/4×6 deck board is 1 × 5-1/2 in, so it is 1 in thick, not 1-1/4 in. PS 20 does not list decking, so that size is from one treated-lumber maker's chart; check the board you are buying.