4x8 actual size
Dry (19% moisture or less)
3-1/2 × 7-1/4 in
88.9 × 184.2 mm
From the standard, NIST PS 20-25
Nominal, dry and green
| Size | Thickness | Width |
|---|---|---|
| Nominal (the name) | 4 in | 8 in |
| Dry, minimum | 3-1/2 in (88.9 mm) | 7-1/4 in (184.2 mm) |
| Green, minimum | 3-9/16 in (90.5 mm) | 7-1/2 in (190.5 mm) |
A 4x8 is dimension lumber (2 in to under 5 in nominal thickness) under the American Softwood Lumber Standard, NIST PS 20-25. The standard names lumber by its nominal size and sets the smallest it may be once it has been dressed (planed smooth). Lumber dried to 19% moisture content or less is held to the dry size; lumber surfaced green, above 19%, is left a little bigger because it shrinks as it dries.
Board feet in a 4x8
Thickness × width ÷ 12 gives the board feet in each foot of length. By the nominal size that is 2.667 bd ft per foot; by the dry dressed size it is 2.115. Ask your yard which one it prices on.
| Length | Nominal bd ft | Actual (dry) bd ft |
|---|---|---|
| 8 ft | 21.33 | 16.92 |
| 10 ft | 26.67 | 21.15 |
| 12 ft | 32 | 25.38 |
| 16 ft | 42.67 | 33.83 |
For a whole order, use the board foot calculator. Every size side by side is on the lumber sizes chart.
Framing a deck? The deck calculator says which joist, beam and post sizes span it, from the AWC DCA 6 tables.
What a 4x8 weighs per foot
One foot of a dry 4x8, 3-1/2 × 7-1/4 in, at 12% moisture content: its volume × the Wood Handbook’s density, 62.43 lb/ft³ × the species’ specific gravity at 12% × 1.12. Multiply by the length for a whole board. Wetter wood weighs more; the wood weight calculator takes any moisture content.
| Species | Specific gravity (12%) | Weight per foot |
|---|---|---|
| Douglas-fir (coast) | 0.48 | 5.91 lb |
| Southern pine (loblolly) | 0.51 | 6.28 lb |
| Western hemlock | 0.45 | 5.54 lb |
| Eastern white pine | 0.35 | 4.31 lb |
| White spruce | 0.36 | 4.44 lb |
One source: USDA Forest Products Laboratory, the authority for species properties it measured
Work it out for your project
- Estimate the lumber for a whole project
List the parts, get the boards and board feet to buy, with waste.
Sources
- NIST (US Department of Commerce), Voluntary Product Standard PS 20-25, American Softwood Lumber Standard, Table 3 (effective January 2025), read 2026-11-07
- Engineering ToolBox, Softwood lumber: nominal and actual (dry) sizes, read 2026-11-07
- WoodBin, Standard dimensions of lumber (hardwood thickness after G. Wengert; softwood dry and green sizes), read 2026-11-07