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Lumber Size Chart
Quick answer: A 2x4 is actually 1-1/2″ × 3-1/2″. Under 8″ nominal, subtract 1/2″; at 8″ and above, subtract 3/4″. So a 2x8 is 1-1/2″ × 7-1/4″, not 7-1/2″.
Nominal versus actual dimensions for every common board, post and sheet good — in inches and millimetres — plus why the difference exists and where it bites.
Dimensional lumber: nominal vs actual
A 2x4 has not measured two inches by four inches since the 1960s. The nominal name describes the rough green size before it is dried and surfaced on four sides (S4S). What you buy is the actual size below.
| Nominal | Actual (in) | Actual (mm) | Typical use |
|---|---|---|---|
| 1x2 | 3/4 x 1-1/2 | 19 x 38 | Furring, trim stock |
| 1x3 | 3/4 x 2-1/2 | 19 x 64 | Furring, strapping |
| 1x4 | 3/4 x 3-1/2 | 19 x 89 | Trim, shelving |
| 1x6 | 3/4 x 5-1/2 | 19 x 140 | Trim, fence boards |
| 1x8 | 3/4 x 7-1/4 | 19 x 184 | Shelving, siding |
| 1x10 | 3/4 x 9-1/4 | 19 x 235 | Shelving, risers |
| 1x12 | 3/4 x 11-1/4 | 19 x 286 | Shelving, stair risers |
| 2x2 | 1-1/2 x 1-1/2 | 38 x 38 | Blocking, balusters |
| 2x3 | 1-1/2 x 2-1/2 | 38 x 64 | Non-bearing partitions |
| 2x4 | 1-1/2 x 3-1/2 | 38 x 89 | Standard wall framing |
| 2x6 | 1-1/2 x 5-1/2 | 38 x 140 | Exterior walls, rafters, decking |
| 2x8 | 1-1/2 x 7-1/4 | 38 x 184 | Joists, headers, rafters |
| 2x10 | 1-1/2 x 9-1/4 | 38 x 235 | Joists, headers, stair stringers |
| 2x12 | 1-1/2 x 11-1/4 | 38 x 286 | Joists, stringers, long-span headers |
| 4x4 | 3-1/2 x 3-1/2 | 89 x 89 | Posts, deck footings |
| 4x6 | 3-1/2 x 5-1/2 | 89 x 140 | Beams, heavy posts |
| 6x6 | 5-1/2 x 5-1/2 | 140 x 140 | Structural posts, retaining walls |
Sizes are for surfaced dry softwood at 19% moisture content or less, per PS 20. Green (unseasoned) lumber is slightly larger, and rough-sawn stock from a small mill may be close to full nominal.
Why a 2x4 is 1-1/2 by 3-1/2
The board starts life at a full 2″ × 4″ when it is rough-sawn and still green. Drying shrinks it. Surfacing all four faces to a consistent, splinter-free finish removes more. What was standardised in PS 20 is the final dimension, so every mill in the country ships the same 1-1/2″ × 3-1/2″ stick and framing math is predictable.
This is why a wall framed with 2x4s and 1/2″ drywall on both faces measures 4-1/2″ thick, not 5″. And why a 2x4 laid flat gives you exactly 3-1/2″ of bearing, which is what the span tables assume.
Sheet goods thickness
| Nominal | Actual | Metric | Common use |
|---|---|---|---|
| 1/4" | 0.250" | 6.4 mm | Underlayment, cabinet backs |
| 3/8" | 0.375" | 9.5 mm | Wall sheathing (older), soffit |
| 15/32" | 0.469" | 11.9 mm | Nominal 1/2" wall sheathing |
| 1/2" | 0.500" | 12.7 mm | Drywall, cabinet sides |
| 19/32" | 0.594" | 15.1 mm | Nominal 5/8" roof sheathing |
| 5/8" | 0.625" | 15.9 mm | Type X drywall, roof deck |
| 23/32" | 0.719" | 18.3 mm | Nominal 3/4" subfloor (T&G) |
| 3/4" | 0.750" | 19.1 mm | Cabinet-grade ply, shelving |
Plywood is the awkward one. What everyone calls 1/2″ sheathing is usually 15/32″, and 3/4″ subfloor is 23/32″. The 1/32″ matters when you are matching a plywood edge to dimensional lumber or setting a router depth.
Board lengths and what is actually stocked
Softwood is stocked in even lengths — 8, 10, 12, 14, 16 ft, and 20 ft in some yards for 2x10 and 2x12. Studs are also sold pre-cut: 92-5/8″ for an 8 ft wall, 104-5/8″ for 9 ft, 116-5/8″ for 10 ft. Those numbers assume a single bottom plate and a double top plate, which together add 4-1/2″ and land the finished wall at exactly 8, 9 or 10 feet.
Frequently asked questions
What are the actual dimensions of a 2x4?
1-1/2 inches by 3-1/2 inches (38 mm × 89 mm) for surfaced dry softwood. The nominal 2x4 name refers to the rough green size before drying and surfacing.
Why is a 2x4 not actually 2 by 4 inches?
It starts at a full 2 × 4 when rough-sawn and green. Drying shrinks it and surfacing all four faces removes more material. PS 20, the American Softwood Lumber Standard, standardises the finished size so every mill ships the same dimension.
Is 3/4 inch plywood really 3/4 inch?
Usually not — most so-called 3/4 in plywood is 23/32 in (0.719 in). Likewise 1/2 in sheathing is typically 15/32 in. Cabinet-grade hardwood plywood is often closer to true 3/4 in, so check before you cut a dado.
How long is a pre-cut stud?
92-5/8 in for an 8 ft wall. Adding a 1-1/2 in bottom plate and two 1-1/2 in top plates gives 97-1/8 in, which with 1/2 in drywall lands the ceiling at 8 ft plus a small tolerance.