Building materials · Reference
A 2x4 is nominal, not actual — a surfaced (S4S) piece of dimensional lumber actually measures 1 1/2 × 3 1/2 in (38 × 89 mm), because softwood is sawn oversize then dried and planed smooth, shrinking on every face.
Lumber is specified by nominal cross-section (thickness × width) plus length; the actual surfaced size is always smaller than the nominal name.
| Nominal | Actual (in) | Actual (mm) |
|---|---|---|
| 1x2 | 3/4 × 1 1/2 in | 19 × 38 mm |
| 1x4 | 3/4 × 3 1/2 in | 19 × 89 mm |
| 1x6 | 3/4 × 5 1/2 in | 19 × 140 mm |
| 2x4The classic example | 1 1/2 × 3 1/2 in | 38 × 89 mm |
| 2x6 | 1 1/2 × 5 1/2 in | 38 × 140 mm |
| 2x8Width loses 3/4 in | 1 1/2 × 7 1/4 in | 38 × 184 mm |
| 2x10 | 1 1/2 × 9 1/4 in | 38 × 235 mm |
| 2x12 | 1 1/2 × 11 1/4 in | 38 × 286 mm |
| 4x4Post | 3 1/2 × 3 1/2 in | 89 × 89 mm |
Source: US DOC / NIST — Voluntary Product Standard PS 20-20, American Softwood Lumber Standard, Table 3 (dry) (2020)
| Length (ft) | Length (m) |
|---|---|
| 8 ft | 2.44 m |
| 10 ft | 3.05 m |
| 12 ft | 3.66 m |
| 16 ft | 4.88 m |
Source: PS 20-20 §5.3.1 (standard lengths in multiples of 0.3048 m / 1 ft) (2020)
| Designation | Size |
|---|---|
| Nominal (marketed) | 4x2 in / 50 × 100 mm |
| Finished (actual) | 38 × 89 mm |
Source: Fulham Timber — CLS Studwork Timber (4x2) 38x89mm finished
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A 2x4 is the nominal (rough-sawn) size. After the green board is dried and planed smooth on all four sides it shrinks to a minimum dressed size of 1 1/2 × 3 1/2 in (38 × 89 mm), set by the US PS 20-20 American Softwood Lumber Standard.
1 1/2 × 3 1/2 inches, i.e. 38 × 89 mm (thickness × width), for surfaced-dry dimension lumber under PS 20-20.
3 1/2 × 3 1/2 inches, or 89 × 89 mm, surfaced-dry (PS 20-20).
For nominal thicknesses/widths up to 6 in the loss is 1/2 in per dimension (a 2x4 loses 1/2 in each way to 1 1/2 × 3 1/2 in); for widths of 8 in and up the loss is 3/4 in, so a 2x8 is only 7 1/4 in (184 mm) wide, not 8 in.
Even 2-ft multiples are most common — 8, 10, 12 and 16 ft (2.44, 3.05, 3.66 and 4.88 m). PS 20-20 sets standard lengths in multiples of 0.3048 m (1 ft) or 0.6096 m (2 ft).
38 × 89 mm — sold as “4x2” CLS (Canadian Lumber Standard) studwork timber, whose finished size is 38 × 89 mm (nominal 50 × 100 mm). This matches the 38 × 89 mm figure listed for a 2x4 in PS 20-20 Table 3.
Every figure on this page is traced to a named source, linked under each table. Standard sizes come from published standards and established industry references; where a size is a typical range or varies by manufacturer, we say so. Sizes vary by region, model and revision — treat these as the standard reference, and confirm the exact spec of the item you are selling.
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