Nominal vs actual lumber sizes
Lumber is named by its nominal size — the rough dimension before drying and planing — but sold at its smaller actual size. The gap is real and fixed: a 2×4 measures 1½ × 3½ in. This chart is every common size, cross-checked against the same dimensions the framing calculators use.
| Nominal | Actual (in) | Actual (mm) |
|---|---|---|
| 1 × 2 | ¾ × 1½ | 19 × 38 |
| 1 × 3 | ¾ × 2½ | 19 × 64 |
| 1 × 4 | ¾ × 3½ | 19 × 89 |
| 1 × 6 | ¾ × 5½ | 19 × 140 |
| 1 × 8 | ¾ × 7¼ | 19 × 184 |
| 1 × 10 | ¾ × 9¼ | 19 × 235 |
| 1 × 12 | ¾ × 11¼ | 19 × 286 |
| 2 × 2 | 1½ × 1½ | 38 × 38 |
| 2 × 3 | 1½ × 2½ | 38 × 64 |
| 2 × 4 | 1½ × 3½ | 38 × 89 |
| 2 × 6 | 1½ × 5½ | 38 × 140 |
| 2 × 8 | 1½ × 7¼ | 38 × 184 |
| 2 × 10 | 1½ × 9¼ | 38 × 235 |
| 2 × 12 | 1½ × 11¼ | 38 × 286 |
| 4 × 4 | 3½ × 3½ | 89 × 89 |
| 4 × 6 | 3½ × 5½ | 89 × 140 |
| 6 × 6 | 5½ × 5½ | 140 × 140 |
Why a 2×4 isn’t 2 by 4
The nominal number is the size of the board when it is first rough-sawn, green and full. Two things shrink it to the size you buy:
- Drying: green lumber is milled oversize, then kiln- or air-dried, and wood shrinks as it loses moisture.
- Surfacing (S4S): the mill planes all four faces smooth to a uniform dressed size — that removes the rest.
- The width rule: subtract ½ in below nominal 8 in, ¾ in at 8 in and over — so a 2×6 is 5½, but a 2×8 is 7¼.
- The thickness rule: nominal 1 in dresses to ¾ in; nominal 2 in dresses to 1½ in — every board and 2× follows it.
- Rough-sawn is different: undressed, full-dimension timber measures its nominal size — that’s why a barn 2×4 is fatter.
When to use nominal, when to use actual
Both numbers have a job — mixing them up is where layout goes wrong:
- Order by nominal: you buy and call out lumber by the nominal name — “grab a 2×6.”
- Lay out by actual: stud walls, spacing and openings work off the real 1½ in thickness — a doubled 2× is 3 in, not 4.
- Board feet use both: the board foot calculator takes nominal thickness and width but the actual length.
- Precut studs: walls use a shorter precut stud so the framed height lands right with the plates — see the table.
- Sheathing gaps: plywood and OSB are a hair under 4×8 ft on purpose, to leave an expansion gap.
| Wall height | Precut stud |
|---|---|
| 8 ft | 92⅝ in |
| 9 ft | 104⅝ in |
| 10 ft | 116⅝ in |
Lumber size FAQ
Is a 2x4 really 2x4 inches?
No — a 2×4 actually measures 1½ × 3½ inches. The 2×4 is the nominal size, the rough dimension before the board is dried and planed smooth. After surfacing on all four sides (S4S), a nominal 2 in becomes 1½ in and a nominal 4 in becomes 3½ in. Every framing lumber size works the same way.
Why is lumber smaller than its nominal size?
The nominal size is the rough-sawn, green (wet) dimension. The mill then dries the lumber, which shrinks it, and planes all four faces smooth to a uniform size — that surfacing removes the rest. The nominal number is a naming and ordering convention that stuck; the actual dressed size is what you build with.
What is the actual size of a 2x6, 2x8, 2x10 and 2x12?
A 2×6 is 1½ × 5½ in, a 2×8 is 1½ × 7¼ in, a 2×10 is 1½ × 9¼ in, and a 2×12 is 1½ × 11¼ in. Notice the rule change: up to nominal 6 in you subtract ½ in, but at nominal 8 in and wider you subtract ¾ in — which is why a 2×8 is 7¼, not 7½.
What is the actual size of a 1x4 and 1x6 board?
A 1×4 board is ¾ × 3½ in and a 1×6 is ¾ × 5½ in. All nominal 1× boards are ¾ in thick once surfaced. The width follows the same rule as dimensional lumber: subtract ½ in below 8 in nominal, ¾ in at 8 in and over, so a 1×8 is ¾ × 7¼ in.
Is a 4x4 post actually 4x4 inches?
No — a 4×4 post is 3½ × 3½ in, and a 6×6 is 5½ × 5½ in. Posts and timbers are dressed the same way as dimensional lumber, so a 4×4 matches the 3½ in width of a 2×4 laid flat. Rough-sawn or full-dimension timbers, sold undressed, do measure their full nominal size.
Does the length of a board shrink too?
No — only the thickness and width are reduced by surfacing. An 8 ft board is a full 8 ft (or a hair over); the nominal and actual length are the same. The nominal-vs-actual difference applies only to the cross-section, which is why board-foot math uses nominal thickness and width but actual length.
What does S4S mean on lumber?
S4S means "surfaced four sides" — the board has been planed smooth on all four faces to the standard dressed dimension. Most lumber sold at home centers is S4S. You may also see S2S (surfaced two sides) or rough-sawn stock, which is left at or near its full nominal size and measures larger.
Sources and standards
The sizes are a national standard; the mill practice is why they exist:
- American Softwood Lumber Standard (PS 20): the voluntary standard that sets nominal and dressed (actual) softwood sizes.
- S4S, surfaced-dry: the standard dressed sizes assume lumber planed four sides and dried — the sizes in Table 1.
- Metric equivalents: the 38 × 89 mm (2×4) figures are the standard hard-converted sizes used in metric markets.
- Rough-sawn & full-dimension: undressed timber is sold at nominal size and measures larger — always check.