Lumber Size Chart — Nominal vs Actual Dimensions (2x4, 2x6 & More)
PS 20 softwood lumber sizes: nominal vs actual dressed dimensions from 1x2 to 8x8 in inches and millimetres, plus board feet per linear foot.
Updated August 18, 2026
A 2x4 is not 2 by 4 inches — it is 1-1/2 by 3-1/2. Nominal sizes name the rough-sawn green dimensions; by the time the board is dried to 19% moisture content and surfaced four sides (S4S), the American Softwood Lumber Standard (US DOC PS 20) fixes the actual dressed size. The reductions are systematic: nominal 1-inch thickness dresses to 3/4 in, nominal 2 to 1-1/2, nominal 4 to 3-1/2; widths through nominal 6 lose 1/2 in, while widths of nominal 8 and up lose 3/4 in — which is why a 2x6 is 5-1/2 in wide but a 2x8 is 7-1/4, not 7-1/2.
Timbers — nominal 5 in and thicker — follow their own rule: they are ordinarily sawn and sold green (unseasoned), dressing 1/2 in off each face, so a 6x6 is 5-1/2 in square and an 8x8 is 7-1/2 in square as sold. Metric equivalents below follow the PS 20 convention of 25.4 mm per inch of the actual dressed size, rounded to the nearest millimetre — the familiar 38 × 89 mm for a 2x4. Lumber is still bought by the board foot on nominal dimensions: one board foot is 1 nominal inch × 12 nominal inches × 1 foot, so BF per linear foot = thickness × width ÷ 12, computed in the second table.
Nominal vs Actual Dressed Sizes (S4S)
| Nominal | Actual (in) | Actual (mm) |
|---|---|---|
| 1x2 | 3/4 × 1-1/2 | 19 × 38 |
| 1x3 | 3/4 × 2-1/2 | 19 × 64 |
| 1x4 | 3/4 × 3-1/2 | 19 × 89 |
| 1x6 | 3/4 × 5-1/2 | 19 × 140 |
| 1x8 | 3/4 × 7-1/4 | 19 × 184 |
| 1x10 | 3/4 × 9-1/4 | 19 × 235 |
| 1x12 | 3/4 × 11-1/4 | 19 × 286 |
| 2x2 | 1-1/2 × 1-1/2 | 38 × 38 |
| 2x3 | 1-1/2 × 2-1/2 | 38 × 64 |
| 2x4 | 1-1/2 × 3-1/2 | 38 × 89 |
| 2x6 | 1-1/2 × 5-1/2 | 38 × 140 |
| 2x8 | 1-1/2 × 7-1/4 | 38 × 184 |
| 2x10 | 1-1/2 × 9-1/4 | 38 × 235 |
| 2x12 | 1-1/2 × 11-1/4 | 38 × 286 |
| 4x4 | 3-1/2 × 3-1/2 | 89 × 89 |
| 4x6 | 3-1/2 × 5-1/2 | 89 × 140 |
| 6x6 | 5-1/2 × 5-1/2 | 140 × 140 |
| 8x8 | 7-1/2 × 7-1/2 | 190 × 190 |
Dry (19% max MC) dressed sizes per PS 20-20 Table 3. Timbers (6x6, 8x8) are surfaced green: the 8x8 is 7-1/2 in square as normally sold; PS 20-20 lists 7-1/4 in as its dry-dressed minimum. mm = 25.4 × dressed inches, rounded to the nearest mm.
Board Feet per Linear Foot (computed, nominal dimensions)
| Nominal | BF per linear ft | BF per 8-ft board | BF per 12-ft board |
|---|---|---|---|
| 1x4 | 0.33 | 2.67 | 4.00 |
| 1x6 | 0.50 | 4.00 | 6.00 |
| 1x8 | 0.67 | 5.33 | 8.00 |
| 1x10 | 0.83 | 6.67 | 10.00 |
| 1x12 | 1.00 | 8.00 | 12.00 |
| 2x4 | 0.67 | 5.33 | 8.00 |
| 2x6 | 1.00 | 8.00 | 12.00 |
| 2x8 | 1.33 | 10.67 | 16.00 |
| 2x10 | 1.67 | 13.33 | 20.00 |
| 2x12 | 2.00 | 16.00 | 24.00 |
| 4x4 | 1.33 | 10.67 | 16.00 |
| 4x6 | 2.00 | 16.00 | 24.00 |
| 6x6 | 3.00 | 24.00 | 36.00 |
| 8x8 | 5.33 | 42.67 | 64.00 |
Computed cells: BF/lf = nominal thickness × nominal width ÷ 12 — board feet always use NOMINAL dimensions by trade convention, never the dressed actual size.
Sources & Further Reading
- US DOC PS 20-20 (American Softwood Lumber Standard), Revision 1, October 2021 — Table 3 nominal and minimum-dressed sizes; Appendix B1 metric convention
- FPL Wood Handbook (Forest Products Laboratory), Chapter 5, Table 5-6 — dressed-size cross-check and the timbers-surfaced-green convention
- Board-foot column: computed from the trade formula BF = nominal t × w × length ÷ 12
Frequently Asked Questions
Why is a 2x4 actually 1-1/2 by 3-1/2 inches?
The nominal size names the rough green board as it leaves the saw. Drying shrinks it and planing all four faces smooth (S4S) removes more, and PS 20 standardizes the finished result: nominal 2 in dresses to 1-1/2 in, nominal 4 in to 3-1/2 in. Every mill dresses to the same standard sizes, so framing layouts and span tables all assume the actual dimensions.
Why does a 2x8 lose more width than a 2x6?
PS 20 takes 1/2 in off nominal widths up through 6 in, but 3/4 in off widths of 8 in and up — wider boards shrink more in drying and need more surfacing to clean up. So a 2x6 dresses to 5-1/2 in but a 2x8 dresses to 7-1/4 in, and 2x10 and 2x12 land on 9-1/4 and 11-1/4.
Is a 6x6 post really 5-1/2 inches square?
Yes. Timbers — nominal 5 in and thicker — dress 1/2 in off each face, so a 6x6 is 5-1/2 × 5-1/2 in. Unlike boards and dimension lumber they are ordinarily surfaced green, because timbers that thick are rarely kiln-dried before sale; an 8x8 runs 7-1/2 in square as sold (its dry-dressed minimum in PS 20-20 is 7-1/4).
Do board feet use nominal or actual dimensions?
Nominal, always — that is the trade convention. A 2x6 counts as 2 × 6 in the formula BF = t × w × length ÷ 12 even though it measures 1-1/2 × 5-1/2, so an 8-ft 2x6 is 8 board feet. Pricing, takeoffs, and the board foot calculator all follow the same nominal rule.
Try the Calculators
Lumber Size Chart — Nominal vs Actual Dimensions (2x4, 2x6 & More) — reuven.tools/reference/lumber-sizes — verified against: US DOC PS 20-20 (American Softwood Lumber Standard), Revision 1, October 2021 — Table 3 nominal and minimum-dressed sizes; Appendix B1 metric convention; FPL Wood Handbook (Forest Products Laboratory), Chapter 5, Table 5-6 — dressed-size cross-check and the timbers-surfaced-green convention; Board-foot column: computed from the trade formula BF = nominal t × w × length ÷ 12