North American softwood lumber is named by nominal size, but sold at a smaller dressed size. The chart gives the actual size of each common label in inches and millimetres, taken from the American Softwood Lumber Standard, PS 20-20 Table 3. A dry 2x4, for example, is 38 × 89 mm.
Lumber size chart in mm
Type a nominal size (2x10) or one dimension (4), an inch size (9-1/4 or 9 1/4) or millimetres (235).
Showing 16 of 16 sizes
| Nominal | Actual (in) | Actual (mm) | Exact (mm) | PS 20 class |
|---|---|---|---|---|
| 1x2 | 3/4 × 1-1/2 | 19 × 38 | 19.05 × 38.10 | Boards, dry |
| 1x3 | 3/4 × 2-1/2 | 19 × 64 | 19.05 × 63.50 | Boards, dry |
| 1x4 | 3/4 × 3-1/2 | 19 × 89 | 19.05 × 88.90 | Boards, dry |
| 1x6 | 3/4 × 5-1/2 | 19 × 140 | 19.05 × 139.70 | Boards, dry |
| 1x8 | 3/4 × 7-1/4 | 19 × 184 | 19.05 × 184.15 | Boards, dry |
| 1x10 | 3/4 × 9-1/4 | 19 × 235 | 19.05 × 234.95 | Boards, dry |
| 1x12 | 3/4 × 11-1/4 | 19 × 286 | 19.05 × 285.75 | Boards, dry |
| 2x2 | 1-1/2 × 1-1/2 | 38 × 38 | 38.10 × 38.10 | Dimension, dry |
| 2x3 | 1-1/2 × 2-1/2 | 38 × 64 | 38.10 × 63.50 | Dimension, dry |
| 2x4 | 1-1/2 × 3-1/2 | 38 × 89 | 38.10 × 88.90 | Dimension, dry |
| 2x6 | 1-1/2 × 5-1/2 | 38 × 140 | 38.10 × 139.70 | Dimension, dry |
| 2x8 | 1-1/2 × 7-1/4 | 38 × 184 | 38.10 × 184.15 | Dimension, dry |
| 2x10 | 1-1/2 × 9-1/4 | 38 × 235 | 38.10 × 234.95 | Dimension, dry |
| 2x12 | 1-1/2 × 11-1/4 | 38 × 286 | 38.10 × 285.75 | Dimension, dry |
| 4x4 | 3-1/2 × 3-1/2 | 89 × 89 | 88.90 × 88.90 | Dimension, dry |
| 6x6 | 5-1/2 × 5-1/2 | 140 × 140 | 139.70 × 139.70 | Timbers, dry or green |
To size a part from your own stock, or to get board feet and metric volume for a length, use the nominal to actual lumber size calculator.
Size by size
What is a 2x4 in mm?
A 2x4 is 38 × 89 mm (1-1/2 × 3-1/2 in) at the PS 20 minimum dressed dry size. Unrounded, that is 38.10 × 88.90 mm.
- If sold green
- 1-9/16 × 3-9/16 in (40 × 90 mm)
- Cross-section
- 5.25 sq in, 66% of the nominal 8
What is a 2x6 in mm?
A 2x6 is 38 × 140 mm (1-1/2 × 5-1/2 in) at the PS 20 minimum dressed dry size. Unrounded, that is 38.10 × 139.70 mm.
- If sold green
- 1-9/16 × 5-5/8 in (40 × 143 mm)
- Cross-section
- 8.25 sq in, 69% of the nominal 12
What is a 2x8 in mm?
A 2x8 is 38 × 184 mm (1-1/2 × 7-1/4 in) at the PS 20 minimum dressed dry size. Unrounded, that is 38.10 × 184.15 mm.
- If sold green
- 1-9/16 × 7-1/2 in (40 × 190 mm)
- Cross-section
- 10.875 sq in, 68% of the nominal 16
What is a 2x10 in mm?
A 2x10 is 38 × 235 mm (1-1/2 × 9-1/4 in) at the PS 20 minimum dressed dry size. Unrounded, that is 38.10 × 234.95 mm.
- If sold green
- 1-9/16 × 9-1/2 in (40 × 241 mm)
- Cross-section
- 13.875 sq in, 69% of the nominal 20
What is a 2x12 in mm?
A 2x12 is 38 × 286 mm (1-1/2 × 11-1/4 in) at the PS 20 minimum dressed dry size. Unrounded, that is 38.10 × 285.75 mm.
- If sold green
- 1-9/16 × 11-1/2 in (40 × 292 mm)
- Cross-section
- 16.875 sq in, 70% of the nominal 24
What is a 1x2 in mm?
A 1x2 is 19 × 38 mm (3/4 × 1-1/2 in) at the PS 20 minimum dressed dry size. Unrounded, that is 19.05 × 38.10 mm.
- If sold green
- 25/32 × 1-9/16 in (20 × 40 mm)
- Cross-section
- 1.125 sq in, 56% of the nominal 2
What is a 1x4 in mm?
A 1x4 is 19 × 89 mm (3/4 × 3-1/2 in) at the PS 20 minimum dressed dry size. Unrounded, that is 19.05 × 88.90 mm.
- If sold green
- 25/32 × 3-9/16 in (20 × 90 mm)
- Cross-section
- 2.625 sq in, 66% of the nominal 4
What is a 1x6 in mm?
A 1x6 is 19 × 140 mm (3/4 × 5-1/2 in) at the PS 20 minimum dressed dry size. Unrounded, that is 19.05 × 139.70 mm.
- If sold green
- 25/32 × 5-5/8 in (20 × 143 mm)
- Cross-section
- 4.125 sq in, 69% of the nominal 6
What is a 4x4 in mm?
A 4x4 is 89 × 89 mm (3-1/2 × 3-1/2 in) at the PS 20 minimum dressed dry size. Unrounded, that is 88.90 × 88.90 mm.
- If sold green
- 3-9/16 × 3-9/16 in (90 × 90 mm)
- Cross-section
- 12.25 sq in, 77% of the nominal 16
What is a 6x6 in mm?
A 6x6 is 140 × 140 mm (5-1/2 × 5-1/2 in) at the PS 20 minimum dressed size, dry or green. Unrounded, that is 139.70 × 139.70 mm.
- Green
- 5-1/2 × 5-1/2 in (140 × 140 mm)
- Cross-section
- 30.25 sq in, 84% of the nominal 36
Why a 2x4 is not 2 by 4 inches
PS 20 defines nominal size as a label for a size category that does not reflect the dressed size, and states that the nominal size is greater than the dressed size (section 2.16). The label is a name, not a measurement. What the standard does fix is a minimum for each label once the piece has been surfaced:
- Thickness drops by a fixed step per class. Nominal 1 in boards dress to 3/4 in (19 mm); nominal 2 in dimension lumber to 1-1/2 in (38 mm); nominal 4 in to 3-1/2 in (89 mm).
- Width drops by 1/2 in up to nominal 7 in, then by 3/4 in. A 1x6 or 2x6 is 5-1/2 in wide, but an 8 in nominal width is 7-1/4 in and a 10 in is 9-1/4 in.
- Timbers follow their own rule. Nominal 5 and 6 in timbers dress 1/2 in under nominal, dry or green, so a 6x6 is 5-1/2 × 5-1/2 in. PS 20 defines timbers as nominal 5 in or more in the least dimension (section 3.4.3).
Dry, green and shrinkage after dressing
Dry lumber under nominal 5 in thick must be at 19% moisture content or less when it is dressed (section 6.2.3); green lumber is above 19% (section 2.12). Green pieces are dressed larger because they will shrink as they dry. PS 20 bases the green minimums on shrinkage of 2.35% in thickness and 2.80% in width from the fibre saturation point to 19% moisture content (Table 3 footnote, section 6.2.5). The "If sold green" line in each card above is that green minimum.
Dry stock can keep shrinking after it leaves the planer. Section 6.2.3.1 allows a tolerance below the dry size of 1% for every 4 percentage points of moisture lost (0.7% for Redwood, Western Red Cedar and Northern White Cedar). A 2x10 dressed at 19% that dries to 11% loses 8 points, so it may measure 2% under 9-1/4 in, down to 9.065 in (230 mm), and still comply.
How the millimetres are rounded
PS 20 Appendix B1 converts at exactly 25.4 mm per inch and treats the nearest millimetre as significant for sizes over 1/8 in. A digit after the decimal point below 5 is dropped and above 5 rounds up; an exact .5 rounds to the even millimetre. That is why 2-1/2 in (63.5 mm) is listed as 64 mm and 7-1/2 in (190.5 mm) as 190 mm. The chart applies the same rule, and the millimetre values it produces match those printed in Table 3. In a dispute over size, the standard gives the inch measurement precedence.
Metric lumber sizes outside North America
This chart covers the North American PS 20 system only. Structural timber sold in Australia, New Zealand, the UK and Europe is specified under each country's own standards rather than PS 20, so a North American nominal label has no defined local equivalent. No local sizes are listed here because they could not be checked against those standards; use the supplier's stated dimensions, or enter them as custom dimensions in the size calculator.
Common questions
Is a 2x4 really 38 × 89 mm?
Yes, for dry lumber. PS 20 section 2.16 states that a dry 2 by 4 is surfaced to 38.1 mm by 88.9 mm (1-1/2 by 3-1/2 in), which rounds to 38 × 89 mm. Green 2x4s have a larger minimum, 40 × 90 mm.
Why does my 2x6 measure wider than 5-1/2 in?
PS 20 sizes are minimums. A 2x6 dressed green must be at least 5-5/8 in (143 mm) wide, and dry stock that has picked up moisture since it was dressed can swell. Measure the boards you will actually use.
How do I convert a lumber size to millimetres?
Multiply the actual dressed size in inches by 25.4. PS 20 Appendix B1 then rounds to the nearest millimetre for sizes over 1/8 in, sending an exact half to the even millimetre.
Are board feet worked out on nominal or actual size?
Nominal. PS 20 section 2.2 defines board measure from nominal thickness and width, so a 2x4 is priced as 2 × 4 in even though it measures 1-1/2 × 3-1/2 in.
Sources
- Voluntary Product Standard PS 20-20: American Softwood Lumber Standard · U.S. Department of Commerce, National Institute of Standards and Technology, PS 20-20 Revision 1, October 2021
Table 3, sections 2.2, 2.16. Table 3 minimum dressed dry thicknesses and widths for the Boards and Dimension classifications. Verified .
- Voluntary Product Standard PS 20-20: American Softwood Lumber Standard · U.S. Department of Commerce, National Institute of Standards and Technology, PS 20-20 Revision 1, October 2021
Table 3, sections 2.12, 6.2.3, 6.2.3.1, 6.2.5, Appendix B1. Table 3: Boards widths 10 and 12 (9-1/4 and 11-1/4 in dry), Dimension width 3 (2-1/2 in dry) and thickness 4 (3-1/2 in dry), and the Timbers rule for nominal 5 and 6 in (1/2 in off, dry or green). Section 3.4.3 defines timbers as nominal 5 in or greater in least dimension. Also the green-size and shrinkage provisions and the millimetre rounding rule. Verified .
Gaps. Sizes for Australian, New Zealand, UK and European structural timber are not listed because they were not verified against the relevant national standards. Hardwood lumber is outside PS 20 and is not covered. Redwood, Western Red Cedar and Northern White Cedar have their own green minimums in PS 20 section 6.2.5 that the green line does not show.