The quick rule
For North American softwood lumber, subtract from the nominal size:
- 1″ nominal thickness becomes ¾″.
- 2″ to 6″ nominal loses ½″ (2″ → 1½″, 4″ → 3½″, 6″ → 5½″).
- 8″ and wider loses ¾″ (8″ → 7¼″, 10″ → 9¼″, 12″ → 11¼″).
These are the standard dry, surfaced sizes set by the American Softwood Lumber Standard (PS 20). Length is not reduced: an 8-foot board is about 96″ long, often slightly more.
Board sizes (1x lumber)
| Nominal | Actual (inches) | Actual (mm) |
|---|---|---|
| 1x2 | ¾ × 1½ | 19 × 38 |
| 1x3 | ¾ × 2½ | 19 × 64 |
| 1x4 | ¾ × 3½ | 19 × 89 |
| 1x6 | ¾ × 5½ | 19 × 140 |
| 1x8 | ¾ × 7¼ | 19 × 184 |
| 1x10 | ¾ × 9¼ | 19 × 235 |
| 1x12 | ¾ × 11¼ | 19 × 286 |
Dimensional lumber (2x lumber)
| Nominal | Actual (inches) | Actual (mm) |
|---|---|---|
| 2x2 | 1½ × 1½ | 38 × 38 |
| 2x3 | 1½ × 2½ | 38 × 64 |
| 2x4 | 1½ × 3½ | 38 × 89 |
| 2x6 | 1½ × 5½ | 38 × 140 |
| 2x8 | 1½ × 7¼ | 38 × 184 |
| 2x10 | 1½ × 9¼ | 38 × 235 |
| 2x12 | 1½ × 11¼ | 38 × 286 |
Posts and timbers
| Nominal | Actual (inches) | Actual (mm) |
|---|---|---|
| 4x4 | 3½ × 3½ | 89 × 89 |
| 4x6 | 3½ × 5½ | 89 × 140 |
| 6x6 | 5½ × 5½ | 140 × 140 |
Pressure-treated lumber follows the same nominal sizes, but it is often sold wet from treatment and can shrink slightly as it dries.
Plywood thickness
Plywood is sold by a nominal thickness, and the real sheet is usually a little thinner. Thickness varies by product and mill, so treat these as typical values and measure your sheet.
| Sold as | Typical actual (inches) | Typical actual (mm) |
|---|---|---|
| ¼″ | 7/32 | 5.6 |
| ½″ | 15/32 | 11.9 |
| ⅝″ | 19/32 | 15.1 |
| ¾″ | 23/32 | 18.3 |
Full sheets are 4 × 8 feet (48 × 96 inches). Hardwood and Baltic birch plywood are often made in metric thicknesses such as 12 mm and 18 mm.
Hardwood thickness (quarter sizes)
Hardwood is sold in quarters of an inch of rough thickness, and widths and lengths vary board to board. Typical thickness after surfacing both faces (S2S):
| Quarter size | Rough | Surfaced (S2S) |
|---|---|---|
| 4/4 | 1″ | 13/16″ (20.6 mm) |
| 5/4 | 1¼″ | 1 1/16″ (27 mm) |
| 6/4 | 1½″ | 1 5/16″ (33.3 mm) |
| 8/4 | 2″ | 1¾″ (44.5 mm) |
Why it matters for your cut list
Plans go wrong when the design uses nominal sizes and the boards don’t. Three 2x4s stacked flat are 4½″ tall, not 6″. A shelf meant to sit between two 1x12 sides needs to be 1½″ narrower than the cabinet’s outside width, not 2″. A dado cut for “¾″ plywood” will be loose on a 23/32″ sheet.
Always design and cut with actual sizes, and measure the boards you bought before the first cut. Our cut list checklist covers the other checks worth doing before you start.
Board feet
Hardwood is priced by the board foot: a volume of 144 cubic inches, such as a board 1″ thick, 12″ wide, and 12″ long. Multiply thickness × width × length in inches and divide by 144. Use the rough (quarter) thickness for hardwood, so a 4/4 board 6″ wide and 96″ long is 1 × 6 × 96 ÷ 144 = 4 board feet.
How TimberLab handles lumber sizes
TimberLab’s cut lists use real stock sizes: a 1x4 slat is planned as ¾″ × 3½″, and a 2x6 rail as 1½″ × 5½″. The shopping list fits the cuts onto standard 8-foot boards and leaves room for each saw cut. You can see this in our free woodworking plans, such as the bunk bed and the garden bench.
Common questions
Why is a 2x4 not 2 inches by 4 inches?
The name comes from the rough-sawn size. After the board is dried and planed smooth on all four sides, a 2x4 measures 1½ × 3½ inches (38 × 89 mm).
What is the actual size of a 4x4?
A 4x4 post measures 3½ × 3½ inches (89 × 89 mm).
How thick is 3/4-inch plywood really?
Most construction and sanded plywood sold as 3/4 inch is about 23/32 inch (18.3 mm) thick. Hardwood plywood is often 18 mm. Measure your sheet before you set up cuts or dadoes.
What does 4/4 mean for hardwood?
Hardwood thickness is sold in quarters of an inch. 4/4 (four quarter) is about 1 inch thick rough sawn and is typically surfaced to 13/16 inch. 8/4 is about 2 inches rough and typically surfaces to 1¾ inches.
Are lumber lengths smaller than the label too?
No. Only thickness and width shrink from the nominal size. An 8-foot board is about 96 inches long and is often slightly longer. Precut studs are the exception: they are made to lengths such as 92⅝ inches for standard wall framing.