Why the name is larger than the stick
Softwood dimension lumber is named for the rough size. After it is dried and surfaced, the American Softwood Lumber Standard (NIST Voluntary Product Standard PS 20) sets a smaller dressed size. A dry board whose nominal thickness is 1 inch dresses to ¾ inch. A nominal 2 inch thickness dresses to 1½ inches. Widths from 2 through 6 inches lose ½ inch. Widths of 8 inches and wider lose ¾ inch.
That is the whole of the 2×4 story. Thickness 2 becomes 1½. Width 4 becomes 3½. A 2×8 is 1½ by 7¼, not 1½ by 7½, because 8 inches of width is in the group that loses ¾. A 1×4 is ¾ by 3½. A 4×4 is 3½ by 3½.
Green lumber can leave the planer larger than the dry size. The chart below is the dry dressed size. If the joint is tight, measure the board you have. The lumber sizes calculator subtracts the standard from that measurement.
United States and Canada are not the same millimetre
The inch is exactly 25.4 millimetres, so 1½ by 3½ inches is 38.1 by 88.9 mm. Canadian dimension lumber is graded to CLS millimetre sizes of its own: 38 by 89 for a 2×4, 19 by 89 for a 1×4, 38 by 184 for a 2×8. Those are the standard, not a rounding of the inch fraction. A board sold that way should be checked against the millimetre column. A board sold as dressed inch stock should be checked against the inch column.
Hardwood is not in this softwood chart. It is sold by the quarter inch of rough thickness, and the width is whatever the sawyer left. That table is under the softwood chart.
Dressed softwood chart
Nominal name, dry dressed size under PS 20, and the CLS millimetre size used for Canadian dimension lumber.
| Nominal | Dressed, dry | CLS |
|---|---|---|
| 1×2 | ¾ × 1½ in | 19 × 38 mm |
| 1×3 | ¾ × 2½ in | 19 × 64 mm |
| 1×4 | ¾ × 3½ in | 19 × 89 mm |
| 1×6 | ¾ × 5½ in | 19 × 140 mm |
| 1×8 | ¾ × 7¼ in | 19 × 184 mm |
| 1×10 | ¾ × 9¼ in | 19 × 235 mm |
| 1×12 | ¾ × 11¼ in | 19 × 286 mm |
| 2×2 | 1½ × 1½ in | 38 × 38 mm |
| 2×3 | 1½ × 2½ in | 38 × 64 mm |
| 2×4 | 1½ × 3½ in | 38 × 89 mm |
| 2×6 | 1½ × 5½ in | 38 × 140 mm |
| 2×8 | 1½ × 7¼ in | 38 × 184 mm |
| 2×10 | 1½ × 9¼ in | 38 × 235 mm |
| 2×12 | 1½ × 11¼ in | 38 × 286 mm |
| 4×4 | 3½ × 3½ in | 89 × 89 mm |
| 4×6 | 3½ × 5½ in | 89 × 140 mm |
| 6×6 | 5½ × 5½ in | 140 × 140 mm |
Hardwood quarters
A 4/4 board is one inch of rough thickness, four quarters. It is not a promise that the board is still an inch after the faces are planed. A common surfaced-two-sides target is 13/16 inch for 4/4. Dealers do not all hit it. Some stop at ¾. Measure.
| Name | Rough | Common S2S target |
|---|---|---|
| 4/4 | 1 in | 13/16 in |
| 5/4 | 1¼ in | 1 1/16 in |
| 6/4 | 1½ in | 1 5/16 in |
| 8/4 | 2 in | 1¾ in |
| 12/4 | 3 in | 2¾ in |
If you are buying the rough board by the board foot, the thickness in that sum is the rough thickness the name means. If you are calculating the volume of a board you already surfaced, use the caliper. Both cases are worked on the board foot guide.
What the chart will not decide
- It is not a grade. Wane, skip, and moisture are not in the size.
- It is not a span, a joist, or a stair. Those are structural questions.
- It does not say a particular board is square. Dressing sets a target, and the stick can still vary along its length.
Questions
Why is a 2×4 not 2 inches by 4 inches?
The name is the size before dressing. Under the American Softwood Lumber Standard, a dry 2×4 is surfaced to about 1½ by 3½ inches. The planer takes about ½ inch off the thickness and about ½ inch off a width of 6 inches or less. Wider stock, such as a 2×8, loses ¾ inch of width and dresses to 1½ by 7¼.
Why is a Canadian 2×4 listed as 38 by 89 mm, not 38.1 by 88.9?
1½ inches is 38.1 mm and 3½ inches is 88.9 mm. Canadian dimension lumber is sold to its own millimetre sizes, not as a conversion of the inch fraction. A 2×4 in that system is 38 by 89 mm. Use the column that matches how the board was sold, and measure when a joint cares about a tenth of a millimetre.
What thickness is 4/4 hardwood after it is surfaced?
Rough, 4/4 means about 1 inch. Surfaced two sides, a common shop target is 13/16 inch, and some dealers dress closer to ¾. The table is that common target, not a rule. The board in the rack is the measurement that counts.
Does the dressed size tell me if a joist or a stair is legal?
No. This is the size of the stick after the planer. Span, notch, and stair sizes belong to the building code and to someone stamping drawings.