Actual and nominal size
The name 2x4 is the rough sawn size. After drying and planing, the American Softwood Lumber Standard (PS 20) sets the dry dressed size at 1-1/2 by 3-1/2 inches. Length is not reduced.
| Measure | Nominal | Actual |
|---|---|---|
| Thickness | 2 in | 1-1/2 in (38 mm) |
| Width | 4 in | 3-1/2 in (89 mm) |
| Cross-section | 8 sq in | 5.25 sq in |
| Board feet per foot | 0.667 | (priced on nominal) |
What the grade stamp tells you
Each piece carries a stamp from a grading agency. It shows the grade, the species group, the mill number and the moisture content at the time of surface.
- Grade. Typical framing grades are Select Structural, No. 1, No. 2, and Stud. "Stud" grade is made for vertical use in walls and is usually sold in precut lengths. No. 2 is the usual grade for general framing.
- Species group. Common groups are SPF (spruce, pine, fir), Southern Pine, Douglas fir-larch and hem-fir. The group affects strength and price.
- Moisture. S-DRY means the wood was surfaced at 19% moisture or less, and KD means kiln dried. S-GRN means surfaced green, at more than 19%, and it will shrink as it dries.
Common uses
| Use | Typical spacing or form | Notes |
|---|---|---|
| Wall studs | 16 or 24 in on center | Stud grade or No. 2, see the stud calculator |
| Wall plates | Long boards, three times the wall length | Use long lengths to limit joints |
| Headers and blocking | Doubled or cut to fit | Size headers by span tables |
| Shelving and benches | Cut to length | Choose straight pieces |
| Outdoor use | Treated lumber | See treated vs untreated lumber |
Structural sizing. Spans, spacing and header sizes come from code span tables and from an engineer for anything unusual. A guide like this one gives sizes, not structural design.
Picking good boards
- Sight down the length for bow, crook and twist, and leave the bad ones.
- Avoid large knots on edges, splits at the ends and wane (bark on the corners).
- Buy 10% extra for culls.
For length options, see standard 2x4 lengths. Convert price with the lumber cost calculator, and see sizes of other lumber in the lumber dimensions chart.
Frequently asked questions
What are the actual dimensions of a 2x4?
1-1/2 by 3-1/2 inches, or about 38 by 89 mm.
What does KD mean on a 2x4?
Kiln dried, at 19% moisture or less. S-DRY also means 19% or less, and S-GRN means more than 19%.
What grade of 2x4 is best for framing?
Stud grade for wall studs, and No. 2 for general framing. Use Select Structural or No. 1 for higher loads.