BuildMeter guide · Structures & Carpentry
Lumber Nominal vs. Actual Size Chart
Compare common North American nominal lumber labels with typical surfaced dimensions and understand which dimensions belong in volume, weight and layout calculations.
Direct answer
What to know first
Nominal lumber names describe the trade size, while surfaced lumber is smaller after drying and planing. For example, a nominal 2 × 4 is commonly about 1.5 × 3.5 inches. Use actual measured dimensions for volume, fit and metric conversion, and nominal names for purchasing and specification references.
Open Lumber Board-Foot CalculatorNominal names and physical dimensions serve different purposes
North American lumber is commonly sold by nominal trade names such as 2 × 4 or 4 × 4. Surfacing and drying reduce the finished cross-section. The trade name helps identify the product category, while actual dimensions determine fit, physical volume, weight and converted metric size.
| Nominal size | Typical actual size | Approximate metric actual size |
|---|---|---|
| 1 × 2 | 3/4 × 1-1/2 in. | 19 × 38 mm |
| 1 × 4 | 3/4 × 3-1/2 in. | 19 × 89 mm |
| 1 × 6 | 3/4 × 5-1/2 in. | 19 × 140 mm |
| 1 × 8 | 3/4 × 7-1/4 in. | 19 × 184 mm |
| 2 × 2 | 1-1/2 × 1-1/2 in. | 38 × 38 mm |
| 2 × 4 | 1-1/2 × 3-1/2 in. | 38 × 89 mm |
| 2 × 6 | 1-1/2 × 5-1/2 in. | 38 × 140 mm |
| 2 × 8 | 1-1/2 × 7-1/4 in. | 38 × 184 mm |
| 2 × 10 | 1-1/2 × 9-1/4 in. | 38 × 235 mm |
| 2 × 12 | 1-1/2 × 11-1/4 in. | 38 × 286 mm |
| 4 × 4 | 3-1/2 × 3-1/2 in. | 89 × 89 mm |
| 6 × 6 | 5-1/2 × 5-1/2 in. | 140 × 140 mm |
The table reflects common surfaced dimensions, not every product. Rough-sawn lumber, heavy timber, decking, specialty boards and regional products can differ. Measure the actual stock or use the grade stamp and supplier data when fit is critical.
Which dimensions belong in which calculation?
| Task | Dimension to use | Reason |
|---|---|---|
| Physical volume and weight | Actual dimensions | The wood occupies the surfaced cross-section. |
| Opening, notch or connector fit | Actual measured dimensions | Clearance depends on physical size and tolerance. |
| Purchasing list | Nominal trade name plus length and grade | Suppliers identify common framing lumber by nominal size. |
| Metric conversion | Actual dimensions | AWC advises converting physical dimensions rather than treating the nominal label as exact inches. |
| Structural capacity | Project specification and design values | Species, grade, size, moisture and use all matter. |
Worked volume example
A nominal 2 × 4 that is actually 1.5 × 3.5 in. and 8 ft long has a physical volume of 1.5 × 3.5 × 96 = 504 in³, or 0.292 ft³. Using the nominal 2 × 4 cross-section would produce 768 in³ and overstate physical volume by more than 50%.
Board feet require context
A board foot is 144 in³. Traditional lumber transactions may refer to rough or nominal conventions, while a physical-volume calculation uses actual thickness and width. Confirm whether a supplier’s board-foot pricing is based on nominal or surfaced dimensions before comparing it with a calculator.
Common mistakes
- Converting “2 × 4” directly to 50.8 × 101.6 mm as though it were a physical finished size.
- Using actual dimensions in a shopping list without also naming the nominal product.
- Assuming pressure-treated, rough-sawn and kiln-dried stock share identical dimensions.
- Using size alone to infer structural capacity.
- Forgetting that length can also be slightly different from the nominal or ordered value.
Use the calculators
Enter the appropriate dimensions in the Lumber Board-Foot Calculator and use actual cross-section plus density in the Lumber Weight Calculator. Framing quantities can be developed with the Stud Wall Framing Calculator.
Sources and limitations
- American Wood Council — weights and measurement
- American Wood Council — metric lumber conversion FAQ
- NIST SI units — length
Structural lumber selection must account for species, grade, treatment, moisture, span, load, connections and applicable design requirements. This chart is a dimension reference, not a span or capacity table.