183
Nominal → actual size converter
Pick a nominal size; the calculator returns the real dressed dimensions.
REFERENCE TABLE · CONSTRUCTION
Lumber Dimensions Chart: Nominal vs Actual
A 2×4 isn’t 2 inches by 4 inches — it hasn’t been for a century. Convert any nominal size to its real dressed dimensions in inches and millimeters, get the cross-section area and board feet, or print the full chart.
Full nominal vs actual chart (printable)
Standard dressed (S4S — surfaced four sides) softwood dimensions. Print this page for the shop wall.
| Nominal size | Actual (in) | Actual (decimal in) | Area (in²) | Actual (mm) |
|---|---|---|---|---|
| 1 × 2 | ¾ × 1 ½ | 0.75 × 1.50 | 1.12 | 19.0 × 38.1 |
| 1 × 3 | ¾ × 2 ½ | 0.75 × 2.50 | 1.88 | 19.0 × 63.5 |
| 1 × 4 | ¾ × 3 ½ | 0.75 × 3.50 | 2.62 | 19.0 × 88.9 |
| 1 × 6 | ¾ × 5 ½ | 0.75 × 5.50 | 4.12 | 19.0 × 139.7 |
| 1 × 8 | ¾ × 7 ¼ | 0.75 × 7.25 | 5.44 | 19.0 × 184.1 |
| 1 × 10 | ¾ × 9 ¼ | 0.75 × 9.25 | 6.94 | 19.0 × 234.9 |
| 1 × 12 | ¾ × 11 ¼ | 0.75 × 11.25 | 8.44 | 19.0 × 285.8 |
| 2 × 2 | 1 ½ × 1 ½ | 1.50 × 1.50 | 2.25 | 38.1 × 38.1 |
| 2 × 3 | 1 ½ × 2 ½ | 1.50 × 2.50 | 3.75 | 38.1 × 63.5 |
| 2 × 4 | 1 ½ × 3 ½ | 1.50 × 3.50 | 5.25 | 38.1 × 88.9 |
| 2 × 6 | 1 ½ × 5 ½ | 1.50 × 5.50 | 8.25 | 38.1 × 139.7 |
| 2 × 8 | 1 ½ × 7 ¼ | 1.50 × 7.25 | 10.88 | 38.1 × 184.1 |
| 2 × 10 | 1 ½ × 9 ¼ | 1.50 × 9.25 | 13.88 | 38.1 × 234.9 |
| 2 × 12 | 1 ½ × 11 ¼ | 1.50 × 11.25 | 16.88 | 38.1 × 285.8 |
| 4 × 4 | 3 ½ × 3 ½ | 3.50 × 3.50 | 12.25 | 88.9 × 88.9 |
| 4 × 6 | 3 ½ × 5 ½ | 3.50 × 5.50 | 19.25 | 88.9 × 139.7 |
| 6 × 6 | 5 ½ × 5 ½ | 5.50 × 5.50 | 30.25 | 139.7 × 139.7 |
Output · formula · meaning
| Output | Formula | What it means |
|---|---|---|
| Actual dimensions | lookup: standard S4S | A 2×4 is really 1.5 × 3.5 in (38.1 × 88.9 mm) — the number that matters for spans and fits. |
| Metric size | inches × 25.4 | The same dimensions in millimeters for metric plans. |
| Cross-section area | thickness × width | 5.25 in² for a 2×4 — the input span tables and strength calculations use. |
| Board feet | t × w × Lft ÷ 144 | How lumber is priced: a 2×4 × 8 ft is 0.29 board feet. |
Why nominal and actual differ
Nominal sizes describe the rough-sawn board before drying and surfacing. Kiln drying shrinks the wood, then the mill planes all four sides smooth (S4S), shaving off more. What reaches the rack is the “actual” or “dressed” size — and it has been standardized for over a century, so a 2×4 bought in 1950 and one bought today measure the same 1.5 × 3.5 inches.
Who uses this
- DIY builders: framing a wall or deck — the 3.5-inch actual width is what your layout math needs.
- Woodworkers: joinery dimensions come from actual sizes, never the nominal name.
- Estimators: board feet from real dimensions is how lumber quotes and takeoffs work.
- Students & teachers: the classic “why isn’t a 2×4 two by four” demo — cite it below.
Accuracy & limitations
These are the standard dressed (S4S) softwood dimensions used across North American framing lumber. Rough-sawn lumber is cut closer to nominal and varies; 5/4 decking is a special case (1 in actual); and individual boards can run a hair under from drying. For structural spans, always use the actual dimensions with a current span table.
Cutting concrete to match your framing? The concrete calculator and concrete tube calculator size footings and piers; the roof pitch chart converts slope for rafters.
Cite this reference tableBright Side Kit. “Lumber Dimensions Chart: Nominal vs Actual.” brightoolbox.com, 2026.
Bright Side Kit. "Lumber Dimensions Chart: Nominal vs Actual." https://brightoolbox.com/lumber-dimensions-chart/ Accessed October 22, 2026.Frequently asked questions
Why is a 2x4 actually 1.5 x 3.5 inches?
The “2×4” name describes the rough board before drying and planing. Kiln drying shrinks it and the mill surfaces all four sides (S4S), leaving 1.5 × 3.5 inches. The name stuck; the dimensions shrank a century ago and were standardized.
What does S4S mean on lumber?
Surfaced four sides: the board was planed smooth on both faces and both edges. Almost all dimensional lumber at home centers is S4S, which is why the chart above applies to it. Rough-sawn lumber skips this step and runs closer to nominal.
Is a 4x4 really 3.5 x 3.5 inches?
Yes — a standard dressed 4×4 measures 3.5 × 3.5 inches, and a 6×6 measures 5.5 × 5.5. Posts follow the same nominal-minus-shrinking rule as framing lumber.
How do I calculate board feet?
Board feet = thickness (in) × width (in) × length (ft) ÷ 144, using actual dimensions. A 2×4 that’s 8 feet long is 1.5 × 3.5 × 8 ÷ 144 = 0.29 board feet. Hardwood is usually priced by the board foot.
Does the length of the board change the cross-section?
No — a 2×4 is 1.5 × 3.5 inches whether it’s 8 feet or 16 feet long. Length only matters for board feet, spans, and your cut list.
Are metric countries’ lumber sizes different?
Somewhat — many metric markets sell the same S4S dimensions labeled in millimeters (38 × 89 mm for a 2×4), while others use different standards. The converter above gives exact millimeters for the North American sizes.