Nominal vs Actual Lumber Dimensions

A 2×4 measures 1½ by 3½ inches. The nominal name describes the rough, green timber before it was milled; surfacing on four sides and drying takes off the difference. The name survives because it is how lumber is ordered, tallied and priced.

Boards

Nominal	Actual
1 × 2	¾″ × 1½″
1 × 3	¾″ × 2½″
1 × 4	¾″ × 3½″
1 × 6	¾″ × 5½″
1 × 8	¾″ × 7¼″
1 × 10	¾″ × 9¼″
1 × 12	¾″ × 11¼″

Dimension lumber

Nominal	Actual
2 × 2	1½″ × 1½″
2 × 3	1½″ × 2½″
2 × 4	1½″ × 3½″
2 × 6	1½″ × 5½″
2 × 8	1½″ × 7¼″
2 × 10	1½″ × 9¼″
2 × 12	1½″ × 11¼″

Timbers

Nominal	Actual
4 × 4	3½″ × 3½″
4 × 6	3½″ × 5½″
6 × 6	5½″ × 5½″
8 × 8	7¼″ × 7¼″

The rule behind the table

It is not arbitrary, and knowing the rule covers the sizes not listed above.

Widths under 8″ nominal: subtract ½″. A 2×6 is 5½.
Widths of 8″ nominal and over: subtract ¾″. A 2×8 is 7¼, a 2×12 is 11¼.
Thicknesses: 1″ → ¾″, 2″ → 1½″, 4″ → 3½″, 6″ → 5½″.

That step at 8 inches is the one that catches people. A 2×6 loses half an inch and a 2×8 loses three-quarters, so the run of widths is not evenly spaced: 3½, 5½, 7¼, 9¼, 11¼.

This table is for surfaced, dry softwood. Rough-sawn lumber is closer to its nominal size, green lumber is milled slightly larger to allow for shrinkage, and hardwood is sized by quarters of an inch in thickness — 4/4, 5/4, 8/4 — rather than by nominal names.

Where it bites

Stacked framing. Three 2×4s laid flat are 4½ inches, not 6. A built-up header of two 2×10s with ½″ plywood between is 3½ inches — which is exactly a 2×4 wall, and that is not a coincidence.

Board feet, though, use nominal. A 2×4×8 tallies as (2 × 4 × 8) ÷ 12 = 5.33 board feet, not the actual volume. Run a cut list through the board foot calculator and it will use the nominal figures for exactly this reason.

Common questions

What are the actual dimensions of a 2x4?

A 2x4 actually measures 1½ by 3½ inches once it has been surfaced and dried. The 2 by 4 is the nominal size, which describes the rough green timber before it was milled, and it is what the piece is called and priced as.

Why is a 2x4 not actually 2 inches by 4 inches?

It was, roughly, when it was rough-sawn and green. Surfacing on four sides and drying to below 19 percent moisture removes the difference. The nominal name stuck because that is how lumber is ordered, tallied and priced.

How do you work out actual lumber size from nominal?

For widths, subtract ½ inch below 8 inches nominal and ¾ inch at 8 inches and above — so a 2x6 is 5½ and a 2x8 is 7¼. For thicknesses, 1 inch nominal is ¾, 2 inch is 1½, 4 inch is 3½ and 6 inch is 5½.

Do board feet use nominal or actual dimensions?

Nominal. A 2x4x8 tallies as (2 × 4 × 8) ÷ 12 = 5.33 board feet, using the nominal 2 and 4. Using the actual 1½ and 3½ would undercount by about a third and put your figure out against any supplier quote.

Check it against the job. Building codes, span tables and product specifications vary by jurisdiction and manufacturer. Your approved drawings and your local code govern, not this page.