Lumber Size Chart (nominal 2×4 vs actual)

Understanding the lumber size chart (nominal vs actual) — concept and principle

In North America, framing and board lumber is named by nominal size, such as 2×4 or 2×6, but the board you buy is noticeably smaller. A "two-by-four" is not 2 by 4 inches; it is 1½ by 3½ inches. Builders, DIYers and anyone in Korea working with imported SPF framing lumber sold under the same names need the actual size to design shelves, studs, decks and joints. Pick a nominal size and the chart shows the actual dressed size in inches and millimetres.

How it is calculated

The calculator looks up a fixed table of standard dry, surfaced (planed) sizes for softwood lumber. The pattern is:

• Nominal 1 in thickness → ¾ in (19 mm)
• Nominal 2 in → 1½ in (38 mm); nominal 3, 4, 5, 6 in → subtract ½ in
• Nominal 8 in and wider → subtract ¾ in (8 → 7¼, 10 → 9¼, 12 → 11¼ in)

Millimetres are actual inches × 25.4, rounded to the nearest millimetre.

The principle

Nominal names describe the rough-sawn size. When a log is sawn, the boards are cut at roughly the nominal dimension; they then shrink as they dry and lose more when they are planed smooth on all four sides (S4S), which also rounds the edges. The American Softwood Lumber Standard (PS 20) fixes the finished dimensions so that a 2×4 from any mill is the same size. Wider boards lose a little more, partly because shrinkage across the grain grows with width. Unseasoned (green) lumber is made slightly larger so that it reaches about the same size once it dries.

Worked example

1. Default input: nominal 2 × 4.
2. Thickness: nominal 2 in → 1½ in.
3. Width: nominal 4 in − ½ in → 3½ in.
4. In millimetres: 1.5 × 25.4 = 38.1 → 38 mm, and 3.5 × 25.4 = 88.9 → 89 mm, so 38 × 89 mm.

How to read the result

Always design with the actual size. Three 2×4 studs side by side are 4½ in thick, not 6 in. Length is not reduced in the same way: an 8 ft board is sold at its full length. Board-foot pricing in the US uses nominal dimensions, so a 2×4 that is 12 ft long counts as 2 × 4 × 12 ÷ 12 = 8 board feet even though it contains less wood.

Common mistakes

• Spacing studs or cutting notches using nominal sizes.
• Assuming hardwood boards follow the same rule; hardwoods are sold by thickness in quarters (4/4, 8/4) and their surfaced sizes differ.
• Expecting metric-sized lumber in Europe or Korea to match North American actual sizes exactly.

Frequently asked questions

What is the actual size of a 2×4?

1½ × 3½ in, about 38 × 89 mm.

Is a 2×6 really 6 inches wide?

No, it is 5½ in (140 mm).

What is a 4×4 actually?

3½ × 3½ in, about 89 × 89 mm.

Why is a 2×8 7¼ inches and not 7½?

Boards 8 in and wider lose ¾ in rather than ½ in under the standard, because wider boards shrink more as they dry.

Sources: NIST Voluntary Product Standard PS 20, American Softwood Lumber Standard · U.S. Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material