Nominal vs Actual Lumber Sizes (and What It Means for Board Feet)

Why nominal sizes exist

Softwood lumber is named by its approximate rough-sawn size. When the log is sawn, a "2×4" is close to 2 × 4 in. It is then dried, which shrinks it, and planed (surfaced) on all four sides to make it smooth and uniform. The final dressed size is standardized by the American Softwood Lumber Standard (PS 20) — and for a 2×4 it is 1½ × 3½ in.

Size chart

Rule of thumb: up to 6 in nominal, subtract ½ in; 8 in and above, subtract ¾ in. Thickness of 1 in nominal is ¾ in; 2 in nominal is 1½ in.

Metric timber

In the UK, Europe and Australia timber is named by its actual size in millimeters, although the old habit persists in conversation — "four by two" for a 100 × 50 mm section, which may be 95 × 45 mm or 89 × 38 mm once planed. Check whether a size is sawn or planed (PSE/PAR) before designing to it.

Hardwood is different

Hardwood is usually sold rough-sawn by thickness in quarter inches: 4/4 is 1 in, 5/4 is 1¼ in, 8/4 is 2 in. Widths and lengths are random. Board feet are calculated from the rough dimensions, and you plane it down yourself — a 4/4 board typically finishes at about ¾–13/16 in.

Board feet and nominal sizes

A board foot is 144 cubic inches — 1 in × 12 in × 12 in. Board feet = thickness (in) × width (in) × length (ft) ÷ 12. For softwood, yards calculate board feet from nominal sizes, so a 2×6 that is 8 ft long is 2 × 6 × 8 ÷ 12 = 8 BF, even though the actual wood is only 1½ × 5½ in. For hardwood, use the rough size.

Moisture and movement

Lumber is sold at a stated moisture content — "KD" (kiln-dried) or "S-DRY" softwood is generally at or below 19%, while green and freshly pressure-treated wood can be much wetter. Wood shrinks as it dries, mostly across its width and thickness rather than along its length. A wet 2×10 can lose a noticeable fraction of an inch in width, which matters when you are spacing deck boards or fitting trim. Let lumber acclimate on site, stickered and covered, before precise work.

Engineered products

Engineered lumber — LVL, glulam, I-joists and plywood or OSB sheets — follows its own sizing. Sheet goods are close to their nominal sizes in length and width (4 × 8 ft, or 1220 × 2440 mm), but thickness is often slightly under the nominal, such as 23/32 in for "¾ in" plywood. I-joists and LVL come in depths chosen to match dimensional lumber so they can be mixed in a frame. Always use the manufacturer's span tables for engineered products.

Buying lumber: grades and lengths

Dimensional lumber is graded for strength and appearance. In North America, common structural grades include Select Structural, No. 1 and No. 2, and stud grade; each grade stamp also shows the species group, mill and moisture condition. Appearance boards for trim and furniture are graded differently. Lengths are usually sold in 2 ft increments from 8 to 16 ft or longer, so plan cut lists to use full lengths efficiently.

In metric markets, timber is sold in lengths in 300 mm steps (2.4, 2.7, 3.0, 3.6, 4.2, 4.8 m and so on) and strength classes such as C16 and C24 in the UK and Europe, or MGP10 and MGP12 in Australia. When converting a plan from one market to another, check both the actual section size and the strength class — a 47 × 100 mm C24 stud is not a direct equivalent to a US 2×4 No. 2 in every application.

Reading a grade stamp

A North American grade stamp typically shows the grading agency, the mill number, the grade, the species or species group (for example SPF or DF-L) and the moisture condition (S-DRY, KD or S-GRN). Treated lumber also carries a tag with the preservative and its use category, such as ground contact. Check the stamp when buying for structural or outdoor use, not just the size.

When the difference matters

• Framing a wall: three 2×4 studs side by side are 4½ in, not 6 in.
• Deck board rows: use 5½ in, not 6 in, or you will run short.
• Shelving: a 1×12 shelf is 11¼ in deep.
• Stair stringers: a 2×12 has 11¼ in of depth to cut the notches from.
• Structural span tables already use actual sizes — but you must use the matching species and grade.