Nominal vs actual lumber sizes: the full chart

Why lumber is smaller than its name

Nominal size is roughly the size of the board when it is first sawn from the log, while still green. Two things take it down to its finished size:

• Drying. Wood shrinks as it dries below about 25–30% moisture content (the fiber saturation point described in the USDA Wood Handbook), mostly across its width and thickness.
• Surfacing. The mill planes all four faces smooth and eases the edges. That is what “S4S” (surfaced four sides) on a tag means.

In the US, the finished dry sizes are standardized by the American Softwood Lumber Standard (NIST PS 20), and Canadian mills follow matching sizes. That is why a 2×4 from any mill measures the same.

Dimensional lumber chart

The rule of thumb: for nominal sizes up to 6 in, the actual size is ½ in less (¼ in less for 1× thickness, which ends up ¾ in). From 8 in and up, subtract ¾ in. Timbers are the exception: 6×6 and larger are normally dressed while still green, ½ in under nominal on each face, and shrink a little more as they dry. PS 20 dresses 5/4 boards to at least 1 in dry; 5/4 shop lumber for millwork is planed thicker, to 1-5/32 in, under the NELMA and WWPA shop rules.

The three sizes that cause the most confusion each have a worked page: what is the actual size of a 2x4 covers studs, precut lengths and wall depth; what is the actual size of a 2x6 covers deck rows and 2×6 walls; and what is the actual size of a 4x4 covers wet treated posts that measure fat. For board feet at every length, see the board foot chart.

Lengths

Lumber lengths are generally true to their nominal length or slightly over — an 8 ft board is at least 96 in, sometimes a fraction more. Common lengths run in 2 ft steps from 8 to 20 ft. Studs are the exception: precut studs are sold at lengths such as 92-5/8 in and 104-5/8 in so that, with top and bottom plates, they frame standard 8 ft and 9 ft walls.

Where the difference bites

• Wall and floor depth. A 2×4 wall cavity is 3½ in deep, and a 2×6 wall is 5½ in — insulation, electrical boxes and window jambs are all sized for that.
• Built-up posts and beams. Two 2×4s nailed together are 3 × 3½ in, not 4 × 4. Three 2×10s make a 4½ in thick beam.
• Shelves and cabinets. A 1×12 shelf is 11¼ in deep. Plan cabinet depths and shelf pins around it.
• Deck layout. Deck rows are counted on the 5½ in face of a 2×6 or 5/4×6, plus the gap.
• Board feet vs weight. Board-foot prices use nominal size; weight and true volume use actual size.

Plywood and sheet goods

Sheet goods have their own version of the same problem. Plywood sold as ¾ in is usually 23/32 in (about 18 mm), “½ in” is often 15/32, and hardwood plywood from overseas may be metric thickness. Sheets are generally a true 4 × 8 ft (1220 × 2440 mm), though some products are slightly undersized so they can be spaced on a wall. Measure the actual sheet before cutting dadoes or grooves for it.

Hardwood is different

Hardwood lumber is not sold in nominal dimensions at all. It is sold by rough thickness in quarter inches — 4/4 is 1 in rough, 8/4 is 2 in — at whatever random width the log gave. Surfaced two sides (S2S), the NHLA standard thicknesses are 13/16 in for 4/4, 1-1/16 in for 5/4, 1-5/16 in for 6/4, 1¾ in for 8/4 and 2¾ in for 12/4, while board feet stay on the rough size. Our hardwood buying guide covers the terms, and the hardwood tally calculator adds up a stack of random-width boards.

Metric timber

In the UK, Europe, Australia and New Zealand, planed timber is usually labelled with its actual finished size in millimetres (for example 18 × 94 mm or 45 × 95 mm), while sawn timber is labelled by its sawn size, which planing reduces. Some imported North American sizes appear too — CLS studs are 38 × 89 mm, the same as a 2×4. As everywhere, measure before you design joinery around a label.