Lumber · Reference

Lumber dimensions: nominal vs actual

A 2×4 is 1.5 × 3.5 in, not 2 × 4. Look up the actual dressed size of any nominal lumber in inches or millimetres — then see what the gap does to your plate stacks and your material order.

Detail A — Nominal → actual

Actual dressed (S4S) size

Actual thickness (in)—

Actual width (in)—

Nominal name—

Cross-section

— Actual cross-section (in)

Stacked & overstatement

Stacked thickness—

Real wood vs the name

01 How the math works

There is no formula to derive — the actual size is a lookup. Softwood dimensional lumber is sawn to the nominal size, then surfaced on all four sides (S4S) and dried, which takes it down to a smaller finished dimension. That finished dimension is fixed by the American Softwood Lumber Standard PS 20, so a 2×4 leaves every mill at 1.5 × 3.5 in.

Actual thickness, width = PS 20 dressed size for the nominal name
Stacked thickness = pieces × actual thickness
Real wood share = (actual thickness × actual width) ÷ (nominal thickness × nominal width)

The difference between the "2×4" on the rack and the 1.5 × 3.5 in of actual wood is one of the few numbers the lumber yard, the framer, and the inspector all agree on. It just never makes it onto the price tag.

The full dressed-size table, both units (PS 20; metric figures are the standard's own whole-millimetre actuals):

1×20.75 × 1.5 in · 19 × 38 mm

1×40.75 × 3.5 in · 19 × 89 mm

1×60.75 × 5.5 in · 19 × 140 mm

1×80.75 × 7.25 in · 19 × 184 mm

2×21.5 × 1.5 in · 38 × 38 mm

2×41.5 × 3.5 in · 38 × 89 mm

2×61.5 × 5.5 in · 38 × 140 mm

2×81.5 × 7.25 in · 38 × 184 mm

2×101.5 × 9.25 in · 38 × 235 mm

2×121.5 × 11.25 in · 38 × 286 mm

4×43.5 × 3.5 in · 89 × 89 mm

6×65.5 × 5.5 in · 140 × 140 mm

5/4×6 decking1 × 5.5 in · 25 × 140 mm

02 Worked example

Take the most common member on any job, the 2×4. Its actual size is 1.5 × 3.5 in (38 × 89 mm). A wall framed with 2×4 studs is 3.5 in thick before drywall — not 4 in.

The stacking matters more than the single piece. A platform-framed wall carries three plates: one bottom plate and a doubled top plate. At the 1.5 in plate thickness, those three stack to 4.5 in (114 mm), not the 6 in the name implies. That is why an 8 ft wall uses a 92-5/8 in (2,353 mm) pre-cut stud: the stud plus its three plates makes a 97-1/8 in (2,467 mm) rough wall, which clears an 8 ft stack of gypsum board with room for the floor and ceiling finish.

Set the calculator to a 2×4 with 3 pieces stacked and it reads back the 4.5 in plate stack directly.

03 When this calculator is wrong

The nominal-vs-actual gap is the single biggest source of estimating errors in DIY framing, and it is empirical fact, not opinion — it lives in PS 20. A 2×4 is 1.5 × 3.5 in, so its cross-section is 5.25 in² against a nominal 8 in²: only 66% of the name is wood, and the name overstates the section by 34%. Compute wall thickness, volume, or coverage from the nominal number and you are wrong on every axis. There is no counter-case here; the only fix is to estimate off the actual dimension.

Where the table itself stops applying:

Rough-sawn and full-dimension lumber. A true rough-sawn 2×4 measures a full 2 × 4 in. The table is for surfaced S4S stock — reclaimed timber, sawmill-direct orders, and some heavy timbers are sold at or near their full nominal size. Measure the actual stock before you cut.
The metric figures are standardized, not converted. PS 20 fixes the metric actuals as clean whole millimetres: 38 × 89 mm for a 2×4. Convert 1.5 in yourself and you get 38.1 mm, which disagrees with the standard by 0.1 mm on every stud. Use the listed figure, not your own conversion.
Engineered and panel products don't follow it. LVL, glulam, and I-joists are made to a called-out actual size, and sheet goods like plywood and OSB run to their own nominal scale, where the labelled thickness and the measured thickness can differ again. None of them read off this table — check the actual spec on the stamp.
Deck boards eat the same gap. A "6 in" board is 5.5 in of face. Lay decking across a 12 ft (144 in) width using the nominal 6 in and you get 24 rows; the real 5-1/2 in face plus a 1/8 in gap needs 26 rows — a 7% under-order from trusting the name.

04 What to do with the result

Estimate off the actual dimension, every time. For a stud wall, order plate stock at three times the wall length (one bottom, two top) and read the stacked thickness for how much of your stud height the plates consume. For headers and built-up beams, the stacked number is the real assembled thickness — three 2× plies make 4.5 in, which is why a built-up beam in a 2×4 wall needs a filler to reach the 3.5 in wall depth from two plies.

When a drawing calls out a "2×," it means the actual 1.5 in member — the actual size is what carries load and what the span tables assume. If you are buying rough-sawn or reclaimed stock, ignore this table and measure the pieces you have, because they won't match it.

05 Common questions

Why is a 2×4 not actually 2 by 4 inches?: It starts life close to 2 × 4 in rough-sawn, then gets surfaced on four sides and dried to a smaller finished size. PS 20 fixes that finished size at 1.5 × 3.5 in (38 × 89 mm). The nominal name is the rough size; the actual is what you buy.
What are the actual dimensions of 2× lumber?: Every 2× is 1.5 in thick. The widths: 2×4 = 1.5 × 3.5 in, 2×6 = 1.5 × 5.5 in, 2×8 = 1.5 × 7.25 in, 2×10 = 1.5 × 9.25 in, 2×12 = 1.5 × 11.25 in.
What is a 2×4 in millimetres?: 38 × 89 mm, as PS 20 lists it. These are the standard's own whole-millimetre figures, not a conversion — converting 1.5 in gives 38.1 mm, which is close but not what the standard says.
What is the actual size of a 4×4 post?: 3.5 × 3.5 in (89 × 89 mm). A 6×6 is 5.5 × 5.5 in (140 × 140 mm). The same nominal-vs-actual gap applies to posts.
What size is 5/4 decking?: 1 × 5.5 in (25 × 140 mm). The 5-1/2 in face is the same as a 2×6, so a "6 in" deck board covers 5.5 in of surface, not 6.