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Measure

Lumber Size Chart

What nominal lumber sizes actually measure.

Nominal and actualReference01
NominalActual (in)Actual (mm)
1 × 20.75 × 1.519 × 38.1
1 × 30.75 × 2.519 × 63.5
1 × 40.75 × 3.519 × 88.9
1 × 60.75 × 5.519 × 139.7
1 × 80.75 × 7.2519 × 184.1
1 × 100.75 × 9.2519 × 234.9
1 × 120.75 × 11.2519 × 285.8
2 × 21.5 × 1.538.1 × 38.1
2 × 31.5 × 2.538.1 × 63.5
2 × 41.5 × 3.538.1 × 88.9
2 × 61.5 × 5.538.1 × 139.7
2 × 81.5 × 7.2538.1 × 184.1
2 × 101.5 × 9.2538.1 × 234.9
2 × 121.5 × 11.2538.1 × 285.8
4 × 43.5 × 3.588.9 × 88.9
4 × 63.5 × 5.588.9 × 139.7
6 × 65.5 × 5.5139.7 × 139.7

Applies to surfaced, dry softwood. Rough-sawn and green lumber measure closer to nominal, and hardwood is sold by quarters of an inch of rough thickness instead.

A 2x4 has not measured two by four inches in a very long time. Surfacing removes material after the board is named, so every nominal size is smaller than it sounds — and the amount removed changes at eight inches, which is the part people forget.

Why use this tool?What it does differently03

Why use this tool?

Every common size

From 1x2 through 6x6, in inches and millimetres.

The eight-inch step

Widths under 8 in lose half an inch; 8 in and over lose three quarters. That change catches people out.

Generated from the rule

Derived from the surfacing convention rather than transcribed, so it is internally consistent throughout.

Scope stated

Surfaced dry softwood. Rough-sawn, green and hardwood all measure differently.

How this worksThe method04

How this lumber size chart works

Lumber is named at its rough-sawn green dimension and then surfaced on all four sides, which removes material. The name sticks; the wood does not.

Thickness loses a quarter inch at one inch nominal and half an inch above it, so a nominal 1 is 3/4 and a nominal 2 is 1 1/2. Width loses half an inch below eight inches nominal and three quarters at eight and above — which is why a 2x8 is 7 1/4 rather than 7 1/2.

That step at eight inches is the detail worth remembering. Assuming a consistent half inch across all widths puts a 2x10 and a 2x12 out by a quarter inch each, which compounds across a floor.

How to use itStep by step05

How to use it

  1. Step 1: Find your nominal size

    What it is called at the yard — 2x4, 2x10 and so on.

  2. Step 2: Read the actual

    In inches or millimetres, whichever your drawings use.

  3. Step 3: Watch the 8 in step

    Widths of 8 and above lose three quarters rather than a half.

  4. Step 4: Check rough stock separately

    Rough-sawn and green lumber measure much closer to nominal.

Example usageWorked figures06

Example usage

The famous one
A 2x4 actually measures 1 1/2 by 3 1/2 inches — 38.1 by 88.9 mm.
Where the rule changes
A 2x6 is 5 1/2 wide but a 2x8 is 7 1/4, not 7 1/2. The extra quarter inch comes off at eight and stays off above it.
Posts
A 4x4 is 3 1/2 by 3 1/2 and a 6x6 is 5 1/2 by 5 1/2 — the same half-inch loss on both dimensions.
Frequently asked questionsCommon questions07

Frequently asked questions

What size is a 2x4 really?

1 1/2 by 3 1/2 inches, or 38.1 by 88.9 mm. It is named at its rough green dimension and surfaced down from there.

Why is lumber smaller than its name?

Because it is named before it is surfaced. The board is sawn to roughly the nominal size green, then dried and planed on all four sides, which removes material the name never accounts for.

Is a 2x8 exactly 7 1/2 inches?

No — it is 7 1/4. Widths of eight inches nominal and above lose three quarters of an inch rather than a half, which is the exception most people miss.

Does this apply to hardwood?

No. Hardwood is sold by quarters of an inch of rough thickness — 4/4, 8/4 — and by the board foot, with actual surfaced thickness less again.

What about rough-sawn lumber?

Rough-sawn and green stock measure much closer to nominal, since the loss comes from surfacing and drying. Buy it expecting full dimensions and mill it yourself.

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