Lumber sizes: nominal vs actual

A 2x4 is not 2 by 4 inches. Lumber is named by its nominal size, and the American Softwood Lumber Standard (NIST PS 20-25) sets the smallest it may measure once it has been dressed, or planed smooth. These are those minimum sizes, dry (19% moisture content or less) and green.

From the standard, NIST PS 20-25

Common sizes

Nominal	Actual, dry (in)	Actual, dry (mm)	Green (in)
1x2	3/4 × 1-1/2	19.1 × 38.1	13/16 × 1-9/16
1x3	3/4 × 2-1/2	19.1 × 63.5	13/16 × 2-9/16
1x4	3/4 × 3-1/2	19.1 × 88.9	13/16 × 3-9/16
1x6	3/4 × 5-1/2	19.1 × 139.7	13/16 × 5-5/8
1x8	3/4 × 7-1/4	19.1 × 184.2	13/16 × 7-1/2
1x10	3/4 × 9-1/4	19.1 × 235	13/16 × 9-1/2
1x12	3/4 × 11-1/4	19.1 × 285.8	13/16 × 11-1/2
5/4x6	1 × 5-1/2	25.4 × 139.7	1-1/16 × 5-5/8
2x2	1-1/2 × 1-1/2	38.1 × 38.1	1-9/16 × 1-9/16
2x3	1-1/2 × 2-1/2	38.1 × 63.5	1-9/16 × 2-9/16
2x4	1-1/2 × 3-1/2	38.1 × 88.9	1-9/16 × 3-9/16
2x6	1-1/2 × 5-1/2	38.1 × 139.7	1-9/16 × 5-5/8
2x8	1-1/2 × 7-1/4	38.1 × 184.2	1-9/16 × 7-1/2
2x10	1-1/2 × 9-1/4	38.1 × 235	1-9/16 × 9-1/2
2x12	1-1/2 × 11-1/4	38.1 × 285.8	1-9/16 × 11-1/2
4x4	3-1/2 × 3-1/2	88.9 × 88.9	3-9/16 × 3-9/16
4x6	3-1/2 × 5-1/2	88.9 × 139.7	3-9/16 × 5-5/8
6x6	5-1/2 × 5-1/2	139.7 × 139.7	5-1/2 × 5-1/2

Every size in the standard

Any listed thickness goes with any listed width, so a 2x10 is the 2 in dimension thickness with the 10 in width. Boards are under 2 in nominal thickness; dimension lumber is 2 in up to 5 in; timbers are 5 in and thicker.

Boards: thickness
Nominal	Dry	Green
3/8 in	5/16 in (7.9 mm)	3/8 in
1/2 in	7/16 in (11.1 mm)	1/2 in
5/8 in	9/16 in (14.3 mm)	5/8 in
3/4 in	5/8 in (15.9 mm)	11/16 in
1 in	3/4 in (19.1 mm)	13/16 in
1-1/4 in	1 in (25.4 mm)	1-1/16 in
1-1/2 in	1-1/4 in (31.8 mm)	1-5/16 in

Boards: width
Nominal	Dry	Green
2 in	1-1/2 in (38.1 mm)	1-9/16 in
3 in	2-1/2 in (63.5 mm)	2-9/16 in
4 in	3-1/2 in (88.9 mm)	3-9/16 in
5 in	4-1/2 in (114.3 mm)	4-5/8 in
6 in	5-1/2 in (139.7 mm)	5-5/8 in
7 in	6-1/2 in (165.1 mm)	6-5/8 in
8 in	7-1/4 in (184.2 mm)	7-1/2 in
9 in	8-1/4 in (209.6 mm)	8-1/2 in
10 in	9-1/4 in (235 mm)	9-1/2 in
11 in	10-1/4 in (260.4 mm)	10-1/2 in
12 in	11-1/4 in (285.8 mm)	11-1/2 in
14 in	13-1/4 in (336.6 mm)	13-1/2 in
16 in	15-1/4 in (387.4 mm)	15-1/2 in

Dimension lumber: thickness
Nominal	Dry	Green
2 in	1-1/2 in (38.1 mm)	1-9/16 in
2-1/2 in	2 in (50.8 mm)	2-1/16 in
3 in	2-1/2 in (63.5 mm)	2-9/16 in
3-1/2 in	3 in (76.2 mm)	3-1/16 in
4 in	3-1/2 in (88.9 mm)	3-9/16 in
4-1/2 in	4 in (101.6 mm)	4-1/16 in

Dimension lumber: width
Nominal	Dry	Green
2 in	1-1/2 in (38.1 mm)	1-9/16 in
2-1/2 in	2 in (50.8 mm)	2-1/16 in
3 in	2-1/2 in (63.5 mm)	2-9/16 in
3-1/2 in	3 in (76.2 mm)	3-1/16 in
4 in	3-1/2 in (88.9 mm)	3-9/16 in
4-1/2 in	4 in (101.6 mm)	4-1/16 in
5 in	4-1/2 in (114.3 mm)	4-5/8 in
6 in	5-1/2 in (139.7 mm)	5-5/8 in
8 in	7-1/4 in (184.2 mm)	7-1/2 in
10 in	9-1/4 in (235 mm)	9-1/2 in
12 in	11-1/4 in (285.8 mm)	11-1/2 in
14 in	13-1/4 in (336.6 mm)	13-1/2 in
16 in	15-1/4 in (387.4 mm)	15-1/2 in

Timbers (5 in and thicker)

Nominal size	Dry: take off	Green: take off
5 to 6 in	1/2 in	1/2 in
7 to 15 in	3/4 in	1/2 in
16 in and up	1 in	1/2 in

So a 6x6 is 5-1/2 × 5-1/2 in, and an 8x8 is 7-1/4 in square dry or 7-1/2 in green. The standard notes that timbers are often sold without drying.

Why lumber is smaller than its name

The nominal size is a name, not a measurement. The standard sets a minimum dressed size for each name, and lumber that has been dried is allowed to be smaller than lumber surfaced green, because wood shrinks as it dries.

Hardwood is different: it is sold rough, by thickness in quarters, and measured in board feet. See the hardwood thickness chart and the board foot calculator.

Sources

How we source these numbers