Lumber Sizes: Actual vs Nominal Dimensions Chart

The name of a piece of lumber is its nominal size: roughly what it measured when it was sawn from the log. By the time it’s dried and planed smooth, it’s smaller. That’s why a 2x4 is actually 1½ × 3½ inches. The rule for softwood lumber is simple once you see the pattern, and the chart below covers every common size.

Dimensional lumber sizes chart

These are the dry, surfaced sizes set by the American Softwood Lumber Standard (PS 20), used for framing lumber such as pine, fir and spruce at US home centers.

Nominal	Actual (inches)	Actual (mm)
1 × 2	¾ × 1½	19 × 38
1 × 3	¾ × 2½	19 × 64
1 × 4	¾ × 3½	19 × 89
1 × 6	¾ × 5½	19 × 140
1 × 8	¾ × 7¼	19 × 184
1 × 10	¾ × 9¼	19 × 235
1 × 12	¾ × 11¼	19 × 286
2 × 2	1½ × 1½	38 × 38
2 × 3	1½ × 2½	38 × 64
2 × 4	1½ × 3½	38 × 89
2 × 6	1½ × 5½	38 × 140
2 × 8	1½ × 7¼	38 × 184
2 × 10	1½ × 9¼	38 × 235
2 × 12	1½ × 11¼	38 × 286
4 × 4	3½ × 3½	89 × 89
4 × 6	3½ × 5½	89 × 140
6 × 6	5½ × 5½	140 × 140
8 × 8	7¼ × 7¼	184 × 184

The pattern behind the numbers

You don’t need the chart once you know the rule:

• 1 inch nominal (thickness of boards) → ¾ inch actual.
• 2, 3, 4, 5 and 6 inch nominal → subtract ½ inch: 2 → 1½, 4 → 3½, 6 → 5½.
• 8 inches and more → subtract ¾ inch: 8 → 7¼, 10 → 9¼, 12 → 11¼.

So a 2x10 is 1½ × 9¼ inches, and a 4x6 is 3½ × 5½ inches.

Lengths, studs and sheets

Lengths are the real length. An 8-foot 2x4 is at least 96 inches long, and often a little more, so it can be cut square.

Precut studs are shorter on purpose. A wall frame has one bottom plate and two top plates, each 1½ inches thick, so a 92⅝ inch stud makes a wall 97⅛ inches tall: just right for 8-foot drywall sheets plus a ceiling. For 9-foot walls, studs are 104⅝ inches.

Plywood and OSB sheets are a full 4 × 8 feet (48 × 96 inches), but the thickness is usually a little under its name:

Sold as	Actual thickness	In mm
¼ in	7/32 in	5.6
⅜ in	11/32 in	8.7
½ in	15/32 in	11.9
⅝ in	19/32 in	15.1
¾ in	23/32 in	18.3

The difference matters when you cut a groove (a dado) for a shelf: a ¾ inch dado will be loose around 23/32 inch plywood.

Exceptions to watch for

• Pressure-treated lumber is often sold wet, so it can be slightly larger at first and shrink as it dries outside.
• Rough-sawn lumber from a sawmill hasn’t been planed, so it’s close to its full nominal size: a rough 2x4 is about 2 × 4 inches.
• Hardwoods (oak, maple, walnut) use a different system called quarters: 4/4 (“four quarter”) is 1 inch thick when rough and about 13/16 inch after planing. Widths are random, so measure each board.
• Decking sold as 5/4 × 6 is 1 × 5½ inches.

Measuring lumber yourself

Measure the thickness and width with a ruler or tape across the face, away from the ends, which can be rough or split. For lengths, hook the tape on one end and read at the other; the tape measure shows how to read the 16 inch stud marks along the way. To convert any size, the inches to mm converter does it exactly (1 inch = 25.4 mm).

Building a board and batten or grid accent wall from 1x3 or 1x4 boards? The board and batten calculator works out the spacing and how many boards to buy.

More DIY charts: plywood and drywall thickness · nail sizes · pipe sizes: nominal vs actual · sandpaper grit · screw and bolt sizes.