Nominal vs actual lumber sizes

What nominal means

The American Softwood Lumber Standard (PS 20) defines it:

The label designation for lumber size categories that does not reflect the dressed size. The nominal size is greater than the dressed size i.e., a dry 2 by 4 is surfaced to 38.1 mm by 88.9 mm (1 1/2 by 3 1/2 inches) [PS 20-20, section 2.16]

And warns against reading a nominal size as a measurement:

The use of "nominal" sizes in the language of this Standard follows the practice of the industry. No inferences shall be drawn that the "nominal" sizes are dressed sizes. [PS 20-20, section 3.4.4]

The Wood Handbook says the same about how lumber is recorded:

Lumber length is recorded in actual dimensions, whereas width and thickness are traditionally recorded in “nominal” dimensions—actual dimensions are somewhat less. [Wood Handbook, ch. 6, p. 6-10]

Common sizes

The standard’s Table 3 is titled “Nominal and minimum-dressed sizes of boards, dimension, and timbers.” Under the title it says:

The thicknesses apply to all widths and all widths apply to all thicknesses. [PS 20-20, Table 3]

The sizes below are read from that table, dry and green. The standard defines the two:

Dry: Lumber of less than nominal 5-inch thickness which has been seasoned or dried to a maximum moisture content of 19 percent. [PS 20-20, section 2.7]

Green: Lumber of less than nominal 5-inch thickness which has a moisture content in excess of 19 percent. [PS 20-20, section 2.12]

The table’s note on the green columns:

except for minimum dressed green sizes for Redwood, Western Red Cedar, and Northern White Cedar [see section 6.2.5]. [PS 20-20, Table 3]

Each cell is thickness by width, in the standard’s minimum dressed sizes. Table 3 lists more sizes than these, including thinner and thicker boards and the timbers.

Counted at the name

PS 20 defines board measure on the nominal size, not the dressed one:

The number of board feet in a piece of lumber is obtained by multiplying the nominal thickness in inches or fraction of an inch by the nominal width in feet by the length in feet. [PS 20-20, section 2.2]

So a 2x4 that is 8 feet long counts as 2 × 4 × 8 ÷ 12 = 5.33 board feet, though the wood in the rack measures 1½″ by 3½″. Counted at what the tape reads it would be 3.50 board feet. The calculator counts the name by default. A store 2×4 is really 1½″ × 3½″. The price uses 2 × 4.

If you type the dressed size (1½″ × 3½″) the calculator recognises it as that store size and counts the name, and a button switches to counting what you measured. Hardwood has no such names: it is usually manufactured to random width, so type what you measure. [Wood Handbook, ch. 6, p. 6-3]

Wider boards lose more

Read down the width column of Table 3 and the dry size drops by a different amount. Widths of nominal 2 through 7 inches are one half inch under; 8 inches and wider are three quarters of an inch under, so a 1x8 is 7¼″ wide, not 7½. Do not assume a uniform half inch.

Try it

Put 2 and 4 into the calculator, or tap the 2×4 button, and it shows the real size under the answer.