Lumber Size Calculator: Nominal vs Actual Dimensions

How to use the lumber size calculator

1. Pick a size from the chips, or type the nominal thickness and width. Use 1.25 for 5/4 decking and 1.5 for 6/4 boards.
2. Choose dry or green. Kiln-dried studs and joists stamped KD or S-DRY are dry. Large timbers and some treated lumber are often surfaced green (S-GRN) and are a touch bigger.
3. Enter the length and number of pieces to get board feet and linear feet for the order.
4. Optionally add a price, either per piece or per board foot, to compare a stud at the home center with a lumber yard’s board-foot price.

Have a board and no label? Switch to Measured → nominal, enter what your tape says, and the tool finds the nominal size within 1/16 in.

Why a 2×4 is not 2 inches by 4 inches

“2×4” is a name, not a measurement. The American Softwood Lumber Standard, PS 20, calls the nominal size “the label designation for lumber size categories that does not reflect the dressed size”, and gives the example itself: a dry 2 by 4 is surfaced to 38.1 by 88.9 mm, or 1½ by 3½ in. The difference comes from sawing a rough piece from the log, drying it, and planing all four faces smooth. PS 20 even sets a floor for rough lumber: it must be at least ⅛ in thicker and wider than the dressed size it will be planed to.

The sizes in the standard are minimums at the time of manufacture. PS 20 notes that lumber shrinks or swells as its moisture content changes, so a stud that has been sitting in a damp yard, or drying in a heated room, can measure a hair over or under the table.

Nominal vs actual lumber size chart

All values are from PS 20-25, Table 3. Notice the pattern. Through 6 in, a dry piece loses ½ in (2 becomes 1½, 6 becomes 5½). From 8 in up it loses ¾ in, so a 2×8 is 7¼ in deep and a 2×12 is 11¼. Boards of 1 in nominal thickness are ¾ in thick, and 5/4 boards are a full 1 in.

Boards, dimension lumber and timbers

PS 20 sorts softwood lumber into three size classes, and the class decides which row of the table applies:

• Boards are less than 2 in nominal thickness: 1×4 trim, 1×6 fence pickets and 5/4 decking.
• Dimension lumber runs from 2 in up to, but not including, 5 in nominal thickness. It is what you frame with: studs, joists, rafters and planks, including 4×4 and 4×6 posts.
• Timbers are 5 in nominal or more in their least dimension: 6×6 posts, beams and girders. Instead of a fixed table, the standard takes a set amount off each face. Dry timbers lose ½ in at 5 and 6 in, ¾ in from 7 to 15 in, and 1 in at 16 in and up. Green timbers lose ½ in at every size, which is why a 6×6 post is 5½ in square either way and a green 8×8 is 7½ in.

Dry, in PS 20, means 19% moisture content or less for lumber under 5 in thick; anything wetter is green, and timber grading rules may set a different limit. Green dimension lumber is surfaced slightly oversize, 1 9/16 in instead of 1½ for a 2×, because the standard bases green sizes on the shrinkage expected as the wood dries to 19%. Redwood, Western Red Cedar and Northern White Cedar have their own green widths in the standard, so the green column here does not apply to them.

Board feet are figured on nominal size

PS 20 defines the board foot using nominal thickness and width, not the planed size. One 2×4 that is 8 ft long is 2 × 4 × 8 ÷ 12 = 5.33 board feet, even though the wood in it is only 1½ × 3½ in. A 1×12 that is 1 ft long is exactly 1 board foot. Ten 2×6s at 12 ft are 120 board feet.

That matters when a lumber yard quotes by the board foot or by the thousand board feet (MBF) and a home center prices by the piece. If an 8 ft 2×4 costs $4.00, that is $0.75 per board foot ($4.00 ÷ 5.33). The calculator does this conversion for you, and also shows the share of nominal volume you actually receive: a dry 2×4 is 1.5 × 3.5 = 5.25 sq in of a nominal 8, or about 66%.

Planning with actual sizes

• Wall thickness. A 2×4 wall is 3½ in of framing before drywall and sheathing, and a 2×6 wall is 5½ in, the cavity depths used in our insulation calculator.
• Deck layout. Board counts depend on the true 5½ in face of a 5/4×6 or 2×6, plus the gap. The deck calculator uses that width, and joist depth for the joist span table follows the 7¼, 9¼ and 11¼ in sizes above.
• Posts and footings. A 4×4 is 3½ in and a 6×6 is 5½ in square, which sets the hole diameter in the fence calculator and the concrete bag calculator.
• Built-up beams. Three 2×10s nailed together are 4½ in wide (3 × 1½), not 6 in, so a beam bracket for a 6× timber will not fit.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Actual sizes are the minimum dressed sizes in PS 20-25, Table 3: dry means 19% moisture content or less, green means more than 19%. Boards are under 2 in nominal thickness, dimension lumber is 2 in to under 5 in, and timbers are 5 in and up.
• Timbers: dry pieces lose ½ in at 5–6 in, ¾ in at 7–15 in and 1 in at 16 in and up; green pieces lose ½ in on every face size.
• Board feet use nominal sizes, as PS 20 defines board measure: thickness (in) × width (in) × length (ft) ÷ 12. “Wood you actually get” uses the dressed size and is shown for comparison only.
• Millimeters are the inch figures × 25.4. The reverse lookup matches a measured size within 1/16 in of a dressed size.

Sources

1. American Lumber Standard Committee / NIST, Voluntary Product Standard PS 20-25, American Softwood Lumber Standard (January 2025). Table 3 nominal and minimum dressed sizes (a dry 2×4 is 1½ × 3½ in, 38.1 × 88.9 mm); board measure on nominal size (§2.2); rough lumber at least ⅛ in over dressed size (§5.1); dry = 19% moisture or less.
2. National Institute of Standards and Technology, Voluntary Product Standard PS 20-20, Revision 1 (October 2021). Previous edition, same dressed sizes in Table 3; used to confirm the table did not change.

Last reviewed October 26, 2026. Product figures vary by brand; the label on what you buy wins.