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    Lumber: What a 2x4 Actually Measures

    A US nominal size is a class, not a dimension. Every dressed size here, plus the dry-versus-green rule that decides which one turns up on the truck.

    Dressed size, dry (19% moisture or less)

    1x2 3/4 x 1-1/2 in
    1x4 3/4 x 3-1/2 in
    1x6 3/4 x 5-1/2 in
    1x8 3/4 x 7-1/4 in
    1x10 3/4 x 9-1/4 in
    1x12 3/4 x 11-1/4 in
    2x2 1-1/2 x 1-1/2 in
    2x4 1-1/2 x 3-1/2 in
    2x6 1-1/2 x 5-1/2 in
    2x8 1-1/2 x 7-1/4 in
    2x10 1-1/2 x 9-1/4 in
    2x12 1-1/2 x 11-1/4 in
    4x4 3-1/2 x 3-1/2 in
    6x6 5-1/2 x 5-1/2 in

    The one that catches people out

    • The number on the label is a CLASS, not a measurement. Nothing sold as a 2x4 has ever measured 2 by 4 inches once dressed.
    • Note the jump: 1x8 and wider lose 3/4 in, not 1/2 in. A 2x8 is 7-1/4, not 7-1/2. Cut a rough opening off the nominal figure and you are a quarter inch out per member.
    • DRY and GREEN are set at the moment of surfacing, not when you use it. Green stock is dressed larger, from about 1/16 in on a 2x4 up to 1/4 in on a 2x12, so it lands near the dry size once it has dried.

    Rough sawn vs S4S

    S4S Surfaced four sides. This is the planed product the table above describes.
    Rough sawn Not planed. At least 1/8 in larger than the dressed minimum, so it is much closer to the nominal figure.
    Why it matters A rough 2x4 and an S4S 2x4 are not interchangeable in a frame. Check which you are being quoted.

    Lengths and studs

    Stock lengths Multiples of 2 ft: 8, 10, 12, 14, 16 ft
    Precut stud, 8 ft wall 92-5/8 in, not 96 in. The difference is the plates.
    Ordering trap A precut stud and an 8 ft board are different products. Ask for the one you mean.
    Dressed sizes come from the American Softwood Lumber Standard (PS 20), maintained by the American Lumber Standard Committee. Timbers of 5 in nominal and larger follow species grading rules that can define the dry/green threshold differently from the 19% used for boards and dimension lumber. What actually arrives is decided by the grade stamp on the piece, so read it: S-DRY, S-GRN, KD and KD-HT are not the same product.

    Sources: alsc.org · nist.gov · awc.org

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