Why is a 2x4 not actually 2 inches by 4 inches?

A 2x4 starts as a 2" x 4" rough-cut board, but after kiln drying (which shrinks it) and planing smooth (which removes material from each face), it ends up at 1.5" x 3.5". This has been the industry standard since 1964. What is the actual size of a 2x6?

What is the actual size of a 2x6?

A 2x6 actually measures 1.5 inches by 5.5 inches. Like all dimensional lumber, it loses 1/2 inch from each named dimension during the drying and planing process. Are lumber lengths actual or nominal?

Are lumber lengths actual or nominal?

Lumber lengths are actual measurements. An 8-foot board is a full 96 inches long, and may even be slightly longer. Only the cross-section dimensions (width and thickness) use the nominal naming convention.

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Lumber Dimensions Explained: Nominal vs Actual Sizes (Complete Chart)

Lumberama Editorial Team, Lumber Industry & Standards Specialists Jul 29, 2025 8 min read

Why Lumber Isn't the Size It Says

When you buy a 2x4, you get a board that measures 1.5" x 3.5". This isn't deception — it's the result of the milling process. The "2x4" name refers to the

nominal size — the rough-cut dimension before the wood is dried and planed smooth. After kiln-drying (which shrinks the wood) and surface planing (which removes about 1/4" from each face), the board reaches its

actual size of 1.5" x 3.5".

This naming convention has been standardized since 1964 when the American Lumber Standard Committee established the current sizing rules. Every lumber yard, home center, and building supply in North America follows the same standard.

Dimensional Lumber Size Chart

Nominal → Actual Thickness:

Notice the pattern: boards under 2" nominal lose 1/4" per face (1/2" total). Boards 2" and larger lose 1/2" per dimension. And boards 8" and wider lose 3/4" in width.

Common Framing Lumber Sizes

2x4 (1.5" x 3.5"): Wall studs, blocking, light framing. The workhorse of residential construction. Available in 8', 10', 12', 14', and 16' lengths. Pre-cut studs are 92-5/8" for 8' walls with a single bottom plate and double top plate.

2x6 (1.5" x 5.5"): Exterior wall studs (for better insulation), floor joists on short spans, deck boards, fence pickets. R-19 insulation fits between 2x6 studs versus R-13 for 2x4 walls.

2x8 (1.5" x 7.25"): Floor joists, roof rafters, headers over windows and doors. Spans up to 12' for floors at 16" on center depending on species and grade.

2x10 (1.5" x 9.25"): Floor joists for longer spans, roof rafters, stair stringers, deck beams. Spans up to 16' for floors.

2x12 (1.5" x 11.25"): Long-span floor joists, ridge boards, stair stringers where a wider board prevents notch-to-depth ratio problems. Spans up to 20' for floors.

Board Lumber Sizes

Board lumber (also called "one-by" or "boards") is nominally 1" thick. After milling, actual thickness is 3/4".

1x2 (3/4" x 1.5"): Furring strips, lattice, trim backing. | 1x3 (3/4" x 2.5"): Window/door casing, battens. | 1x4 (3/4" x 3.5"): Trim, fence boards, craft projects. | 1x6 (3/4" x 5.5"): Shelving, fence boards, paneling. | 1x8 (3/4" x 7.25"): Wide shelving, shiplap, siding. | 1x10 (3/4" x 9.25"): Cabinet sides, wide trim. | 1x12 (3/4" x 11.25"): Shelving, cabinet sides, craft projects.

Timber Sizes

Posts and timbers (5" x 5" nominal and larger) follow different rules — they lose only 1/2" per dimension:

4x4 post: 3.5" x 3.5" actual | 6x6 post: 5.5" x 5.5" actual | 4x6 beam: 3.5" x 5.5" actual | 6x8 beam: 5.5" x 7.5" actual | 8x8 timber: 7.5" x 7.5" actual

Length Standards

Unlike cross-section dimensions, lumber lengths are actual. An 8-foot 2x4 measures a full 96 inches long (sometimes slightly longer, never shorter). Standard lengths: 6', 8', 10', 12', 14', 16', 18', 20', 22', 24'. Lengths over 16' are often special-order and carry a premium.

How This Affects Your Projects

Wall thickness: A 2x4 wall with 1/2" drywall on each side = 4.5" total (3.5" stud + 0.5" + 0.5"). A 2x6 wall = 6.5" total. This matters for door jambs, window trim reveals, and outlet box depths.

Shelf spans: A 3/4"-thick (1x) pine shelf sags over 24" without support. A 1.5"-thick (2x) shelf spans 36-48" depending on load. This is actual thickness, not nominal — plan accordingly.

Joist calculations: When calculating floor or ceiling heights, use actual dimensions. A floor built on 2x10 joists with 3/4" subfloor and 3/4" finish floor = 10.75" total (9.25" + 0.75" + 0.75").

Hardwood Lumber: Different Rules

Hardwood lumber uses the "quarter" system: 4/4 ("four-quarter") = 1" thick rough, 13/16" surfaced. 5/4 = 1.25" rough, 1-1/16" surfaced. 6/4 = 1.5" rough, 1-5/16" surfaced. 8/4 = 2" rough, 1-3/4" surfaced. Hardwood is sold by the board foot (1" x 12" x 12" = 1 board foot) rather than by the linear foot.