Lumber Actual Dimensions
A 2x4 is not actually 2 inches by 4 inches. The nominal size differs from the actual finished size because lumber is sawn at nominal dimensions, then dried and planed smooth. A "2x4" measures 1.5" × 3.5" actual, and every other dimensional lumber size is similarly reduced.
Dimensional Lumber Size Chart
| Nominal | Actual Size | Width (in) | Depth (in) |
|---|---|---|---|
| 1x2 | 3/4" x 1-1/2" | 0.75 | 1.5 |
| 1x3 | 3/4" x 2-1/2" | 0.75 | 2.5 |
| 1x4 | 3/4" x 3-1/2" | 0.75 | 3.5 |
| 1x6 | 3/4" x 5-1/2" | 0.75 | 5.5 |
| 1x8 | 3/4" x 7-1/4" | 0.75 | 7.25 |
| 1x10 | 3/4" x 9-1/4" | 0.75 | 9.25 |
| 1x12 | 3/4" x 11-1/4" | 0.75 | 11.25 |
| 2x2 | 1-1/2" x 1-1/2" | 1.5 | 1.5 |
| 2x3 | 1-1/2" x 2-1/2" | 1.5 | 2.5 |
| 2x4 | 1-1/2" x 3-1/2" | 1.5 | 3.5 |
| 2x6 | 1-1/2" x 5-1/2" | 1.5 | 5.5 |
| 2x8 | 1-1/2" x 7-1/4" | 1.5 | 7.25 |
| 2x10 | 1-1/2" x 9-1/4" | 1.5 | 9.25 |
| 2x12 | 1-1/2" x 11-1/4" | 1.5 | 11.25 |
| 2x14 | 1-1/2" x 13-1/4" | 1.5 | 13.25 |
| 4x4 | 3-1/2" x 3-1/2" | 3.5 | 3.5 |
| 4x6 | 3-1/2" x 5-1/2" | 3.5 | 5.5 |
| 4x8 | 3-1/2" x 7-1/4" | 3.5 | 7.25 |
| 6x6 | 5-1/2" x 5-1/2" | 5.5 | 5.5 |
| 6x8 | 5-1/2" x 7-1/2" | 5.5 | 7.5 |
| 8x8 | 7-1/4" x 7-1/4" | 7.25 | 7.25 |
Sheet Goods Thickness
| Material | Nominal | Actual |
|---|---|---|
| Plywood (3/4") | 3/4" | 23/32" (0.719") |
| Plywood (1/2") | 1/2" | 15/32" (0.469") |
| Plywood (3/8") | 3/8" | 11/32" (0.344") |
| Plywood (1/4") | 1/4" | 7/32" (0.219") |
| OSB | 7/16" | 7/16" (0.4375") |
| OSB | 3/4" | 23/32" (0.719") |
| Drywall (standard) | 1/2" | 1/2" (0.5") |
| Drywall (fire-rated) | 5/8" | 5/8" (0.625") |
Why Nominal Doesn't Match Actual
The difference traces back to how lumber was originally sold and processed. Before the 1960s, lumber was sold in rough green (wet) condition at roughly the nominal size. A 2x4 actually measured close to 2" × 4" when delivered rough. Over time, three factors gradually shrunk the actual dimensions:
Drying Shrinkage
Green lumber loses dimension as it dries — typically 3-8% depending on species and moisture content. A green 2x4 that measured 2" × 4" might shrink to 1.8" × 3.7" after drying.
Planing
Modern dimensional lumber is planed smooth (S4S, or "surfaced four sides") after drying, removing roughly 1/4" from each face. This produces the dimensional accuracy needed for modern construction.
Standardization (1964)
The American Lumber Standards Committee formalized the nominal-to-actual relationship in 1964. Before this, different regions and mills sold slightly different actual sizes. The 1964 standard locked it at 1.5" × 3.5" for a 2x4 and similar reductions for other sizes.
Rough Sawn vs S4S Lumber
When buying lumber from a sawmill rather than a big-box store, you may encounter rough sawn (not planed) or S2S(surfaced two sides) lumber. Rough sawn lumber is typically sold at or very close to its nominal size — a rough sawn 2x4 might actually measure 1.875" × 3.875" or even a full 2" × 4". Always verify actual dimensions before planning your project.
Quarter Measurement for Hardwood
Hardwood from sawmills is often sold by quarter-inch thickness: 4/4 = 1", 5/4 = 1.25", 6/4 = 1.5", 8/4 = 2". These are rough dimensions before planing. After S4S planing, 4/4 lumber typically finishes to 13/16" (about 0.81") and 8/4 finishes to about 1.75".
Why This Matters
- Layout.A wall built with 2x4s has 3.5" stud depth, not 4". This affects door and window rough opening calculations, drywall corner bead placement, and trim returns.
- Span calculations.All IRC span tables use actual dimensions. A 2x10 floor joist spans based on its 9.25" actual depth, not 10".
- Material takeoffs. A board foot calculation uses nominal dimensions (a 2x4x8 = 5.33 bf), but a physical fit calculation uses actual.
2x4 Actual Dimensions
A 2x4 is actually 1.5" × 3.5". It’s the most common framing member in residential construction — used for wall studs, plates, headers under 4’, and light framing. The 3.5" depth is what defines the standard 2x4 wall cavity, which fits R-13 fiberglass batts and standard electrical box depths. When laying out window and door rough openings, always use 3.5", not 4".
2x6 Actual Dimensions
A 2x6 is actually 1.5" × 5.5". 2x6s are used for taller stud walls (commonly in northern climates where R-19 insulation is required), floor joists in shorter spans, roof rafters, and heavier headers. The 5.5" depth gives enough cross-section for moderate spans and accepts 1/2" or 5/8" sheathing without modification. When framing exterior walls with 2x6s instead of 2x4s, remember the wall thickness becomes 5.5" — door jambs, window jambs, and trim returns all change.
2x8 Actual Dimensions
A 2x8 is actually 1.5" × 7.25". Note the dimension reduction is now 0.75" off the depth instead of 0.5" — boards 8" nominal and wider lose more depth in planing. 2x8s are common for floor joists in 12-foot rooms, rafters in moderate-pitch roofs, and headers over wider openings. The 7.25" depth determines maximum span per the IRC tables — see our floor joist span tables for code-approved spans by species and grade.
2x10 Actual Dimensions
A 2x10 is actually 1.5" × 9.25". This is the workhorse joist for residential floor framing — most 16-foot living-room spans use 2x10s on 16" centers. The 9.25" depth provides enough cross-section to span ~14’ in #2 SPF without intermediate support, depending on live load. 2x10s are also common as ceiling joists with attic storage and as the bottom chord of roof framing. The nominal 2x10 is also commonly written as “2 by 10” or “2-by-10” in older blueprints.
2x12 Actual Dimensions
A 2x12 is actually 1.5" × 11.25". 2x12s are the standard material for stair stringers because the 11.25" depth provides adequate “throat” material left after notching for treads and risers. Code generally requires a minimum of 3.5" of remaining throat depth at the narrowest point. 2x12s are also used for the longest-span floor joists, beam built-ups (e.g., a triple 2x12 header), and stringer-style roof rafters in heavy-snow regions.
4x4 Actual Dimensions
A 4x4 is actually 3.5" × 3.5". 4x4s are most commonly used for deck posts (under loads where 4x4 is code-approved — verify with local IRC adoption), pergola posts, fence posts, and light railing posts. For load-bearing structural posts in a deck under heavy snow load or supporting beams over 8 feet, code typically requires 6x6 (5.5" × 5.5") or larger.
Frequently Asked Questions
What is the actual size of a 2x4?
A 2x4 is actually 1.5 inches by 3.5 inches. The nominal size (2 inches by 4 inches) refers to the rough-sawn green dimensions before drying and planing. After kiln-drying and surfacing on four sides (S4S), the finished dimensions are 1.5" × 3.5".
What is the actual size of a 2x6?
A 2x6 is actually 1.5 inches by 5.5 inches. Like all dimensional softwood lumber, the nominal name (2x6) refers to the rough green dimensions. After drying and planing, the finished S4S dimensions are 1.5" × 5.5".
What is the actual size of a 2x8?
A 2x8 is actually 1.5 inches by 7.25 inches. The dimension reduction from nominal increases for wider boards: a 2x4 loses 0.5" off each face dimension, but a 2x8 loses 0.75" off the larger dimension because of additional planing tolerances at wider sizes.
What is the actual size of a 2x10?
A 2x10 is actually 1.5 inches by 9.25 inches. This is the standard size used in IRC span tables for floor joists and rafters. Note: when calculating span capacity, span tables use the actual 9.25" depth, not the nominal 10".
What is the actual size of a 2x12?
A 2x12 is actually 1.5 inches by 11.25 inches. 2x12s are most commonly used for stair stringers, floor joists, and beams. Stair stringer layout calculations use the 11.25" depth, which gives enough material to cut tread and riser notches with adequate remaining web.
What is the actual size of a 4x4?
A 4x4 is actually 3.5 inches by 3.5 inches. 4x4 posts are commonly used for deck posts, fence posts, and light pergola work. For load-bearing structural posts, code typically requires 6x6 (5.5" × 5.5") or larger.
Why isn't a 2x4 actually 2 inches by 4 inches?
The nominal size refers to the rough-sawn green (wet) dimensions before processing. Lumber loses 3-8% during kiln drying, then 1/4" is removed from each face during S4S planing to produce smooth, dimensionally consistent boards. The American Lumber Standard PS 20 formalized the 1.5" × 3.5" actual size for 2x4s in 1964.
Do IRC span tables use nominal or actual lumber dimensions?
Span tables always use actual dimensions. A 2x10 floor joist's span capacity is calculated based on its 9.25" actual depth, not 10". This is also true for engineered lumber (LVL, glulam, I-joists) which list their actual dimensions directly.
Are rough-sawn boards from a sawmill actually 2 inches thick?
Often yes, or close to it. Rough-sawn lumber sold directly from sawmills typically measures at or near nominal size — a rough 2x4 might be 1.875" × 3.875" or even a full 2" × 4". Always measure rough lumber before planning your project, since dimensions vary by mill.
Why are sheet goods like plywood also smaller than the nominal thickness?
Plywood and OSB face the same drying and surfacing reduction. Nominal 3/4" plywood actually measures 23/32" (0.719"). Nominal 1/2" plywood is 15/32" (0.469"). Drywall is the exception — it's manufactured to nominal thickness, so 1/2" drywall is actually 0.5".