Dimensional lumber is sold by nominal size, but a dry 2x4 measures 1-1/2 by 3-1/2 inches and a dry 1x4 measures 3/4 by 3-1/2 inches under the American Softwood Lumber Standard (PS 20). The 2021 International Residential Code (IRC) labels its span tables with the nominal names, not these actual sizes.
Dimensional lumber is softwood sawn to a nominal thickness from 2 inches up to, but not including, 5 inches and a nominal width of 2 inches or more. PS 20 also calls this class "dimension" and notes it is sold as framing, joists, planks, rafters or studs. Nominal size is the name on the tag. Actual size is what a tape measure finds.
Key facts
- A dry nominal 2x4 is surfaced to 1-1/2 inches by 3-1/2 inches (PS 20 Table 3), and the same size dressed green is 1-9/16 by 3-9/16 inches.
- Dry lumber under a nominal 5-inch thickness is lumber seasoned to a maximum moisture content of 19 percent, and green lumber is above that figure (PS 20 Sections 2.7 and 2.12).
- A dry nominal 1x4 board is 3/4 inch thick by 3-1/2 inches wide (PS 20 Table 3).
- The 2021 IRC floor joist tables, such as Table R502.3.1(2), are headed with nominal sizes 2x6, 2x8, 2x10 and 2x12.
- Standard lengths are multiples of 1 foot or 2 feet as given in the certified grading rules (PS 20 Section 5.3.1).
What is the actual size of dimensional lumber?
Dry dimension lumber is dressed to the minimum sizes in PS 20 Table 3, so a nominal 2x4 is 1-1/2 by 3-1/2 inches and a 2x12 is 1-1/2 by 11-1/4 inches. The chart below lists the common dimension sizes, with green sizes beside them.
Nominal and minimum dressed sizes of dimension lumber, PS 20 Table 3 (National Institute of Standards and Technology). Dry means 19 percent maximum moisture content. Inches.
| Nominal size | Actual dry thickness x width | Actual green thickness x width |
|---|---|---|
| 2x4 | 1-1/2 x 3-1/2 | 1-9/16 x 3-9/16 |
| 2x6 | 1-1/2 x 5-1/2 | 1-9/16 x 5-5/8 |
| 2x8 | 1-1/2 x 7-1/4 | 1-9/16 x 7-1/2 |
| 2x10 | 1-1/2 x 9-1/4 | 1-9/16 x 9-1/2 |
| 2x12 | 1-1/2 x 11-1/4 | 1-9/16 x 11-1/2 |
| 2x14 | 1-1/2 x 13-1/4 | 1-9/16 x 13-1/2 |
| 2x16 | 1-1/2 x 15-1/4 | 1-9/16 x 15-1/2 |
| 4x4 | 3-1/2 x 3-1/2 | 3-9/16 x 3-9/16 |
| 4x6 | 3-1/2 x 5-1/2 | 3-9/16 x 5-5/8 |
| 4x8 | 3-1/2 x 7-1/4 | 3-9/16 x 7-1/2 |
The American Wood Council (AWC) prints the same dry sizes for 2x4 through 2x12 at the front of its Span Tables for Joists and Rafters, which are based on PS 20.
What is the actual size of a 1x4, 1x6 and other boards?
A dry nominal 1x board is 3/4 inch thick, and its width follows the same pattern as dimension lumber. Boards are lumber under nominal 2 inches thick, and PS 20 Table 3 gives them their own thickness rows.
Nominal and minimum dressed sizes of boards, PS 20 Table 3. Inches.
| Nominal size | Actual dry thickness x width | Actual green thickness x width |
|---|---|---|
| 1x4 | 3/4 x 3-1/2 | 25/32 x 3-9/16 |
| 1x6 | 3/4 x 5-1/2 | 25/32 x 5-5/8 |
| 1x8 | 3/4 x 7-1/4 | 25/32 x 7-1/2 |
| 1x10 | 3/4 x 9-1/4 | 25/32 x 9-1/2 |
| 1x12 | 3/4 x 11-1/4 | 25/32 x 11-1/2 |
Thicker boards follow the same table: a nominal 1-1/4 inch board is 1 inch thick dry and a nominal 1-1/2 inch board is 1-1/4 inches thick dry. Table 3 states that thicknesses apply to all widths and widths apply to all thicknesses, which is why the 1x4 above shares its 3-1/2 inch width with the 2x4.
Timbers, meaning nominal 5-inch and thicker pieces, are handled differently. PS 20 Table 3 gives them as a reduction from nominal (1/2 inch off for 5 and 6 inch dimensions, 3/4 inch off for 7 to 15 inches) instead of listing each size, so read the table for a 6x6 or larger post.
Why is a 2x4 not 2 inches by 4 inches?
A 2x4 is smaller than its name because the name is a nominal label, and the finished piece is dried and planed smooth. PS 20 Section 2.16 defines nominal size as the label designation for lumber size categories that does not reflect the dressed size, and it gives the dry 2 by 4 as the example.
Sawmills cut oversize rough boards, dry them, then run them through a planer to give four smooth faces. Each step removes wood, so the product comes out below the label. PS 20 Section 3.4 states that the nominal sizes in the standard follow industry practice and that no one should infer the nominal sizes are dressed sizes. Because the label is the common language, framers, suppliers and span tables all keep using it.
What is the difference between green and dry lumber sizes?
Green lumber is dressed slightly larger than dry lumber because it will shrink as it dries. PS 20 sets the dividing line at 19 percent moisture content for lumber under nominal 5 inches thick.
In Table 3, a green 2x4 is 1-9/16 by 3-9/16 inches while a dry 2x4 is 1-1/2 by 3-1/2. Table 3 footnote 5 explains the green sizes are based on shrinkage factors of 2.35 percent in thickness and 2.80 percent in width from the fiber saturation point down to a 19 percent maximum moisture content. After that, PS 20 Section 6.2.3.1 recognizes that dry lumber can still shrink or expand with moisture content, so a stud that sat in the rain is not guaranteed to measure exactly 1-1/2 inches.
The grade stamp records which condition applies. PS 20 Section 6.2.6 requires the mark S-GRN if surfaced green, S-DRY if surfaced dry, or KD if kiln dried to a maximum of 19 percent at time of surfacing. The lumber grades guide shows where those letters sit on the stamp.
Where does the IRC use nominal sizes?
The 2021 IRC uses nominal sizes in its prescriptive span tables and in many installation rules, and uses actual measurements where a rule depends on real dimensions. Look at how each section words the size.
- Span tables. Table R502.3.1(2) lists joists as 2x6, 2x8, 2x10 and 2x12. Section R502.3 sends other grades and species to the AWC span tables. The floor joist span table and the rafter span chart reproduce the 2021 IRC values, and the stud spacing guide covers Table R602.3(5).
- Plates and blocking. Section R602.3.4 requires studs to have full bearing on a nominal 2-by or larger plate or sill. Section R502.7 calls for full-depth solid blocking not less than 2 inches nominal in thickness.
- Bridging threshold. Section R502.7.1 requires bridging for joists exceeding a nominal 2 inches by 12 inches.
- Bearing. Section R502.6 requires not less than 1-1/2 inches of bearing on wood or metal, an actual dimension.
The second group matters when you draw. A nominal callout such as 2x10 names the member, while an actual 1-1/2 inch thickness controls how a joist, a plate or a ledger sits in a detail. Use the nominal name on the callout, and use actual sizes for any dimension that has to close.
What are standard lumber lengths?
Standard lengths are multiples of 1 foot or 2 feet as specified in the certified grading rules (PS 20 Section 5.3.1). Unlike thickness and width, length is not reduced by planing.
PS 20 Section 5.3.2 trims lumber to a length not less than nominal and not more than 3 inches over nominal. For nominal 2 inch dimension over 12 inches wide or over 20 feet long, the overlength tolerance is 12 inches. Which lengths a yard stocks varies by region, and a note to the 2021 IRC span tables says to check sources for availability of lumber in lengths greater than 20 feet, so check supply before drawing a single long joist.
How do lumber sizes show up on a framing plan?
On a drawing, joists and beams are called out by nominal size, such as 2x10 at 16 inches on center, with the grade and species beside them. Dimensions that must close, such as a stair opening or a wall thickness built up from plates and sheathing, use actual sizes. If you draw in millimeters, PS 20 gives the metric equivalent of the dry 2 by 4 as 38.1 mm by 88.9 mm for information, with the inch values as the units of record.
Framing Plan draws joist layouts, beams and openings and exports PNG, PDF and DXF. It does not check lumber sizes or code compliance: its sample profile uses demo span figures and metric member sizes, and the editor works in millimeters. The adopted code edition and any local amendments govern, and engineered products such as I-joists and LVL use the manufacturer's tables, not the PS 20 dimension lumber sizes. For nail sizes that go with these members, see the framing nail size chart.
Summary
Dimensional lumber is named by nominal size and dressed smaller: dry 2x4 is 1-1/2 by 3-1/2 inches and dry 1x4 is 3/4 by 3-1/2 inches under PS 20 Table 3. Green lumber is cut slightly larger because it shrinks to dry size at 19 percent moisture content. The 2021 IRC span tables use the nominal names, while rules such as joist bearing in Section R502.6 use actual measurements. The authoritative table is PS 20 at NIST, and the AWC span tables repeat the dry sizes.
Open the framing studio to lay out a preliminary joist and beam plan, then add nominal callouts for your designer or plan reviewer.