Lumber Dimensions: Nominal vs Actual Sizes
Lumber is sold by its nominal (name) size, but the actual dimensions are smaller due to drying and planing. This guide covers every standard size with actual measurements in inches and millimeters.
Quick Reference: Nominal → Actual Rules
- Under 2" nominal — subtract 1/4 inch (e.g. 1" becomes 3/4")
- 2" to 6" nominal — subtract 1/2 inch (e.g. 2" becomes 1-1/2", 4" becomes 3-1/2")
- 8" and above nominal — subtract 3/4 inch (e.g. 8" becomes 7-1/4", 10" becomes 9-1/4")
- Length is always actual — an 8-foot board is truly 96 inches
Board Lumber (1-by)
Board lumber has a nominal thickness of 1 inch (actual 3/4 inch). Used for trim, shelving, siding, fencing, and non-structural applications.
Dimension Lumber (2-by)
Dimension lumber is the backbone of residential construction. Used for wall studs, floor joists, rafters, headers, and all structural framing.
Timber (4×4 and Larger)
Timbers are large-dimension lumber used for posts, beams, headers, and heavy structural applications. Sizes 5 inches and larger are classified as timber.
Decking Profiles (5/4)
Decking lumber uses the 5/4 (five-quarter) thickness — nominally 1-1/4 inches, actually 1 inch. The 5/4×6 is the standard residential deck board.
Standard Lumber Lengths
Unlike width and thickness, lumber length is actual — a board sold as 8 feet is truly 96 inches (often with a small trim allowance). Standard lengths come in 2-foot increments. The most commonly stocked lengths are 8, 10, 12, and 16 feet. Longer lengths (18, 20, 24 ft) are typically special order and may have longer lead times.
| Feet | Inches | Meters |
|---|---|---|
| 6' | 72" | 1.83 m |
| 8' Stocked | 96" | 2.44 m |
| 10' Stocked | 120" | 3.05 m |
| 12' Stocked | 144" | 3.66 m |
| 14' | 168" | 4.27 m |
| 16' Stocked | 192" | 4.88 m |
| 18' Special Order | 216" | 5.49 m |
| 20' Special Order | 240" | 6.10 m |
| 22' Special Order | 264" | 6.71 m |
| 24' Special Order | 288" | 7.32 m |
Pressure Treated Lumber Sizes
Pressure treated lumber has the same nominal and actual dimensions as untreated lumber. However, PT boards weigh 50-80% more due to chemical preservatives forced into the wood under high pressure. Weights shown are for treated lumber.
Hardwood Quarter System
Unlike softwood lumber which uses nominal sizing (e.g. "2×4"), hardwood lumber is sold using the "quarter system" where thickness is expressed as the number of quarter-inches. A "4/4" board (said "four-quarter") is 1 inch thick in the rough. After surfacing (planing) on two sides (S2S), the finished thickness is reduced. Widths and lengths are random — you buy hardwood by the board foot, not in standardized widths like softwood.
| Quarter Designation | Rough Thickness | Surfaced Thickness (S2S) |
|---|---|---|
| 4/4 | 1" | 13/16" |
| 5/4 | 1-1/4" | 1-1/16" |
| 6/4 | 1-1/2" | 1-5/16" |
| 8/4 | 2" | 1-3/4" |
| 12/4 | 3" | 2-3/4" |
| 16/4 | 4" | 3-3/4" |
Glulam Beam Dimensions
Glue-laminated (glulam) beams are engineered wood products made by bonding layers of dimensional lumber together. Unlike sawn lumber, glulam widths are actual — a 5-1/8" glulam is truly 5-1/8" wide. Standard depths increase in 1-1/2" increments (matching the thickness of each lamination layer).
| Standard Width | Available Depths |
|---|---|
| 3-1/8" | 6" to 18" |
| 3-1/2" | 6" to 18" |
| 5-1/8" | 6" to 30" |
| 5-1/2" | 6" to 30" |
| 6-3/4" | 9" to 36" |
| 8-3/4" | 9" to 42" |
| 10-3/4" | 12" to 48" |
Why Nominal Size ≠ Actual Size
When lumber was first standardized in the early 1900s, a "2×4" was exactly 2 inches by 4 inches — rough-sawn straight from the log. Over time, the industry adopted kiln drying and surface planing as standard processes, and the nominal names stuck even though the finished dimensions shrank.
The three-step process that reduces dimensions:
- Rough Cutting — The log is sawn into boards at the nominal size (a 2×4 truly measures about 2" × 4" at this stage).
- Kiln Drying — The green lumber is dried in a kiln to a moisture content of 15-19%. This process causes shrinkage of approximately 1/16" to 1/8" per dimension.
- Planing (Surfacing) — All four faces are run through a planer to produce smooth, consistent surfaces. This removes another 1/8" to 1/4" per face, bringing the final actual size to 1-1/2" × 3-1/2".
The nominal name is kept as an industry convention because it represents the starting lumber volume and provides a simple, universally understood naming system. The American Lumber Standard Committee (ALSC) and the National Institute of Standards and Technology (NIST) formalized the current actual dimensions in 1964, and they've remained unchanged since.
Lumber Calculators
Use our free calculators to estimate board feet, weight, and cost for your project.