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
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.
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