Lumber Size Explained
Why is a 2×4 Not 2 Inches by 4 Inches?
**Quick answer:**
A 2×4 starts as roughly 2″ × 4″ when cut green from the log.
It shrinks during drying and is further reduced when planed smooth — ending at **1.5″ × 3.5″**.
The history of the 2×4 name
Lumber sizing traditions date back to the late 1800s. Back then, a “2×4” was exactly 2 inches by 4 inches when sold. As milling technology improved, lumberyards began **drying and surfacing** boards to make them more uniform and easier to work with — reducing their final size.
How the shrinking happens
- Rough cutting: Board is cut from the log at or slightly above 2″ × 4″.
- Drying: Kiln or air drying causes the wood to lose moisture and shrink.
- Planing: A machine shaves the board smooth, removing more material and creating uniform dimensions.
Nominal vs actual size comparison
| Label | Nominal Size | Actual Size | Metric |
|---|---|---|---|
| Standard kiln-dried 2×4 | 2″ × 4″ | 1.5″ × 3.5″ | 38 × 89 mm |
Why they didn’t just change the name
The lumber industry kept the traditional nominal names for consistency in design, trade, and sales. Changing labels every time processing methods evolved would cause confusion for builders, suppliers, and building codes.
FAQ
Is every 2×4 exactly 1.5″ × 3.5″?
Most are, but green (unseasoned) lumber can be slightly larger because it hasn’t fully dried.
Do other sizes have this problem?
Yes. All dimensional lumber — 1×, 2×, etc. — has a nominal size and a smaller actual size.