Nominal vs Actual Lumber Size Converter
The gap between the name on the ticket and the size on the tape is where most estimating errors come from. This shows the actual milled dimension for every common nominal size and what that difference does to your material count.
Results
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Questions people ask
Why are lumber sizes nominal?
Hardwood and softwood lumber is priced and sold by historical volume units, not by actual dimensions. The nominal name is a rough-sawn reference standard that never changed, even though mills have consistently milled boards smaller.
What is the difference between nominal and actual?
Nominal is the rough-sawn size; actual is the finished size after drying and planing. A nominal 2×4 comes out 1½ × 3½ inches — 25 percent less thickness and 12.5 percent less width than the name suggests.
Does this affect structural calculations?
Yes. Joist and beam sizing is based on actual cross section. The joist span tables published by lumber suppliers are already calculated from actual dimensions, so use those tables directly rather than nominal sizes.
Where does the shrinkage come from?
Rough lumber is cut oversize on purpose — 4/4 is sawn about 1 inch thick so that after drying shrinkage and surfacing it lands near the finished ¾ inch. The oversize allowance is the difference.
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