Nominal vs Actual Lumber Sizes Explained
Every dimension lumber name is wrong. A 2×4 is not 2 by 4 inches. Knowing this one fact prevents more material errors than any other single piece of information in this site.
The short answer
A nominal 2×4 is milled to 1½ × 3½ inches. A nominal 1×6 is milled to ¾ × 5½ inches. The name is a historical pricing standard that has never matched the physical board.
Why lumber is cut oversize
Rough lumber is sawn with an intentional allowance for two reasons: drying shrinkage and machining. A board called 4/4 is cut about 1 inch thick. After drying it shrinks, and after surfacing the planer takes another cut off the surface. What is left is 13/16 or ¾ inch — but it is still billed as 4/4 because that is what the rough board was.
This is not a defect. It is a pricing convention that predates modern milling, and it persists because it works — everyone in the trade has always used the same names.
What the difference costs you
| Using | Coverage of 1×6 | Boards for 320 sq ft |
|---|---|---|
| Nominal 6 inch width | 100% | 80 |
| Actual 5½ inch width | 91.7% | 88 |
Eight boards short. For 8 foot stock, that is 64 linear feet you will have to buy twice — and the second trip costs you time and often a delivery fee.
When nominal is the right number
- Board foot pricing of rough lumber — the formula uses nominal thickness, because that is how it is sold.
- Ordering rough stock — when you buy rough lumber to mill yourself, the board is closer to nominal.
- Structural tables — published joist and beam tables are already calculated from actual dimensions. Use them as published.
When actual is the right number
- Coverage and tiling calculations — anything that measures how much surface boards cover.
- Fit and clearance — when a board has to fit an opening, the actual width is what goes in the saw.
- Floor height budgeting — a 2×4 laid flat adds 1½ inches, not 2. Over three layers of subfloor that is half an inch you may not have.
- Furniture proportion — the difference between a 3½ inch face and a 4 inch face is visible.
Measure rather than assume
Widths vary between mills and grades. Southern pine tends to be closer to nominal than SPF, and some premium mills hold closer to nominal than others. For a critical fit, measure a board from the lot you bought before you cut the whole stack.
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