Nominal vs Actual Lumber Sizes, Explained
July 6, 2026
Grab a tape measure, run it across the face of a 2x4, and you get 3.5 inches. Not 4. Run it across the thickness and you get 1.5, not 2. Nobody lied to you at the lumberyard. That gap between the name and the measurement is the difference between nominal and actual size, and it trips up every new woodworker at least once.
Here is the short version. Nominal size is the name. Actual size is what you measure. And board feet, the way lumber gets priced and sold, are figured at the nominal size.
Why a 2x4 is really 1.5 x 3.5
A board starts life bigger. When a green 2x4 gets sawn from the log, it really is close to 2 by 4. Then it dries, which shrinks it, and then it runs through a planer to get those smooth, flat faces. Planing shaves material off all four sides.
By the time the board hits the shelf, it lost about a half inch off the thickness and width. The 2 became 1.5. The 4 became 3.5. The name stuck around from the rough-sawn stage, so we still call it a 2x4.
Length is the exception. An 8-foot board is actually 8 feet, sometimes a hair over. Only thickness and width get trimmed down.
The chart: nominal vs actual dimensions
Here are the common sizes you'll see stacked at any yard. Thickness and width are in inches.
| Nominal | Actual (thickness x width) |
|---|---|
| 1x2 | 0.75 x 1.5 |
| 1x4 | 0.75 x 3.5 |
| 1x6 | 0.75 x 5.5 |
| 1x8 | 0.75 x 7.25 |
| 1x10 | 0.75 x 9.25 |
| 1x12 | 0.75 x 11.25 |
| 2x2 | 1.5 x 1.5 |
| 2x4 | 1.5 x 3.5 |
| 2x6 | 1.5 x 5.5 |
| 2x8 | 1.5 x 7.25 |
| 2x10 | 1.5 x 9.25 |
| 2x12 | 1.5 x 11.25 |
| 4x4 | 3.5 x 3.5 |
| 6x6 | 5.5 x 5.5 |
Two patterns make this easy to remember. Anything nominal 1-inch thick planes down to 0.75. Anything nominal 2-inch thick planes down to 1.5. And once a board is 8 inches wide or more, it loses 3/4 inch off the width instead of 1/2 inch.
For a deeper walkthrough of these dimensions and where they come from, the full board foot chart breaks it down further.
Why board feet use nominal size
This is the part that saves you money and confusion. A board foot is figured at the nominal dimensions, not the actual planed size. So a 2x4 counts as a full 2 by 4 when you run the numbers.
The formula is simple:
board feet = (thickness x width x length in inches) / 144
Run a standard 2x4x8. That's 8 feet, or 96 inches long.
(2 x 4 x 96) / 144 = 5.33 board feet
If you used the actual 1.5 x 3.5 instead, you'd get 3.5 board feet, and every price and tally in the country would be wrong. The whole trade agrees to price at nominal, so everyone's math lines up.
A few more to lock it in, all at nominal size:
| Board | Board feet |
|---|---|
| 1x6x8 | 4 |
| 1x4x8 | 2.67 |
| 1x12x8 | 8 |
| 4x4x8 | 10.67 |
That 1x6x8 works out clean: (1 x 6 x 96) / 144 = 4. The 4x4x8 is (4 x 4 x 96) / 144 = 10.67. Dimensional softwood at the big-box store often gets sold by the linear foot instead, but hardwood and anything sold rough gets priced by the board foot at nominal.
Where this actually bites you
Two places. First, cut lists. If your plan calls for a 3.5-inch-wide piece and you buy a 1x4 expecting 4 inches, you're short a half inch. Design around actual sizes, buy by nominal names.
Second, pricing. When you compare a hardwood quote to a home-center 2x4, remember one is priced at nominal board feet and the other might be linear feet of a smaller actual board. If that split confuses you, the breakdown in board foot vs linear foot sorts it out.
Keep a size chart handy until the numbers stick, then you'll stop second-guessing every board you pick up.