Hardwood lumber is described differently from the softwood "2×4" you buy at a big-box store, and the difference trips up almost every new woodworker. Hardwood is sold rough, by the board foot, and its thickness is quoted in quarters of an inch. Get comfortable with the system and ordering becomes straightforward.
Hardwood thickness is written as a fraction over four, where the numerator is the rough thickness in quarter-inches:
| Nominal | Spoken | Rough thickness |
|---|---|---|
| 4/4 | four-quarter | 1″ |
| 5/4 | five-quarter | 1.25″ |
| 6/4 | six-quarter | 1.5″ |
| 8/4 | eight-quarter | 2″ |
So 4/4 oak is nominally 1 inch thick in the rough. This nominal thickness is what board-foot pricing is based on, even if the board has been surfaced thinner.
Rough lumber is sawn slightly oversize, then loses material when it is planed flat. A typical surfaced 4/4 board ends up around 13/16″ (about 0.81″) thick once it is dressed on both faces — but you still pay for it as a full 1″ (4/4) board foot. Likewise, 8/4 stock surfaces to roughly 1-3/4″. The gap between nominal and actual exists because you are buying the rough volume; the planing is what makes it usable and flat.
The more a yard surfaces a board, the more wood is removed and the higher the price per board foot, because you are paying for both the labor and the lost thickness.
Rough hardwood usually comes in random widths and lengths rather than fixed sizes, because boards are cut to follow the log. That is precisely why it is sold by the board foot — the volume measure adapts to whatever width and length you actually receive.
Always calculate board feet from the nominal rough dimensions, not the dressed ones. A 4/4 × 6″ × 8 ft board is figured as (1 × 6 × 96) ÷ 144 = 4.00 board feet, even though it finishes around 13/16″ thick. Using the planed thickness would undercount what the yard charges you.
Open the free Board Foot Calculator →Enter the decimal thickness (1, 1.25, 1.5, 2) in the calculator and it handles the rest, including a full project cut list. Next, see how much hardwood lumber costs.