What Do 4/4, 5/4 and 8/4 Mean in Lumber?

Updated October 2026

Hardwood thickness is described in quarters of an inch. “4/4” (said “four-quarter”) is four quarters: 1”. “8/4” is eight quarters: 2”.

These numbers describe the rough-sawn thickness, the board as it comes off the sawmill before planing. Once surfaced, every board is thinner than its quarter size.

Quarter sizes and typical surfaced thickness

Surfaced dimensions vary a little between mills. If you need an exact finished thickness, ask the yard what they plane to, or buy rough and plane it yourself. See rough vs. surfaced lumber.

Which thickness for which part?

• 4/4: the workhorse. Cabinet sides, drawer fronts, aprons, shelves, face frames. Finished ¾”.
• 5/4: tabletops and shelves that need to look a bit heavier. Finished about 1”.
• 6/4: heavier tabletops and bench seats.
• 8/4: table legs, bench legs, thick tops, chair parts.
• 12/4 and 16/4: turning blanks, thick legs and newel posts. Often sold by the piece.

Price goes up with thickness

Thick stock comes from bigger logs, takes far longer to dry, and checks more often, so dealers charge more per board foot for it. 8/4 commonly costs 20–40% more per BF than 4/4 of the same species and grade, and 12/4 or thicker can run 50–80% more.

Board feet are still calculated on the quarter (rough) thickness: an 8/4 board counts as 2” thick even after planing. See how to calculate board feet.

5/4 deck boards are different

At home centers, “5/4 decking” (often written 5/4 × 6) is a pressure-treated softwood deck board about 1” thick and 5½” wide with rounded edges. It is not the same product as 5/4 hardwood from a lumber dealer, which is planed to about 1 1/16” with square edges.

Price your boards

The board foot calculator lets you choose 4/4 through 16/4, applies typical thick-stock premiums, and shows weight and cost by species.