What 4/4, 5/4 and 8/4 Mean (Lumber Thickness)
Hardwood is sold rough and its thickness is quoted in quarters of an inch: 4/4 is one inch, 5/4 is 1¼, 8/4 is 2. Here is the full quarter chart, how rough differs from the surfaced thickness you end up with, and which figure to use for board feet.
By the Calculate My Reno Team / Published
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4/4 means one inch thick. Hardwood is sold rough-sawn and its thickness is quoted in quarters of an inch, so “four-quarter” is 4 ÷ 4 = 1 inch, “five-quarter” (5/4) is 1¼ inches, and “eight-quarter” (8/4) is 2 inches. You say it out loud as “four-quarter,” not “four over four.”
Why hardwood uses quarters
Softwood like a 2x4 comes pre-surfaced to a standard size. Hardwood is different: it is sold rough so you can plane it to the exact thickness your project needs. The quarter system is just a tidy way to name that rough thickness before any wood is removed. It is the National Hardwood Lumber Association (NHLA) convention.
The quarter chart
The “call” is the rough thickness off the saw; the surfaced figure is what you get after planing both faces (S2S):
| Call | Said as | Rough thickness | Surfaced (approx.) |
|---|---|---|---|
| 4/4 | four-quarter | 1 in | ~13/16 in |
| 5/4 | five-quarter | 1¼ in | ~1‑1/16 in |
| 6/4 | six-quarter | 1½ in | ~1‑5/16 in |
| 8/4 | eight-quarter | 2 in | ~1¾ in |
| 12/4 | twelve-quarter | 3 in | ~2¾ in |
| 16/4 | sixteen-quarter | 4 in | ~3¾ in |
5/4 lumber
5/4 is 1¼ inches rough. It is the go-to for deck boards and stair treads - stiffer than a 1x board but lighter and cheaper than a full 2x. Surfaced, it lands near an inch, so plan your joinery around the finished thickness, not the 1¼ call.
8/4 lumber
8/4 is 2 inches rough, surfacing to about 1¾. It is used for table legs, thick tabletops and chunky frames. Expect to pay more than double the per-board-foot price of 4/4: thick stock dries slowly, checks and cracks more, and yields fewer clear boards.
Quarters and board feet
Board feet are figured on the rough quarter thickness, not the thinner surfaced size - that is the wood you pay for. So a 4/4 board is 1 inch in the formula and 8/4 is 2 inches. The full method is in how to calculate board feet, and the definition in what is a board foot. Enter the quarter thickness as inches in the board foot calculator: 4/4 = 1, 5/4 = 1.25, 6/4 = 1.5, 8/4 = 2.
This quarter system is for hardwood; pre-surfaced softwood uses the nominal 2x4-style sizes instead.
Frequently asked questions
What does 4/4 mean in lumber?
It means one inch thick. Hardwood thickness is quoted in quarters of an inch, so 4/4 ("four-quarter") is 4 ÷ 4 = 1 inch, measured rough off the saw. It is read aloud as "four-quarter". The same way, 5/4 is 1¼ in and 8/4 is 2 in.
How thick is 5/4 lumber?
5/4 ("five-quarter") lumber is 1¼ inches thick rough-sawn. After it is surfaced smooth it finishes around 1 to 1‑1/16 inches. It is the common thickness for deck boards and stair treads, where it is stronger than a 1x board but lighter than a full 2x.
How thick is 8/4 lumber?
8/4 ("eight-quarter") lumber is 2 inches thick rough, surfacing to about 1¾ inches. It is used for table legs, thick tops and anything that needs a substantial section. It costs more than twice as much per board foot as 4/4 because thick stock is slower to dry and loses more to defects.
Why is hardwood measured in quarters?
It is the National Hardwood Lumber Association convention for rough lumber. Hardwood is sold rough so the buyer can surface it to the exact thickness they need, and quarters give a simple way to specify the rough thickness - 4/4, 5/4, 6/4, 8/4 - before any wood is planed off.
Do I use the rough or surfaced thickness for board feet?
Use the rough quarter thickness. A 4/4 board is figured as 1 inch and 8/4 as 2 inches for board feet, even though the surfaced board is thinner, because that is the thickness you are charged for. Enter 4/4 as 1, 5/4 as 1.25 and 8/4 as 2 in the board foot calculator.