4/4 lumber thickness
Rough (as sawn and sold)
1 in
25.4 mm
Surfaced two sides (S2S)
13/16 in
20.6 mm
From the standard, NHLA Rules (2019)
What 4/4 means
Hardwood thickness is written in quarters of an inch, so 4/4 is 4 quarters: 1 inch, the thickness the board was sawn to. The hardwood grading rules of the National Hardwood Lumber Association set the standard surfaced thickness for each rough thickness: planed on both faces, 4/4 comes to 13/16 inch, 3/16 inch less than rough.
You pay for the rough thickness. The rules measure surfaced lumber before it was surfaced, so a 13/16 inch S2S board is billed as 4/4. Surfacing one side only has no standard thickness in the rules: it is agreed between buyer and seller.
A board that is still rough will flatten to less than its rough size once you joint and plane it yourself, and how much less depends on how much it has cupped or twisted, so measure your own stock before you plan joinery around a number.
Board feet per foot of 4/4
Counted at the rough 1 inch, thickness × width ÷ 12 for each foot of length:
| Width | bd ft per foot |
|---|---|
| 4 in | 0.333 |
| 6 in | 0.5 |
| 8 in | 0.667 |
| 10 in | 0.833 |
For a pile of random-width boards, the board foot calculator tallies them the way a hardwood yard does. All quarters from 4/4 to 16/4 are on the hardwood thickness chart.