Rough Lumber Quarter System
Hardwood is sold by rough thickness in quarters, not inches — 8/4 means a nominal 2 inches rough, measured before any surfacing has touched the board yet.
Hardwood lumber isn't sold as "1 inch" or "2 inch" the way construction lumber is — it's sold in quarters of an inch measured at the rough, un-surfaced stage: 4/4 ("four quarter") is nominally 1" rough, 8/4 ("eight quarter") is nominally 2" rough, and the number describes the sawn thickness before any planing has touched the board at all.
Why hardwood uses a different system entirely
Dimensional softwood lumber (see /reference/nominal-vs-actual-2x4/) is milled to a fixed, standardized actual size regardless of what species or grade you buy — every 2x4 comes out at 1.5" x 3.5" no matter the mill. Hardwood for furniture work doesn't work that way, because furniture-grade lumber is sold rough or only lightly surfaced, and the buyer typically does their own final dimensioning to whatever thickness the specific project needs. A fixed actual-size standard would be pointless for that use case — a furniture maker building a 7/8"-thick tabletop and one building a 3/4"-thick drawer side both start from the same rough board and mill it down themselves, so the useful number to publish is the board's rough thickness before any of that customer-side milling happens.
The quarter-system table
| Quarter | Rough thickness (in) | Typical S2S (surfaced two sides) thickness (in) | Common use |
|---|---|---|---|
| 4/4 | 1.0 | 0.8125 (13/16") | General furniture parts, panels |
| 5/4 | 1.25 | 1.0625 (1 1/16") | Deck boards, thicker shelving |
| 6/4 | 1.5 | 1.3125 (1 5/16") | Table legs, aprons |
| 8/4 | 2.0 | 1.75 (1 3/4") | Turning blanks, heavy furniture parts |
| 10/4 | 2.5 | 2.25 (2 1/4") | Chair and table legs needing real bulk |
| 12/4 | 3.0 | 2.75 (2 3/4") | Heavy timber-frame and thick slab work |
The S2S (surfaced two sides) column is a typical figure, not a guarantee — a mill or supplier planing a 4/4 board to remove cup and get two flat reference faces commonly takes it down to around 13/16", but exactly how much comes off depends on how much the rough board needed to be flattened in the first place. A board with more cup, twist, or surface roughness loses more to reach flat.
Board feet, computed both ways
Board footage for rough hardwood is calculated from the rough thickness, not the surfaced one, because you're paying for the rough board and doing your own milling — an 8/4 board, 6" wide, 8' long, rough, works out to 8 board feet, a figure the board foot calculator confirms directly. Once you surface that same board down to its typical 1.75" S2S thickness without changing its width, the real usable material drops to 7 board feet — the missing board foot is what came off in flattening and thicknessing, gone for good, and worth accounting for when estimating how much rough stock to buy for a project's finished dimension.
Why 5/4 sits between framing and furniture
5/4 stock (1.25" rough, roughly 1.0625" S2S) is a genuine crossover size — it's thick enough to serve as exterior decking, where the rough-to-surfaced distinction barely matters because deck boards are typically used closer to their rough thickness than furniture parts are, and thin enough to work as furniture-grade shelving stock milled down further. If you're buying 5/4 for decking, confirm with the supplier whether it's true rough-sawn hardwood quarter stock or dimensional decking stock following the softwood nominal-vs-actual convention instead — the two use the same "5/4" shorthand in casual conversation but aren't guaranteed to be the same product.
Where this connects
S4S (surfaced four sides, the standard finish for dimensional softwood lumber) and S2S (surfaced two sides, more common for rough hardwood sold partially dressed) describe how many faces a mill has already planed flat before it reaches you — a board sold S2S still needs its edges jointed and ripped to final width by whoever's using it, while S4S stock arrives ready to cut to length. Don't assume a quoted thickness for hardwood already accounts for surfacing loss unless the listing explicitly says S2S or S3S; a "4/4 walnut" listing with no further qualifier is describing the rough board, and the buyer's own planer takes the rest.
A buying rule that avoids the most common mistake
Order rough thickness at least one full quarter-inch step above the finished dimension you actually need, not the exact number — a project needing a true 3/4" finished thickness should be bought as 4/4, not thinner, because 4/4's typical 13/16" S2S figure already assumes some material is spent flattening the board, and a board bought exactly at 3/4" rough leaves nothing to remove for flattening at all. This is the single most common estimating mistake in a first hardwood order: treating the quarter number as if it already were the finished thickness, the same category of mistake /reference/nominal-vs-actual-2x4/ describes for construction lumber, just with the assumption running in the opposite direction — nominal softwood sizes are inflated relative to actual, while a quarter number understates how much rough material a given finished thickness really requires.