Lumber Thickness in Quarters and Surfacing Codes: Rough, S2S, S4S and Edge Options

Hardwood lumber thickness is named in quarters of an inch of the rough sawn board: 4/4 is 1 in, 5/4 is 1 1/4 in, 8/4 is 2 in, up to 16/4 at 4 in. Surfacing codes say which faces and edges were planed: RGH (rough), S1S, S2S, S1E, S2E and S4S. A board surfaced two sides loses about 3/16 in up to 6/4 and 1/4 in from 7/4 up, but it is normally still tallied on its rough nominal thickness.

Quarters	Rough nominal	Metric (rough)	Surfaced two sides (S2S)
3/8 in (not quartered)	3/8 in	10 mm	3/16 in (5 mm)
1/2 in	1/2 in	13 mm	5/16 in (8 mm)
5/8 in	5/8 in	16 mm	7/16 in (9 mm)
3/4 in	3/4 in	19 mm	9/16 in (14 mm)
4/4	1 in	25.4 mm	13/16 in (21 mm)
5/4	1 1/4 in	31.8 mm	1 1/16 in (27 mm)
6/4	1 1/2 in	38.1 mm	1 5/16 in (33 mm)
7/4	1 3/4 in	44 mm	1 1/2 in (38 mm)
8/4	2 in	50.8 mm	1 3/4 in (44 mm)
10/4	2 1/2 in	63.5 mm	2 1/4 in (57 mm)
12/4	3 in	76.2 mm	2 3/4 in (70 mm)
14/4	3 1/2 in	89 mm	3 1/4 in (83 mm)
16/4	4 in	101.6 mm	3 3/4 in (95 mm)
18/4 to 24/4	4 1/2 to 6 in	114 to 152 mm	By contract

Standard thickness values from the Wood Handbook Table 6-4, with the metric rough sizes from the American Hardwood Export Council. Typical values: the NHLA rulebook and your contract govern.

How the quarter system works

The quarter number is the rough thickness in quarter inches over four. It is a sawing target, not a finished size. American Hardwood Export Council material notes that most US hardwood lumber is sawn between 1 in and 2 in, so 4/4 to 8/4 is where the volume and the choice of grades are.

Thickness	Typical availability	Typical uses
4/4	Every species and grade, the bulk of production	Cabinet doors, face frames, drawer parts, flooring, mouldings
5/4	Common	Stair risers, table aprons, parts that finish at a full 1 in
6/4	Common in the main species	Frames, thicker rails, some treads
8/4	Common in the main species, smaller grade choice	Legs, treads, thick tops, turning squares
10/4 to 16/4	Fewer suppliers, longer drying	Butcher block, newel posts, thick legs

Thick stock takes much longer to dry, so mills saw it in smaller volumes. The NHLA species guides note, for example, that not every white oak supplier offers 10/4 and 12/4.

Rough, surfaced and the scant allowance

Rough lumber comes off the saw and the kiln. The Wood Handbook notes that it can run over target size by variable amounts, depending on the mill’s equipment. Kiln drying also shrinks it. AHEC gives a scant allowance for rough kiln dried lumber: boards specified 3/8 to 1 3/4 in may be 1/16 in scant of nominal, and 2 in and thicker may be 1/8 in scant.

Surfaced lumber has been planed. For S2S the standard table above applies. The S1S thickness is subject to contract agreement. When the width is specified, edged lumber (S1E or S2E) is typically 3/8 in scant of nominal width under 8 in wide and 1/2 in scant at 8 in and wider, according to the Wood Handbook.

That leads to a practical rule: order the thickness that leaves room for your own planing. A part that must finish at a full 1 in comes from 5/4, not 4/4. A 3/4 in part comes comfortably from 4/4 rough. If you buy S2S, check that the table size suits your part before you order.

Surfacing and edge codes

Code	Meaning	Faces planed	Edges
RGH	Rough	None	Sawn or wane
S1S	Surfaced one side	One	Sawn or wane
S2S	Surfaced two sides	Both	Sawn or wane
S1E	Surfaced one edge	None	One planed
S2E	Surfaced two edges	None	Both planed
S1S1E	Surfaced one side, one edge	One	One planed
S2S1E	Surfaced two sides, one edge	Both	One planed
S1S2E	Surfaced one side, two edges	One	Both planed
S4S	Surfaced four sides	Both	Both planed
SLR1E	Straight line ripped one edge	As stated	One sawn straight
SLR2E	Straight line ripped two edges	As stated	Both sawn straight, parallel
SL2E (trade use)	Straight line two edges	As stated	Both sawn straight
GR (gang ripped)	Ripped to fixed widths	As stated	Both, to width

The surfaced codes are listed in the Wood Handbook. Straight line rip and gang rip codes are trade usage and vary by mill, so define them on the order. Our guide to straight line rip and gang rip edge options covers when an edge is worth paying for.

Tally: nominal or surfaced

Board feet are normally counted on the nominal rough thickness, even when the lumber ships surfaced. AHEC describes the tally: widths rounded to the nearest whole inch (exact halves alternately up and down), lengths rounded down to the whole foot, thickness on the nominal basis. A 4/4 board planed to 13/16 in is still counted as 1 in thick.

Two quotes on the same 4/4 lumber can therefore differ by nearly 19% (13/16 against 1) if one counts surfaced size and the other nominal. Ask every supplier to state the basis. The NHLA measurement rules page explains surface measure and rounding, and board feet and hardwood pricing shows the effect on price.

Metric orders

Buyers who order in millimeters should name both the quarter and the metric size, and say whether the metric figure is rough or finished, and at what moisture. A European contract may set thickness at a reference moisture and allow for shrinkage. Our page on EN 1313-2 permitted deviations covers the European size rules.

Metric order wording	What it can mean
26 mm	4/4 rough, slightly over 25.4 mm at the saw
21 mm planed	4/4 S2S at 13/16 in
52 mm	8/4 rough
45 mm planed	8/4 S2S, close to 1 3/4 in

What to write on the RFQ

State the quarter, rough or surfaced code, the finished size if surfaced, the minimum thickness you can accept, the tally basis and the moisture. Then send your lumber RFQ and we pass it to up to 3 mills or yards that carry the thickness. The lumber grades hub links the grade rules that go with it, and the hardwood lumber terms glossary defines the other trade abbreviations.

RFQ line: Hard maple, FAS, 5/4 S2S to 1 1/16 in, SLR1E, KD 6-8%, tally on nominal 5/4, minimum net 1 1/32 in accepted, 2,000 BF.