One of the first things I needed to figure out when I started cutting rough lumber on a sawmill was what thickness to make it. I could generally determine if I wanted it to be thick or thin, but just how thick or thin? 4/4 lumber is 1″ thick, so it should be rough cut at 1″ thick, right? Not exactly. For hardwoods, the commercial target for 4/4 lumber is actually 1-1/8″, which allows enough margin to produce dried and planed lumber at a thickness of 13/16″ or 3/4″ (3/4″ is acceptable, but the extra 1/16″ of thickness in 13/16″ material allows room for additional planing or sanding after panels or doors are glued up).

The crazy thing is that back then I couldn’t find solid information on lumber thicknesses anywhere and when I referred to the NHLA (National Hardwood Lumber Association) guide, the thicknesses didn’t match up with what I was finding from hardwood producers.  The NHLA guide doesn’t include the bonus 1/8″ of thickness – 4/4 lumber, for example, is specified at a minimum of 1″.

In my experience, 4/4 hardwood lumber cut at 1″ is too thin to consistently produce flattened and planed lumber at 13/16″ thick and will even have trouble producing 3/4″ thick unless the boards are very flat. The only way 1″ thick rough-cut hardwood lumber can plane out completely to 13/16″ or 3/4″  thick is to skip the flattening and just plane the lumber. This will produce thicker finished lumber, but it won’t be flat and straight since the planer will simply follow the curves of any crooked boards. From a woodworkers perspective this is a horrible practice and makes woodworking much more difficult. For this reason, I cut my 4/4 hardwood lumber like all other quality producers at 1-1/8″ thick and don’t accept anything from other sawmills or wholesalers at 1″ thick.

Starting with the lumber measurement and adding 1/8″ for the final thickness is how all of the hardwood measurements go, with a target for 4/4 lumber at 1-1/8″, 5/4 lumber at 1-3/8″, 6/4 at 1-5/8″ and 8/4 at 2-1/8″. These are the commercially accepted numbers, and except for 8/4 lumber the ones that I shoot for. The problem with 8/4 lumber is that since there is more wood it shrinks more than thinner lumber and 2-1/8″ thick just isn’t enough thickness to flatten and plane lumber to consistently finish at 1-3/4″, which is the target for 8/4 lumber. When I flatten and plane batches of 8/4 lumber milled at 2-1/8″ thick it isn’t uncommon for half of the lumber to finish at 1-5/8″ thick instead of 1-3/4″.

Because I think 2-1/8″ is a little thin, I commonly cut 8/4 lumber at 2-3/8″ thick. 2-3/8″ thick is twice that of 4/4 lumber, plus the 1/8″ saw kerf that would have been between the two imaginary cuts. The extra thickness not only impresses the ladies, but it assures a final dried and planed thickness of at least 1-3/4″ and officially uses no extra wood when compared to cutting 4/4 lumber (to keep things simple, a friend of mine simply calls it “double four quarter” lumber). As I mentioned though, 8/4 is commercially sawn at 2-1/8″ thick, so if you cut it at that measurement it isn’t wrong, 2-3/8″ is just better for the end user (none of my customers have ever been upset that the wood is a little thicker).

The previous examples were for hardwoods, but softwoods, like white pine, can be cut thinner since they shrink less and dry straighter overall, plus softwoods are commonly used for construction purposes instead of furniture, which don’t need the extra thickness for secondary planing or sanding, so 3/4″ final thickness is common for 4/4 softwood lumber. For 4/4 white pine for example, I cut 1″ thick, which will finish at 3/4″. And, for cedar, which shrinks very little and is very straight and stable, I will go even thinner, down to 7/8″. In general though, softwoods are cut on the standard quarter scale with 4/4 lumber measuring 1″.

The scale below shows the target hardwood lumber thicknesses for commercially produced, rough-cut lumber and their planed thickness counterparts. These are the sizes you should expect to find when shopping for hardwoods.
Hardwood Lumber Measurements

Quarter-scale measurement    Rough cut thickness    Planed thickness
4/4                                                1-1/8″                            13/16″
5/4                                                1-3/8″                            1-1/16″
6/4                                                1-5/8″                            1-1/4″
8/4                                                2-1/8″ (or 2-3/8″*)         1-3/4″

*2-3/8″ is a better thickness to consistently finish at 1-3/4″ thick, but 2-1/8″ is the norm.