What do these lumber fractions mean? 4/4, 5/4, 6/4, 8/4, 10/4, 12/4

In short, these fractions refer to the thickness of hardwood lumber. The thickness is read in increments of quarters of an inch.

Why those funny fractions? I’m used to standard sizes like 1×6, 1×12, 2×8 etc.

While you might be used to seeing sizes like 1×6 or 2×8 in lumberyards for softwoods like fir, pine and cedar, the hardwood industry takes a different approach. The softwood industry is vastly different from the hardwood industry. That’s because the primary users of hardwoods (such as red oak and cherry, for example) build custom or made-to-fit products – like furniture and cabinetry – where uniform sizes in the raw material is unnecessary. For example, there’s no standard size for kitchen tables, you can make one any size you want. So sawmills cut hardwood logs to get the best yield from a log, which means all boards will be various in width rather than the same width. If they were cut for specific sizes or uniform widths, there would be more waste, more time, and therefore the resulting boards would cost you more money. Softwoods that get sold in “standard” sizes like 1×6 and 2×8 are cut for particular applications that require those sizes.  In building construction you’ll find standard and uniform sizes from building to building. Door jambs and wall studs for example.

So, the hardwood industry standard for indicating the size starts with lumber thickness, and it’s expressed as a fraction: 4/4, 5/4, 6/4, 8/4 and so on.

rough hardwood lumber board

With rough lumber, it’s hard to see the grain and the boards are frequently slightly cupped or warped from the drying process. Surfacing fixes both of these, but it does remove thickness.

Let’s explain more, starting with the most common, 4/4

If we were talking you’d hear me say it as “four quarters,” which is short for four quarters of an inch.  If you’re pretty snappy with numbers, you’ve already put together that 4/4 is equal to one inch.

4/4 = 1″ thick
5/4 = 1-1/4″ thick
6/4 = 1-1/2″ thick
8/4 = 2″ thick
12/4 = 3″ thick

There’s another designation that might confuse you: S2S.  That means “surfaced two sides.” A saw that cuts lumber from a log is very large and aggressive, so the resulting lumber is known as “rough sawn” and the surfaces of the boards are . . . yes, rough.

Enter S2S. Lumber that is S2S has been planed smooth and flat. Naturally, that process removes some thickness and answers the age old question, “Why isn’t 4/4 lumber a full one inch thick?”

Actual measured thickness on 4/4 lumber that’s S2S is 13/16″.  Make sense?

Rough lumber needs to be dressed with a planer and/or jointer and this is where a lumber dealer like ourselves has to make a tough decision. Have the lumber surfaced smooth and clean, or sell it rough? There are benefits to both, but it’s impractical to stock both. Rough lumber costs less, and the woodworker is often able to yield thicker boards. Surfaced lumber, on the other hand, is much easier to see the grain and color when selecting boards, and the woodworker doesn’t necessarily need a planer in order to work with the material, making it easier on the less experienced woodworkers out there.

We’ve decided to stock surfaced lumber.

Here are pictures of each lumber thickness to help you out (left column shows each with a ruler, the right column without):