The lumberyard said it was a four by four. You get it home, pull out a board from the pile and start measuring. The tape measure something different than what you purchased. What gives? How can that be?
Well, the wood is graded and processed before it ever reaches your workbench. The graphic below show how much larger a “four by four” really is compared to its surface size at the store. Knowing the difference will help you plan your projects actualy.
Why Wood Shrinks After Buying
Back in the colonial mills they used the phrase “quarters of an inch,” which refers to how the size system divide up an inch. A board coming off the saw called a four-four is actually an inch thick. Eight-four is a two-inch-thick board. When you see these numbers, it mean the board has its full potential raw thickness (before it’s dried or even planed).
Why does this matter? Each time it go through a thickness planer, it takes some material away. You’re buying rough lumber, so you know there’ll be some thickness left over as an insurance policy against shrinkage and other errors. The more thickness left over, the less likely it is you’ll wind up with boards that is already too small for what you want, wasting your money.
With some grades, like a simple panel, the rough stock begins at one-inch thickness, but after being surfaced on all four sides you’re left with three quarters of an inch. That’s a big deal if you’re doing any kind of precise joinery. You’ll notice in the chart below how much material you lose depending on grade, from simple panels to massive table slab. A rough twelve-four board, for example, will measure three inches thick but only two and a half once it’s been finished all around.
That gives you something to consider: should I purchase slightly thicker wood and just plane it myself down to size? That is a perfect fit. Similar reasoning applies to width, though there are some oddities here as well. Boards that are narrow will have their nominal width subtracted by about half-an-inch when they surface. Wider boards may have this amount cut off them by anywhere between a half- to three-quarter-inch. This difference is due to the irregular shape of the board (see above), which the mill trims down to square it off.
When laying out your cabinet or piece of furniture, you shouldn’t count on nominal width. Rather, use the surfaced dimension found in the chart below as your guide. The rough estimate printed on a plank’s price tag will be inaccurate for your needs. Whether you are planning to be right on the mark depends a good deal on what species you’re using.
Premium woods such as cherry and black walnut is very pricey and typically sold rough to reduce the cost of high-end wood. Structural framing, on the other hand, commonly uses cedar or some sort of pine (southern pine). This wood is often OK with slight planing since small amounts of shrinkage don’t alter the strength of the wood.
But even structural-grade softwood will still shrink. And there’s the key point: moisture content. Freshly felled lumber can contain between 20 percent and sometimes far more moisture, depending on when it was harvested. That means when it loses that water, the lumber will shrink across both thickness and width. Stock from the kiln stabilizes this by reducing moisture content to something closer to six or eight percent, perfect for furnitures intended for indoors. If you’re patient and have some extra space, air drying can work (about one year for each inch of wood thickness).
This photo shows why you must sticker the stacks correctly so they can dry out evenly from both sides. Without proper airflow between boards, your wood will warp and get moldy, making it unusable. Sealant like wax will trap the moisture in the ends of the boards quickly, leading to cracking called end checking.
Working with rough-sawn lumber means understanding the physics of wood and managing expectations. It means thinking through the end product size and planing allowance and knowing it will all change as the wood shrinks. You’ll be your own best teacher at learning to read the wood and adapt accordingly, but the chart helps lay out that roadmap to turning raw timber into something useful.
Next time you order up some stock, remember these dimensional realities. Knowing exactly what amount of wood you really do have would of put an end to that first panic over measuring discrepancies.
