How long does wood take to acclimate?
Not a week. The measured rates for solid-piled stock are slow enough to be startling, and thickness makes it worse than you would guess.
The rule has no source
"Let it acclimate for a week" is everywhere, in various lengths — 48 hours, 72 hours, three days, a week, two weeks. None of it comes from a standard. The National Wood Flooring Association's installation guidelines run to 184 pages and name no number of days anywhere. What they say instead is that flooring delivered at a moisture content matching its expected in-service conditions may be installed immediately, and that stock which does not match should be cross-stacked on stickers until equilibrium is reached.
That is a completely different instruction. The criterion is a moisture content, not a duration. A week is only the right answer by coincidence.
What a week actually achieves
The Forest Products Laboratory measured the rates. Kiln-dried one-inch hardwood, piled solid under a good roof in warm humid weather, gains about 2% moisture content per month over the first 45 days. In an open roofed shed it is about 1% a month. In a closed shed, 0.3%.
Take the fastest of those, the outdoor summer case, and a week moves the stock roughly half a percentage point. In a closed shed indoors — which is what trips people actually stack timber — a week moves it about a tenth of a point. If your boards arrived four points away from where they need to be, a week has closed one eightieth of the gap.
Thickness is worse than linear
Moisture moves through wood by diffusion, and diffusion times scale with roughly the square of the distance. The Dry Kiln Operator's Manual puts it directly: theoretically, doubling the thickness would multiply the time by four, and in practice it multiplies it by three to four because other transport mechanisms contribute.
So 8/4 stock is not twice the wait of 4/4. It is between three and four times. Nothing else about acclimation is worth as much as this, and none of the flooring advice mentions it — every version hands you a fixed number of days whatever the thickness.
Solid piles barely exchange at all
The rates above are for solid-piled timber, and the reason they are so slow is that only the outer faces of the pile are exposed. Boards in the middle of a tight stack are sealed by their neighbours. FPL measured a pile that after a full year still differed by nearly a point between its inside and outside.
Stickering — laying strips between every course so air reaches all four faces of every board — is what the guidelines actually call for, and it changes the picture completely. A carton of flooring sitting closed on a floor, with the recommended four inches of air underneath, is not acclimating. Air under a sealed box does nothing for the boards inside it.
The target is the room, not the workshop
The last error is the most common. Wood conditioned to your shop reaches equilibrium with your shop, and if the shop is an unheated garage at 65% humidity while the house runs at 35%, you have carefully brought the stock to the wrong number. Condition it to the room the finished work will stand in, or if that is impractical, build with the gap in mind and detail the joinery so it can move.
Asked at the bench
Does temperature speed it up?
Only indirectly, and this catches people. Equilibrium moisture content is set overwhelmingly by relative humidity — temperature shifts it by well under a point across the ordinary range. Warming a space lowers its relative humidity, and it is that which dries the wood. Heat alone, with the moisture staying in the room, achieves little.
What if I have no meter?
Weigh a sample board and keep weighing it every few days. When the weight stops falling it has stopped exchanging moisture, whatever the number is. That is the original method and it needs no instrument beyond scales.
My flooring supplier insists on 72 hours. Now what?
Follow the manufacturer’s instructions, because your warranty depends on them, but measure as well. If the boards arrive within a point of the room, the 72 hours is harmless and unnecessary. If they arrive four points out, the 72 hours will not fix it and the floor will move after you have laid it, whatever the paperwork says.
Is there any point acclimating at all, then?
Yes, when it is done properly and to the right target. Stickered stock in the destination environment genuinely converges, and the tool above gives a rough duration for your thickness. What does not work is a fixed number of days applied to a sealed stack in the wrong room, which is what the advice usually amounts to.