Equilibrium moisture content
Wood left long enough in steady conditions arrives at a moisture content set by the air around it, and then stops. This works out where that is.
What the number means
Moisture content in wood is expressed against the ovendry weight, not the total weight. A board at 12% carries water equal to 12% of what it would weigh with every trace of moisture baked out. This trips people constantly, because the intuitive reading — that 12% of the board is water — gives 10.7%, and the two get mixed in the same article all the time.
The definition has a consequence that sounds wrong and is not: moisture content can exceed 100%. Green western redcedar sapwood averages 249%. Sugar pine sapwood averages 219%. There is nothing paradoxical about it; the water simply outweighs the dry wood. Any calculator that caps the field at 100 has the wrong denominator, and at least one prominent one does.
How this is calculated, and how you can check it
The Wood Handbook publishes a large table of moisture content against temperature and humidity, and states plainly that the table was calculated from a particular equation — the Hailwood-Horrobin two-hydrate sorption model. That makes the whole thing verifiable rather than merely plausible: implement the equation, run it against the printed table, and see whether it agrees.
This implementation reproduces all twelve published reference values to four decimal places, and stays finite and monotonic across the entire range of temperature and humidity. It also lands at 28.8% at 70°F and 100% humidity, which is the fibre saturation point — the model agreeing with itself at its own boundary, which nobody arranged.
The caveat nobody passes on
That table was built mainly from Sitka spruce, measured under conditions described as midway between wetting and drying. Two things follow. First, it is a compromise curve rather than a species-specific one, and the Handbook says so — for most practical purposes it may be applied to wood of any species, which is an approximation flagged as an approximation. Second, and more useful: wood taking moisture up sits about 20% lower in moisture content than the same wood giving it off at the same humidity. That is hysteresis, it is real, and it means this number is the middle of a band rather than a point.
Temperature matters much less than you would think
Move from 50°F to 90°F at constant humidity and the equilibrium moisture content barely shifts — about half a percentage point. Humidity is doing nearly all the work. This is why heating a damp shed does not dry the timber in it unless the warm air is also changing; you have raised the temperature, the relative humidity has fallen as a result, and it is that second effect doing the drying.
Asked at the bench
How long until wood actually reaches this?
Much longer than most advice suggests. Solid-piled one-inch stock gains about 2% a month in warm humid conditions outdoors under a roof, 1% a month in an open shed, and 0.3% a month in a closed one. Stickering the pile so air reaches every face is far quicker. There is a separate tool for this.
What moisture content should I build furniture at?
Somewhere between 6% and 9% for most heated interiors, which is what the calculator returns for ordinary indoor conditions. The number that matters is not a received figure but the equilibrium moisture content of the room the work will stand in — work that out here, then aim the stock at it.
Is the reading on my moisture meter the same thing?
It is trying to measure the same quantity, with caveats. Pin meters read the outer few millimetres unless the pins are insulated and driven deep, so a thick board that has been in a different climate will read its shell, not its core. Readings also need correcting for species and temperature. The core of a board that left a kiln at 7% can still be near 7% long after its surface has drifted.
Does the temperature range here have a limit?
The Handbook tabulates the equation from 30°F to 270°F and states no formal validity range beyond that. Outside those bounds the equation stays numerically well behaved — its only singularity sits above 117% humidity, which cannot occur — but there is no published data to check it against, and the tool says so when you go outside.