Wood movement calculator
Describe the board and the two climates it lives between. You get the width change, the direction it will cup, and what to leave for it.
Reading the drawing
The rings tell you which way it goes. In a flatsawn board they arc across the wide face, so the face that was nearer the bark has more ring length running along it than the face nearer the pith. Wood shrinks roughly twice as much along the rings as across them, so the bark side loses more width than the pith side, the arcs try to straighten, and the board cups away from the heart. That is the mechanism behind the old bench maxim about which way a board curls.
Turn the same log ninety degrees and the rings run through the thickness instead. Now both faces shrink by the same amount and there is nothing to make it bend — which is why quartersawn stock stays flat, and why it gives up that advantage in thickness rather than for free.
The two ways to describe the climate
Moisture content is the honest input, but it needs a meter, and most people do not have one. So the default here works from temperature and humidity instead, using the equation the Wood Handbook publishes for exactly that purpose. Wood left long enough in still conditions settles at a moisture content determined by the air around it, and that relationship is well characterised.
Of the calculators surveyed, only one other chains weather through to movement. The rest demand a moisture content and leave you to find it, which of everything here is the hardest figure for anyone to produce.
Why the coefficient changes with the cut
There are two shrinkage figures for every species, not one. The radial figure applies across the rings, the tangential figure along them, and which one governs your board's width depends entirely on how it was sawn. The Handbook is explicit: use tangential for flatsawn width, radial for quartersawn width, and tangential again when the grain is mixed or you cannot tell — assume the worst.
That last instruction is the one every calculator drops. If you are looking at a stack of rough boards and cannot say what you have, the safe answer is the tangential one.
Asked at the bench
Does the length change too?
Barely. Longitudinal shrinkage runs 0.1% to 0.2% for most species, which is a millimetre across a metre and generally ignorable. The exception is reaction wood and juvenile wood near the pith, which can shrink ten to twenty times that. It is why an otherwise good board sometimes twists as it dries, and why the boards nearest the centre of the log are the ones that misbehave.
Can I stop it by finishing the wood?
No. The Forest Products Laboratory tested 91 finishes and found that no coating applied to wood entirely prevents moisture uptake. A film finish slows exchange down, which helps by damping the seasonal peaks, but the wood still arrives at the same place. Coat both faces the same way, though — finishing one side only makes the board exchange moisture faster on the bare face, and that really does make it cup.
My board is 8/4. Does thickness change the width movement?
Not the amount, no — the percentages are the same whatever the thickness. Thickness matters for how fast the board responds, not how far it goes. A thick board lags the season and never quite reaches either extreme; a thin one tracks the weather closely.
Why does it warn me about my numbers?
Because a large moisture swing produces a correct answer to the wrong question. A heated, occupied house moves wood three to four points across a year. Entering ten points is a legitimate figure for an unheated barn and nonsense for a dining room, and those two differ by several times the movement allowance.