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Wood movement calculator
Wood follows the air's humidity: wider in summer, narrower under winter heating — across the grain, whether you allow for it or not. Enter the board and your indoor humidity swing; get the movement in millimetres and the gaps and slots to build in.
Your board
Central-heated rooms drop to 25–35% in winter; the same rooms sit at 55–65% in summer. The defaults are that normal indoor swing.
The movement
Drawn to true scale — solid is the board at its driest, dashed at its widest.
Build it in
The result appears here — millimetres of seasonal movement, and the clearances to leave.
Wood movement from https://sketchmethis.com/wood-movement — free, no signup.
Why wood moves
Below its fibre saturation point, wood exchanges moisture with the air until it reaches equilibrium — and as its moisture content changes, it swells and shrinks across the grain. Along the grain it barely moves at all. That asymmetry is the whole craft problem: a tabletop 800 mm wide is 800 mm of cross-grain, and it will change width with the seasons no matter what the finish, the glue or the screws have to say about it.
The estimate used here is the standard one: ΔD = D · (S/30) · ΔMC — board width times the species' shrinkage coefficient times the moisture swing. Moisture content follows relative humidity via the published equilibrium table: roughly 6% MC in a heated winter room, 11% in a humid summer.
Movement by species
Full shrinkage from green to oven-dry, in percent — tangential is what a flat-sawn face does, radial is quarter-sawn. Divide by 30 for the per-1%-MC coefficient. Individual boards vary; these are the published species averages:
| Species | Radial | Tangential |
|---|---|---|
| Beech | 5.5 | 11.9 |
| Maple (hard) | 4.8 | 9.9 |
| Birch | 7.3 | 9.5 |
| Oak | 4.3 | 8.9 |
| Ash | 4.9 | 7.8 |
| Walnut | 5.5 | 7.8 |
| Pine | 4.0 | 7.7 |
| Spruce / larch | 3.3–3.6 | 7.8 |
| Douglas fir | 4.8 | 7.6 |
| Cherry | 3.7 | 7.1 |
| Teak | 2.5 | 5.8 |
| Plywood (in plane) | ≈0.45 — the reason it exists | |
Two readings worth keeping: quarter-sawn moves roughly half as much as flat-sawn of the same species, and beech — the classic European workbench wood — moves more than almost anything else on the list. A wide flat-sawn beech top in a centrally heated flat can move well over a centimetre through the year.
The three classic mistakes
Screwing a top down rigid. The top loses the argument with the seasons and splits — or tears the screws out. Fasten wide tops with slotted holes or buttons that let the wood travel.
Gluing a panel into its frame. Frame-and-panel exists precisely so the panel can float. Glue it and you have built a crack, scheduled for February.
Trusting the width on glue-up day. The board you measured in a humid August workshop is not the board that lives beside a radiator in January. Build for the swing, not for today's number.
Questions
Does the finish stop the movement?
No — film finishes only slow the exchange. They flatten the daily spikes but the seasonal swing arrives regardless. Sealing all faces evenly matters for keeping a board flat; it does not keep it the same width.
What about length?
Longitudinal movement is roughly a hundredth of tangential — a 2 m board changes fractions of a millimetre in length. This is why only the cross-grain direction is computed.
My humidity numbers are guesses. Does it matter?
A cheap hygrometer settles it, but the defaults (30–60%) match most heated homes in a temperate climate. If the piece is headed somewhere harsher — a bathroom, a cabin, underfloor heating — widen the range and let the tool show you the worst case.
Where do the numbers come from?
The species shrinkage values and the humidity-to-moisture table are the published wood-science standards used in every serious reference. The calculation runs entirely in your browser; nothing is uploaded.
This is one dimension of one board. A whole piece has dozens.
Movement gaps, kerf, dados that eat width — every piece of furniture is a pile of small sums with expensive failure modes. Sketch Me This is a tool being built to carry all of them: sketch or describe the piece, get dimensioned drawings and the full cut list with the joinery already subtracted.