Wood movement estimator
Solid wood moves with humidity. The question is how much — and along which axis. This calculator uses species-specific tangential and radial shrinkage coefficients from the USDA FPL Wood Handbook and your local indoor humidity range to give you a real number for how wide a panel will get between January and August.
Calculator
Calculations use shrinkage coefficients from green to oven-dry divided by 30 (FPL convention). EMC table by temperature used internally; default 70°F.
Worked example: a 24″ cherry tabletop
You glued up a 24″-wide cherry tabletop from 8/4 stock (surfaced to ~1.5″). Your shop is 28% RH in winter and 65% RH in summer. Flat-sawn cherry moves 9.7% tangentially per 30% RH change.
Between 28% and 65% RH you're looking at roughly a 0.3″ change across the panel — about 1/8″ each direction from your spring-clamped centre. If the top is screwed tight to the apron with no slot, the top will either crush the screws or crack itself. If you cut a 3/8″-wide slot in the apron screws, you've got room.
Why wood moves (and why it moves more one way)
Wood is a bundle of long cells, mostly aligned with the trunk. When the cell walls gain or lose moisture, the cells change dimension — far more across the cells (tangential) than along them (radial). End-grain (longitudinal) is essentially stable for our purposes.
That's why:
- A flat-sawn board moves about twice as much across its width as a quarter-sawn board.
- A board's thickness barely changes (it's mostly measuring the radial direction).
- Tabletop breadboard ends need elongated slots so the top can slide underneath.
- Raised-panel cabinet doors float in their frames — the frame moves very little, the panel does the moving.
Limitations
- Shrinkage coefficients are species averages. Two boards of "walnut" can move 15% differently.
- The calculator assumes an indoor environment. Outdoor projects see 50–90% RH swings and behave very differently.
- Flat-sawn boards often contain a mix of tangential and radial orientation; the calculator picks the dominant one based on your selection.
- This is a one-dimensional estimate. Real panels can cup, bow and twist — those are 3D effects the calculator doesn't capture.
Frequently asked questions
Is quarter-sawn wood really more stable?
Yes — typically about half the tangential movement. That's why antique furniture with quarter-sawn white oak has survived 150 years of seasonal cycling while flat-sawn pieces from the same era often split at the joints. The trade-off is cost and yield: quarter-sawing wastes more log volume.
How do I find the actual humidity in my house?
A $10 hygrometer at the lumber yard is good enough. Measure in the room where the finished piece will live, ideally across a full year, to get your real winter/summer swing. If you don't want to wait a year, the defaults (28% / 65%) are conservative for a climate-controlled home in most of the US and Europe.
Does plywood move seasonally?
Much less than solid wood — about 1/10 as much. The cross-banded plies in plywood cancel most of the movement. That's the main reason plywood is used for cabinet sides and large casework panels despite solid wood looking nicer.
Should I "balance" the humidity of my shop and my house?
If you can, yes. A hygrometer in the shop is worth the $10. If the shop runs at 50% year-round and the house runs 28% in winter, every piece you build will move significantly once it goes home. Ideally, build at the humidity the piece will live at — or expect to dimension the final piece on site.