Wood Movement Calculator
Estimate how much a solid wood panel will expand or contract across the grain by species, width, and seasonal humidity swing.
Default of 6 points of moisture content covers a typical indoor winter-to-summer swing; homes without central heat or in humid climates can see more. This is flat-sawn movement. Quarter-sawn boards of the same species move roughly half as much across the width, which is why quarter-sawn stock is prized for wide panels.
How it works
Solid wood keeps absorbing and releasing moisture from the air for as long as it exists, and it swells or shrinks across the grain as that moisture content changes, even after a project is finished and assembled. The amount of movement depends on the board's width, the species (some woods move a lot more than others), and how much the moisture content actually swings between your driest and wettest season. This calculator uses published flat-sawn (tangential) shrinkage coefficients per species and scales them by width and your seasonal moisture change.
Worked example: a flat-sawn red oak panel 12 inches wide, with a typical 6-point seasonal moisture swing, moves about 12 × 0.00369 × 6, or 0.27 inches across its width. That's close to 9/32 inch, enough that a panel glued rigidly into a frame on all four edges will either crack itself apart or split the frame as the seasons change. An 8-inch-wide hard maple board under the same swing moves about 0.17 inches, or roughly 5/32 inch, since maple has a lower movement coefficient than oak.
FAQ
Why does this matter if my project is already glued together?
Wood keeps moving after assembly, it just does it slowly, tied to the seasons rather than the day it was cut. A door panel, tabletop, or drawer bottom that's trapped rigidly on all sides has nowhere to go when it swells, so something gives: the wood cracks, the glue joint fails, or the frame bows. That's the reason solid panels get set in a groove with room to move, or fastened with slotted hardware, instead of glued solid into a frame.
What's the difference between flat-sawn and quarter-sawn movement?
Flat-sawn (tangential) boards, where the growth rings run roughly parallel to the wide face, move the most across their width. Quarter-sawn boards, cut so the rings run closer to perpendicular to the face, move roughly half as much for the same species and width. That's why quarter-sawn stock costs more and gets picked for wide tabletops and doors.
Does the direction of the grain along the length of a board matter?
Movement along the length (with the grain) is negligible for practical purposes, close to zero. Nearly all the expansion and contraction that matters for design happens across the width and, to a lesser extent, the thickness. That's why this calculator only asks for width.
How do I know my local seasonal moisture content swing if it's not 6%?
Six points is a reasonable default for a heated home in a moderate climate. If you live somewhere humid with big seasonal swings, or your shop and finished piece will live in an unconditioned space like a garage or cabin, the real swing can run higher. A moisture meter checked on scrap wood across a few months gives you a real number to enter instead of the default.
For more on why this happens and how it affects joinery, see how to read wood grain and why it matters, the mortise and tenon joint, explained, and softwood vs hardwood.