Wood Movement Calculator
Estimate how much a board will expand or shrink across its width with a seasonal change in moisture content, using species dimensional-change coefficients.
How it works
ΔW = W × Cv × (MC_end − MC_start), where Cv is the species’ dimensional-change coefficient and MC is moisture content in %
Wood is dimensionally stable along its length but expands and shrinks across its width as it gains or loses moisture with the seasons. The change equals the width times the change in moisture content times a species-specific dimensional-change coefficient (Cv), tabulated in the USDA Wood Handbook. Flatsawn boards move roughly twice as much as quartersawn ones of the same species, which is why quartersawn stock is prized for stable panels and instrument tops. The practical lesson: a wide solid-wood panel can move an eighth of an inch or more between summer and winter, so tabletops must be fastened with slotted or button attachments and frame-and-panel doors must float.
Worked example
A 12″ flatsawn red oak board (Cv ≈ 0.00365) dries from 12% to 7% moisture over winter — a −5% change. Movement = 12 × 0.00365 × (−5) = −0.219″, so it shrinks a bit under 1/4″ across its width. Fasten a top made of it so that quarter-inch can come and go without splitting.
Frequently asked questions
What moisture content should I design around?
Furniture indoors typically cycles between roughly 6% in a dry heated winter and 9–12% in a humid summer. Using your local seasonal extremes gives the worst-case movement to design clearances for.
Why does quartersawn move less?
Tangential shrinkage (flatsawn) is about twice radial shrinkage (quartersawn) for most species, because of how the wood’s ray cells and growth rings are oriented. That is why quartersawn boards have a smaller dimensional-change coefficient and stay flatter.
Does length change too?
Longitudinal movement is tiny — typically under 0.1% — so for practical purposes boards do not change length. All the meaningful seasonal movement is across the width and, to a lesser degree, the thickness.
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This calculator is for educational purposes. Double-check important results before acting on them.