Wood Movement Calculator
Estimate how much a board will expand or shrink across the grain with seasonal humidity changes.
Estimated dimensional change
How seasonal wood movement is estimated
Solid wood expands and contracts across its width (and thickness) as its moisture content changes with the seasons, even after it's fully dried and installed. This movement happens almost entirely across the grain, not along its length, which is why tabletops, wide panels, and flooring need room to move but a board's length is treated as essentially stable.
Wood science expresses this behavior with a species-specific dimensional-change coefficient: the fraction of a board's width that it gains or loses per one percentage point change in moisture content. The formula used here is Dimensional change = Width × Coefficient × ΔMC, where ΔMC is the expected swing in moisture content between your driest and most humid season.
Because species differ considerably in cellular structure, their movement coefficients differ too — woods like red oak and hard maple move noticeably more than woods like western red cedar for the same width and moisture swing, which is why species selection matters as much as board width in this estimate.
Where these numbers come from
The formula and default values in this calculator are based on:
- Dimensional-change coefficient method matches wood movement calculation approaches published in wood technology references derived from USDA Forest Products Laboratory shrinkage/movement data (e.g. as compiled in R. Bruce Hoadley's "Understanding Wood").
These are planning estimates. Check your local building code, product specifications, and a licensed contractor before you buy materials or build.
Frequently asked questions
Why does wood move so much more across the grain than along its length?
Wood's cell structure is aligned along the length of the tree trunk, so moisture-driven swelling and shrinking happens almost entirely perpendicular to those long cells -- across the board's width and thickness -- while the length dimension barely changes at all.
How much moisture content swing should I plan for?
This varies by climate and whether the piece lives in a humidity-controlled interior or a garage, shed, or covered porch. A common design assumption for typical interior US climates is a 4-6 percentage point seasonal swing, but check local humidity data for your specific project location and interior conditions.
Why do species like oak move more than cedar for the same width?
Wood movement is driven by cellular density and structure, which vary by species. Denser, tighter-grained hardwoods like oak and maple generally have higher movement coefficients than lighter softwoods like cedar, meaning they change dimension more for the same moisture content swing.
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