Wood Movement Calculator
Predict how much a wood board will expand or contract in width when relative humidity changes, helping woodworkers plan for seasonal movement and avoid joint failure.
Last updated: September 2026
Formula below · 2 sources (fpl.fs.usda.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Wood moves across the grain as its moisture content (MC) follows the surrounding humidity. The calculator first converts each relative humidity to the wood's equilibrium moisture content (EMC) at 70 °F using the Hailwood–Horrobin equation from the USDA Forest Products Laboratory Wood Handbook (for example 30% RH ≈ 6.2% EMC, 50% RH ≈ 9.2%, 70% RH ≈ 13.1%). The width change is then: Movement = boardWidth × DCC × grainFactor × (EMC_final − EMC_initial), where DCC is the species' tangential dimensional change coefficient per 1% MC (oak 0.00369, hard maple 0.00353, cherry 0.00248, walnut 0.00274, eastern white pine 0.00212) and the grain factor is 1.0 for flat-sawn (tangential) and 0.55 for quarter-sawn (radial) boards. A positive result is swelling, a negative result shrinkage. Real boards take weeks to reach a new EMC, so short humidity spikes move them less than this full-equilibrium figure.
How to use
1. Enter Board Width: 10 inches. 2. Initial Relative Humidity: 30% (winter interior, EMC ≈ 6.2%). 3. Final Relative Humidity: 70% (humid summer, EMC ≈ 13.1%). 4. Wood Species: Oak (0.00369). 5. Grain Orientation: Tangential (flat sawn, 1.0). 6. Movement = 10 × 0.00369 × 1.0 × (13.09 − 6.17) ≈ 0.26 inches. This 10-inch flat-sawn oak board widens about 1/4 inch — enough to crack a panel glued rigidly in a frame. Quarter-sawn (0.55) it would move about 0.14 inches, and in cherry (0.00248) flat-sawn about 0.17 inches.
Frequently asked questions
How much should I account for wood movement in a tabletop design?
A typical 24-inch wide solid hardwood tabletop can move about 1/4 to 3/4 inch across its width between dry winter and humid summer conditions. Designers use figure-8 clips, wooden buttons, or slotted brackets to attach the top to the base, allowing this movement to occur without stressing joints. Never glue a wide solid panel to a rigid base. The exact allowance depends on wood species, grain orientation, and the specific humidity range in your region.
What is the difference between radial and tangential wood shrinkage?
Radial shrinkage occurs perpendicular to the growth rings (quartersawn boards), while tangential shrinkage occurs parallel to the growth rings (flatsawn boards). Tangential shrinkage is typically 1.5 to 2 times greater than radial shrinkage, which is why quartersawn lumber is prized for stability in instrument soundboards, flooring, and fine furniture. Flatsawn boards are less expensive and show more figure, but they move more and are more prone to cupping. Choosing the right grain orientation for your application is as important as choosing the right species.
Why does wood expand and contract with humidity changes?
Wood is hygroscopic, meaning its cell walls absorb and release water vapor to reach equilibrium with surrounding air. Below the fiber saturation point (around 28–30% MC), changes in moisture content cause the cell walls to swell or shrink, which translates directly into dimensional change. Above that point, free water fills cell cavities without causing movement. This is why properly dried lumber still moves seasonally — it continues to respond to ambient humidity even after kiln drying. Climate-controlled workshops and proper acclimation of lumber before milling reduce but cannot eliminate this natural behavior.