Wood Movement Allowance Calculator
Solid wood expands and contracts across its width as humidity changes, and a design that ignores this will cup, crack, or break joints over the seasons. The movement equals the panel width multiplied by the species movement coefficient per percent of moisture change multiplied by the expected change in moisture content. This calculator returns that movement so you can size expansion gaps, float panels, and detail breadboard ends. Enter the coefficient for your species and grain from the Wood Handbook.
Wood movement formula
movement = width * coefficient * moisture content change
gap each side = movement / 2
The coefficient is the fractional dimensional change per one percent of moisture change for your species and grain. Splitting the total movement in half gives the room to leave on each side of a floating panel.
Worked example
A 24 inch wide panel with a coefficient of 0.0025 per percent MC, across a 6 percent seasonal moisture swing: movement = 24 * 0.0025 * 6 = 0.36 inches. Allowing half each side gives a 0.18 inch gap per side in a frame so the panel can float without splitting.
Wood movement allowance: frequently asked questions
How much does wood move?
Wood moves mainly across the grain (in width) as its moisture content changes, while length stays nearly constant. The amount depends on the species and whether the board is flatsawn (tangential) or quartersawn (radial). Movement equals the panel width times the shrinkage coefficient per percent moisture change times the change in moisture content.
Where do I get the shrinkage coefficient?
The USDA Forest Products Laboratory Wood Handbook publishes tangential and radial shrinkage values for many species, given as total percent shrinkage from green to oven-dry. Dividing the total by about 30 (the fibre saturation point in percent) gives an approximate movement per percent of moisture change, which you enter here.
Why leave a gap for wood movement?
If a wide panel is fixed rigidly at both edges and then expands with humidity, it can cup, crack, or push joints apart. Designs like frame-and-panel float the panel so it can move, and breadboard ends and tabletop fasteners allow expansion. Knowing the expected movement tells you how much room to leave.
Does grain orientation matter?
Yes. Tangential (flatsawn) movement is roughly double radial (quartersawn) movement for most species. Quartersawn boards are more dimensionally stable. Use the coefficient that matches your board's grain orientation for an accurate allowance.
Official sources
- USDA Forest Service Forest Products Laboratory: Wood Handbook and wood shrinkage research, covering shrinkage and dimensional change.
- USDA Forest Service Forest Products Laboratory: Wood shrinkage and moisture research. Enter the species coefficient from the handbook.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.