Materials
Timber movement estimate
- where
- movement (mm)
- original width (mm)
- movement coefficient (% of dimension per percentage point MC)
- change in moisture content, final − initial (percentage points)
Explain this equation
Estimates how much a board will shrink or swell across the grain when its moisture content changes.
W is the original width in millimetres. c is the movement coefficient: the percentage change in width per one percentage-point change in moisture content. It depends on the species and on radial or tangential grain. ΔM is the final minus the initial moisture content. ΔW is the change in width; positive means swelling.
Within the normal working range, movement is roughly proportional to the moisture change. So the width changes by c% for every point of ΔM. The /100 turns the percentage into a fraction.
200 mm at 0.25% per point going from 12% to 18% swells about 3 mm, to 203 mm. Doubling the width doubles the movement.
Watch out: this is a straight-line estimate that only applies below fibre saturation. It does not replace a manufacturer's expansion gaps.
Estimate across-grain movement using a supplied species/direction coefficient.
Enter your values to see the result.
Good to know
Linear approximation over a suitable moisture range below fibre saturation. Coefficient depends on species and radial/tangential direction. Does not predict engineered flooring or replace manufacturer movement gaps.
Limits
Timber movement requires the actual species/direction coefficient and a suitable moisture range; the example coefficient is not universal.
Sources
- Product/species coefficient required
This is a calculation aid, not a structural design or compliance certificate. Every default is an example, not a specification for your project.