Calculator
Wood Movement Calculator
Estimate seasonal expansion and contraction across the grain from sourced species shrinkage data, board width, grain orientation, and a moisture-content range.
Wood movement calculator
Estimate how far a solid-wood board expands or contracts across its width as moisture content changes. Movement along the grain is comparatively small and is not modelled here.
Result
Enter a board width, species, grain orientation and moisture-content range, then calculate the estimated seasonal movement.
These results are estimates only. Verify measurements, material specifications, structural requirements, safety requirements, and local building rules before buying materials or building.
How this calculator works
Solid wood exchanges moisture with the surrounding air and changes dimension as its moisture content changes. This calculator estimates that change across the grain, which is the direction that matters for tabletops, panels, drawer fronts and fitted doors. Movement along the grain is comparatively small and is not modelled.
Species presets use radial and tangential shrinkage values published by the USDA Forest Products Laboratory in Wood Handbook Chapter 4, Table 4-3. Those values describe total shrinkage from green to ovendry, so the calculator converts them into a change per one percentage point of moisture content.
Choose tangential for flat-sawn or plain-sawn stock and radial for quarter-sawn stock. Tangential movement is generally the larger of the two, which is why quarter-sawn boards are valued where width stability matters.
Formula
- Dimensional change coefficient = total shrinkage percent / 100 / 30.
- Dimensional change = initial dimension x coefficient x (final moisture percent - initial moisture percent).
- Final dimension = initial dimension + dimensional change.
- Moisture inputs are clamped to the 0 to 30 percent range because wood does not continue to swell above the fibre saturation point.
Worked example
A 900 mm wide flat-sawn white oak tabletop moving from 6% to 12% moisture content uses the tangential value of 10.5%.
The coefficient is 10.5 / 100 / 30 = 0.0035 per 1% moisture content.
Estimated expansion is 900 x 0.0035 x 6 = 18.9 mm across the width, so the attachment method has to allow roughly 19 mm of movement.
The same board in quarter-sawn stock uses the radial value of 5.6%: 900 x (5.6 / 100 / 30) x 6 = 10.08 mm.
Common mistakes
- Entering board length instead of the dimension that crosses the grain.
- Using a tangential value for quarter-sawn stock, which overstates the expected movement.
- Treating the estimate as an exact prediction rather than a clearance to design for.
- Assuming a film finish or kiln drying removes the need for a movement allowance.
- Applying a solid-wood coefficient to plywood or MDF, which are far more stable across their faces.
FAQs
Which dimension should I enter?
The one that crosses the grain — panel width, tabletop width, drawer-front height. Length along the grain changes very little and is not modelled by this calculator.
Where do the shrinkage percentages come from?
USDA Forest Products Laboratory Wood Handbook Chapter 4, Table 4-3, which publishes radial and tangential shrinkage from green to ovendry for domestic species.
Why is the result clamped at 30 percent moisture content?
Below roughly 30% moisture content — the fibre saturation point — wood changes dimension as it gains or loses water. Above it, additional water sits in the cell cavities and the board does not keep swelling.
How accurate is the estimate?
It is a planning figure. Individual boards vary, grain is rarely perfectly oriented, and indoor humidity is not a single fixed value. Use the result to size a clearance or choose a fastening method, not to predict a width to the thousandth of an inch.
Calculator disclaimer
These results are estimates only. Verify measurements, material specifications, structural requirements, safety requirements, and local building rules before buying materials or building.