Enter the board’s cross-grain width in metres at its initial moisture content. For example, 300 mm is 0.300 m. Enter moisture contents as percentage-point numbers: enter 8 for 8%, not 0.08. These inputs use plain numeric values so their difference is directly in percentage points.
Supply a coefficient C in fractional width change per one percentage-point change in moisture content, equivalently m/m per percentage point. A hypothetical C of 0.0025 means 0.25% width change per moisture-content percentage point. If your source gives 0.25% per point, divide by 100 before entering it. No species coefficient is preselected; obtain one appropriate to your wood and measurement direction. Do not enter total green-to-oven-dry shrinkage as C.
A positive result indicates expansion; a negative result indicates shrinkage. Multiply a result in metres by 1,000 to read millimetres.
For expansion planning, use the installation moisture content as the initial value and the expected wettest service condition as the target. A positive width change is the estimated additional width that the assembly must accommodate. A negative result is not a negative clearance: rerun with the wettest expected target to assess expansion.
For a freely moving panel fixed at its centre, half the predicted expansion reaches each edge. With one edge fixed, the full change reaches the opposite edge. These are geometric interpretations of the calculated width change, not a complete joint design. Add any construction tolerance separately; none is included in the result.
| Case | Initial width | Moisture change | Entered C | Signed width change | Final width |
|---|---|---|---|---|---|
| Expansion | 0.300 m | 8% to 12%: +4 points | 0.0025 per point | 0.300 × 0.0025 × 4 = +0.003000 m (3 mm) | 0.303000 m |
| Shrinkage | 0.300 m | 12% to 8%: −4 points | 0.0025 per point | 0.300 × 0.0025 × (−4) = −0.003000 m (−3 mm) | 0.297000 m |
| No moisture change | 0.300 m | 10% to 10%: 0 points | 0.0025 per point | 0.000000 m | 0.300000 m |
Use a tangential coefficient for movement tangent to growth rings and a radial coefficient for movement perpendicular to them. Flatsawn width is approximately tangential; quartersawn width is approximately radial. This calculator does not estimate movement along the grain.
The USDA Wood Handbook, Chapter 13 describes this coefficient approximation for moisture contents from 6% to 14%; both moisture inputs are restricted to that interval. It is an estimate for solid wood, not a prediction of warping, restraint stresses or movement in plywood and other composites.
Width is limited to 0.001–10 m and C to 0–0.01 per percentage point as calculator input bounds, not material-property guarantees. C = 0 gives zero movement by assumption. Values outside the bounds need a separate assessment; do not clamp them to obtain an answer. The calculator does not derive moisture content from air humidity or select coefficients. Displayed decimal places do not indicate measurement accuracy. An unavailable result is not zero.