Wood Movement Calculator
How much a board changes width as its moisture content changes, with the gap to allow.
Results
What this tool does
Wood never stops moving: it takes up and gives off moisture until it matches the air around it, and indoor air swings between about 12 percent equilibrium in a damp winter and 6 in a heated one. A negative result is shrinkage, positive is swelling. The last result adds half again as a working allowance, because panels that fit exactly in summer split in January.
Formula
Δ size = size × coefficient × (final humidity − initial)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
dm | Dimension across the grain | mm |
m1 | Initial moisture content | % |
m2 | Final moisture content | % |
cf | Shrinkage coefficient | /% |
DD | Dimension change | mm |
ND | Dimension across the grain | mm |
PC | Percentage | % |
GP | Dimension change | mm |
Worked example
- Dimension across the grain200 mm
- Initial moisture content12 %
- Final moisture content8 %
- Shrinkage coefficient0.00365 /%
- Dimension change-2.9200 mm
- Dimension across the grain197.080 mm
- Percentage-1.4600 %
- Dimension change4.380 mm
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
- Density changes with temperature, moisture and composition. Use a value from the supplier's data sheet whenever precision matters.
Frequently asked questions
Which coefficient should I use?
It depends on the species and, just as much, on the direction. Wood moves roughly twice as much tangentially — across the growth rings, which is the face of a flatsawn board — as it does radially, and almost nothing along the grain. Published tangential coefficients run around 0.0036 for oak, 0.0027 for pine and 0.0040 for beech, with the radial figure roughly half. A wide flatsawn tabletop in a heated room in winter can move several millimetres, which is why tabletops are never glued rigidly to their frames.