Estimate Wood Movement

This calculator helps you estimate the dimensional change in wood, either shrinkage or expansion, based on changes in relative humidity or moisture content.

There are many factors that will affect the dimensional change to a specific piece of wood. Your actual results will be close but somewhat different from the projected results.

For additional information see the Wood Movement and Equilibrium Moisture Content.

INSTRUCTIONS

1. Enter the wood species, (it's green to ovendry shrinkage data will be automatically presented.
2. Select the calculation basis as either Relative Humidity OR Moisture Content.
3. Enter the settings and initial dimension.
4. Press the Compute button

Some Handy Numbers:

Dimensions: The calculator assumes no specific dimensional units such as inches or centimeters - if your input dimensions represent inches, the output dimensions will also represent inches. Dimensions with a fractional component may be specified as a standard fraction or as a decimal value. Example: 24 3/4 or 24.75.

Moisture content: The moisture content of wood is affected by the temperature and relative humidity of the environment in which the wood is stored. For unfinished, kiln-dried wood stored indoors, its moisture content will fluctuate seasonally from about 4% to 14% for most parts of North America. Valid range: 0 to 40.

Relative Humidity: The relative humidity option is useful if you don't have a moisture meter to measure the wood's moisture content directly but you are able to measure the relative humidity of your shop. The wood's moisture content will be calculated using the specified relative humidity and an assumed shop temperature of 70 degrees Fahrenheit. Your wood must have reached equilibrium at this humidity for the calculator to work properly. Valid range: 0 to 100.

Fiber saturation point (FSP): This is the moisture content at which the wood's cell cavities are emptied of free water but the cell walls are still fully saturated. As the wood dries and its moisture content drops below the FSP, it will shrink and increase in strength. The FSP differs for each species, ranging from about 22% to 35%, with an average of around 28%. Valid range: 20 to 40.

Shrinkage green to ovendry:  laboratory tests have been conducted for most commercial woods to measure how much the wood shrinks from the green to the ovendry condition. In this context, "green" refers to wood at or above the fiber saturation point. Shrinkage is greatest in the tangential direction (along the growth rings), ranging from about 6% to 12% with an average of 8%. Shrinkage in the radial direction (perpendicular to the growth rings) ranges from about 3% to 5% with an average of 4%. Shrinkage in the longitudinal direction is minimal, only about 0.1 %, and can generally be ignored for most furniture applications.

Keep in mind that the green to ovendry shrinkage values are maximum shrinkage values; kiln-dried wood that is stored indoors will only shrink perhaps one-fourth to one-half of the maximum amount.

References

Forest Products Laboratory. 1999. Wood Handbook - Wood as an Engineering Material. Gen. Tech. Rep. FPL-GTR-113.  U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI.

Understanding Wood: A craftsman's Guide to Wood Technology. 1980. R. Bruce Hoadley.  This is an excellent resource for learning about wood movement, how to predict it, and how to measure moisture content using your oven. It also contains maps of regional and local EMC.