Construction & DIY · Woodworking
Wood equilibrium moisture content calculator.
Enter temperature and relative humidity for a dry and a humid season to see the moisture content your wood settles at, and how far it swings.
Inputs
Equilibrium moisture content
Enter a temperature and relative humidity.
What is the equilibrium moisture content of wood?
The USDA Forest Products Laboratory defines equilibrium moisture content (EMC) as the moisture content at which wood is neither gaining nor losing moisture. It depends on the relative humidity and temperature of the surrounding air. This tool uses the Wood Handbook equation that the Handbook publishes for calculating EMC (Equation 4-5, with temperature in degrees Celsius); the same equation produced the Handbook's Table 4-2.
Worked example. At 21.1 C (70 F) the Handbook table gives 6.2% at 30% relative humidity and 12% at 65%. Wood in a heated room in winter and in a humid room in summer therefore swings by about 5.8 percentage points, and that swing is what makes panels move.
How accurate is it? The Handbook says the table values suit practical applications but are not reliable for scientific work. EMC also varies with species and with sorption hysteresis, and species differences are minor at low and moderate humidity but become considerable at high humidity. Treat the result as a planning figure and confirm with a moisture meter.
Frequently asked questions
What is the EMC of wood at 50% relative humidity?
At 21.1 C (70 F) the Wood Handbook table gives 9.2% at 50% relative humidity. Use the tool for other temperatures; the change with temperature is small in the indoor range.
What moisture content should wood be for indoor furniture?
The Wood Handbook says the objective of drying is to bring the moisture content close to the value the finished product will have in service, which is the EMC of the room it will live in. This tool shows that value for your temperature and humidity.
Does the species change the EMC?
Slightly. The Handbook says differences among species are minor at low and moderate relative humidity and larger at high relative humidity, and that species with more extractives tend to have lower EMC at high humidity.
Does a finish stop wood from changing moisture content?
No. The Handbook says coatings such as varnish or lacquer can slow moisture content changes but cannot prevent them.
Sources
Checked
Equation 4-5 and Table 4-2 of the Wood Handbook give the EMC relationship; the page statements on species, hysteresis and coatings come from the same chapter. The tool reproduces Table 4-2 to within about 0.1 percentage point (the table rounds from a Fahrenheit-based calculation). Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Estimates for planning woodwork, not a substitute for measuring the moisture content of your lumber with a meter or by weighing. Not engineering advice.