Project notes
Understand this estimate
Equilibrium moisture content estimates the moisture level wood may approach after extended exposure to stable temperature and relative humidity.
How to use the calculator
Enter air temperature within the handbook range
The implemented USDA equation is limited to 30–120°F.
Inputs Air temperature
Enter relative humidity
Use a percentage from 0 to 100. Zero humidity produces a zero estimate in the implemented equation.
Inputs Relative humidity
Formula and variables
USDA Wood Handbook Eq. 4–5, Fahrenheit form: W=330+0.452T+0.00415T²; K=0.791+0.000463T−0.000000844T²; K1=6.34+0.000775T−0.0000935T²; K2=1.09+0.0284T−0.0000904T²; h=RH/100; EMC=1800/W × [Kh/(1−Kh) + (K1Kh+2K1K2K²h²)/(1+K1Kh+K1K2K²h²)].
- TAir temperature
- Fahrenheit input used by the USDA equation form.°F
- RHRelative humidity
- Ambient equilibrium condition.%
- EMCEquilibrium moisture content
- Estimated moisture content approached under stable conditions.%
What the result means
Result interpretation
The output is an equilibrium estimate under the specified conditions. It is not a wood moisture-meter reading, drying-time estimate or finish-readiness threshold.
Limits and assumptions
This is an equilibrium estimate within the handbook table’s 30–120°F range. Wood must remain under stable conditions long enough to approach equilibrium; this is not a current moisture-meter reading, drying-time prediction or finish-readiness rule.
Worked example
Estimate EMC at 70°F and 35% RH
Assumptions
- Temperature is within the USDA equation’s stated range.
- Conditions are assumed stable long enough for equilibrium.
Calculation
- Using the USDA Wood Handbook Fahrenheit form of Equation 4–5 gives 6.9449% EMC.
| Estimated equilibrium moisture content | 6.945 % |
|---|
Common questions
- Will wood reach this value immediately?
- No. It approaches equilibrium over time, and the rate depends on the material and conditions.
- Is EMC the same as a current meter reading?
- No. It is a model estimate, not a direct measurement of a wood sample.