Equilibrium moisture content of wood, from temperature and humidity
Type the air temperature and relative humidity and get the moisture content wood settles at, with the USDA Wood Handbook equation. Compare two seasons to see how much your wood will swing.
Every book I read says to let wood acclimate, but none told me to what number. Wood ends up at a moisture content set by the air around it. Type the conditions of your room, or of two seasons, and see where it lands.
Quick table at 21 °C (70 °F)
| Relative humidity | Wood moisture content |
|---|---|
| 20% | 4.5% |
| 30% | 6.2% |
| 40% | 7.7% |
| 50% | 9.2% |
| 60% | 11% |
| 70% | 13.1% |
| 80% | 16% |
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How it works
Wood takes moisture from the air or gives it back until it stops changing. That point is the equilibrium moisture content (EMC), and it depends on relative humidity and temperature.1
This page uses the equation the Wood Handbook uses to build its EMC table: M = 1800/W × [Kh/(1 − Kh) + (K₁Kh + 2K₁K₂K²h²)/(1 + K₁Kh + K₁K₂K²h²)], where h is relative humidity as a decimal and W, K, K₁ and K₂ are polynomials in temperature printed in the chapter.1 I checked the results against the handbook's Table 4–2: 21.1 °C (70 °F) and 50% gives 9.2% in the table and 9.25% here.
The handbook says the values can be applied to wood of any species for most practical purposes. They were derived mostly from Sitka spruce, in a condition midway between drying and wetting, so wood that is drying ends a bit higher and wood that is taking up water a bit lower.1
Common questions
What moisture content will wood reach indoors?
It depends on your room. At 21 °C (70 °F), 30% relative humidity gives about 6%, and 50% gives about 9%, according to the Wood Handbook table. Measure your room with a hygrometer and type it in.
Does the species change the result?
Only a little. The handbook says the table can be used for any species for most practical purposes.
Where this comes from
- Samuel V. Glass, Samuel L. Zelinka (USDA Forest Products Laboratory). Moisture relations and physical properties of wood. Wood handbook, chapter 4 (FPL-GTR-190, 2010). US government technical handbook. Samuel V. Glass, Samuel L. Zelinka (USDA Forest Products Laboratory) Consulted 1 Nov 2026.