Equilibrium Moisture Content Calculator

How it works

Wood exchanges moisture with the air around it until it neither gains nor loses water. The moisture content at that balance is the equilibrium moisture content (EMC), and it depends mainly on the relative humidity and, to a lesser degree, on temperature.

The calculator uses the equation the USDA Forest Products Laboratory uses to generate its EMC table: the Hailwood–Horrobin sorption equation with Simpson's coefficients (Wood Handbook FPL-GTR-282, 2021, Eq. 4-5 and Table 4-2). At 70 °F it gives 6.2% at 30% relative humidity, 9.2% at 50% and 16.0% at 80%, matching the table.

Enter a moisture meter reading to compare a board with the space it will live in. A board wetter than the EMC tends to dry and shrink there; a drier board tends to take up moisture and swell. The difference is rounded to 0.1 point, as shown, before it is compared with the band. The size of the band that counts as near equilibrium is an input; the Wood Handbook's interior recommendation for most of the US is an 8% average with individual pieces from 6% to 10% (Table 13-2), a band of ±2 points.

Formula

h  = relative humidity ÷ 100;  T in °F
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
EMC (%) = 1800/W × [Kh/(1 − Kh) + (K1Kh + 2K1K2K²h²)/(1 + K1Kh + K1K2K²h²)]
difference = reading − EMC, rounded to 0.1
near if |difference| ≤ tolerance

Example

At 70 °F and 45% relative humidity the EMC is 8.5%. A board reading 11% is 2.5 points above it, outside a ±2 point band, so it is wetter than EMC and will tend to lose moisture and shrink in that room.

At 70 °F and 50% relative humidity the equation gives 9.2%, the value in Wood Handbook Table 4-2.

Assumptions and limitations

• EMC values are averages for wood in general. The Wood Handbook table was derived mainly for Sitka spruce, and species differ, especially at high humidity. Its 0.1% precision only illustrates trends.
• Sorption hysteresis is ignored: wood drying down settles slightly wetter than wood taking up moisture at the same humidity. The tabulated values sit between the two.
• Temperature is limited to 30–210 °F (about −1.1 to 98.9 °C) and relative humidity to 0–95%, the range of Table 4-2 used here.
• Hot-pressed panels such as plywood, particleboard and hardboard reach a lower moisture content than solid wood at the same humidity.
• Moisture meter readings depend on the meter, species setting, temperature and depth of measurement. The comparison is only as good as the reading entered.
• The ±2 point default is a typical figure from Wood Handbook Table 13-2; flooring, cabinet and furniture makers may publish their own acceptance limits, and those take precedence over this estimate.

Frequently asked questions