EMC calculator

What is the equilibrium moisture content of my wood?

Equilibrium moisture content ("EMC") is the moisture content ("MC") that lumber will eventually reach if left in a given environment indefinitely. EMC is determined largely by the relative humidity and temperature of the air surrounding the lumber. Other factors such as circulation and specific gravity will affect drying speed, i.e., time it takes to reach EMC, but will have no or minimal effect on EMC.

EMC calculator

You can look up the local relative humidity and temperature on most weather apps. For example, live relative humidity measurements can be found here.

Please note that indoor conditions may differ from outdoor conditions, e.g., lower relative humidity, based on several factors like air conditioning or heater usage. The EMC will vary accordingly and it is best to measure the relative humidity and temperature directly in the intended environment (e.g., dining room for a dining table) as a result.

Additional details

Hailwood-Horrobin equation

This calculator below uses the Hailwood-Horrobin equation to estimate the EMC of lumber based on relative humidity and temperature.

$M_{\mathrm {eq} }={\frac {1800}{W}}\left[{\frac {kh}{1-kh}}\,+\,{\frac {k_{1}kh+2k_{1}k_{2}k^{2}h^{2}}{1+k_{1}kh+k_{1}k_{2}k^{2}h^{2}}}\right]$

where M_eq is the equilibrium moisture content (percent), T is the temperature (degrees Fahrenheit), h is the relative humidity (fractional) and:

W=330+0.452\,T+0.00415\,T^{2}

k=0.791+4.63\times 10^{-4}\,T-8.44\times 10^{-7}\,T^{2}

k_{1}=6.34+7.75\times 10^{-4}\,T-9.35\times 10^{-5}\,T^{2}

EMC chart

A chart representation of this relationship can be seen below (source).

Dependence of equilibrium moisture content of wood on relative humidity and temperature