Wood EMC Calculator
Equilibrium moisture content from temperature and RH (Hailwood–Horrobin, Simpson 1973).
Enter the air temperature (°C) and relative humidity (0–100%). The calculator returns the EMC on the desorption branch (drying) using Simpson's four-constant form of the Hailwood–Horrobin sorption equation, matched to the Wood Handbook 2021 Table 4-2. Optional reverse mode: enter a target MC and the tool returns the RH you need at the given temperature (useful for kiln operators: 'what RH at 60 °C hits 8% MC?').
FAQ
What is EMC and why does it matter?
EMC (equilibrium moisture content) is the moisture content wood will eventually reach when stored in a given climate. If you build furniture at 12% MC and ship it into a 9% EMC room, the wood loses another 3 pp and may shrink/crack; into a 15% EMC room it swells. Match the build MC to the in-service EMC, not to a fixed number.
Why does desorption sit above adsorption?
Sorption hysteresis: at the same RH, wood that is drying holds more water than wood that is absorbing water from a dry state. The difference is 1–2 percentage points MC across the practical range. For kiln schedules you should use the desorption curve (the high one).
Is the formula accurate at high temperature?
The Simpson 1973 constants are fitted to the −20 °C to +95 °C range; above 100 °C EMC collapses to very small values and the math is less accurate. For high-temperature kiln schedules above 100 °C, treat the answer as a guide and verify against a moisture meter.