Wood Moisture Equilibrium Calculator

The Hailwood-Horrobin equation's four constants (W, K, K1, K2) from the Forest Products Laboratory's Wood Handbook are defined as functions of temperature in Celsius, so the calculator converts your Fahrenheit input internally before deriving them and solving for EMC. The result is converted back to the percentages and inches you entered, so you never need to think in Celsius yourself.

No. The Wood Handbook publishes the Hailwood-Horrobin fit twice — once with constants written as functions of degrees Celsius (the set used here) and once with constants written for degrees Fahrenheit directly (W = 330 + 0.452T + 0.00415T², and so on, from Simpson 1973). They describe the same sorption isotherm, and evaluated across this calculator's whole 30-200 °F and 5-99% relative humidity range they agree to within 0.014 percentage points of moisture content. If you see a materially different EMC from another tool, the difference is coming from something other than which of these two constant sets it used.

Both effectiveCurrentMC and effectiveTargetMC are capped at 28% before the dimensional-change math runs, because above roughly 28% MC — the fiber saturation point — wood cell walls are already fully saturated and any additional water just sits as free water in the cell cavities without causing further swelling. Only moisture content changes within the cell walls, below that threshold, actually change the board's dimensions.

dimensionalChange multiplies board width by the shrinkage coefficient (as a fraction) by the effective MC change, and the shrinkage coefficient is species-specific — oak's tangential coefficient (about 0.37) is roughly 68% higher than pine's (about 0.22). So identical MC swings on identically sized boards of different species can produce noticeably different width changes; leaving shrinkageCoeff at 0 falls back to a 0.28 average-hardwood value that won't be exact for any specific species.

No — acclimationDays is a rule-of-thumb figure (roughly 2 days per quarter-inch of board thickness per percent of MC change), not a lab-measured diffusion calculation, and the code itself notes actual air-drying rates can run slower. Treat it as a planning floor and always confirm the wood has actually reached equilibrium with a moisture meter before final glue-up, especially for thick stock or large MC swings.