Wood Equilibrium Moisture Content (EMC) Calculator
Find the moisture content wood will settle at for a given temperature and relative humidity (USDA Wood Handbook formula), and whether your boards will shrink or swell compared with a meter reading.
How it works
EMC (%) = 1800 ÷ W × [Kh ÷ (1 − Kh) + (K₁Kh + 2K₁K₂K²h²) ÷ (1 + K₁Kh + K₁K₂K²h²)] · h = RH ÷ 100 · W = 349 + 1.29T + 0.0135T² · K = 0.805 + 0.000736T − 0.00000273T² · K₁ = 6.27 − 0.00938T − 0.000303T² · K₂ = 1.91 + 0.0407T − 0.000293T² (T in °C)
Wood constantly trades moisture with the air until it reaches equilibrium moisture content (EMC), which depends mostly on relative humidity and a little on temperature. The formula is the Hailwood–Horrobin fit by Simpson published in the USDA Forest Products Laboratory Wood Handbook, accurate to within about 1 percentage point for most species. Build furniture from wood dried to the EMC of the room it will live in — typically 6–9% in heated homes and 9–12% in humid climates — or joints will open and panels crack as the wood shrinks or swells.
Worked example
At 20 °C and 50% relative humidity wood settles at about 9.3% moisture. A board reading 12% on a meter is 2.7 points wetter, so it will dry and shrink before it stabilises.
Sources: USDA FPL — Wood Handbook, chapter 4 (moisture relations) · Wikipedia — Equilibrium moisture content
Frequently asked questions
What moisture content should wood be for furniture?
Match the EMC of where it will be used: about 6–8% for heated homes in the dry northern US or Canada, 8–10% for most of Europe and coastal areas, and 11–13% in humid tropical climates.
What is the EMC at 70 °F and 50% RH?
About 9.2%. At 30% RH it drops to about 6.2% and at 80% RH it rises to about 16%.
Does wood species change the EMC?
Only slightly. Most species fall within about 1 point of the formula; some tropical woods and heat-treated or thermally modified wood stay a few points drier.
How long does wood take to reach EMC?
Weeks to months, depending on thickness. A rule of thumb is about a week per 25 mm (1 in) of thickness after a large humidity change, longer for thick or sealed boards.
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This calculator is for educational purposes. Double-check important results before acting on them.