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Vriksai Timber Intelligence

EMC CalculatorEquilibrium Moisture Content Predictor

Calculate the Equilibrium Moisture Content wood will reach at any temperature and relative humidity. Based on the Hailwood-Horrobin equation used by the USDA Forest Products Laboratory worldwide.

Why this calculation matters

EMC is where the timber ends up in service. It decides the kiln target, the acclimation period, and how much the finished piece will move across a year.

Hailwood-Horrobin ModelTemperature + RH InputCelsius and FahrenheitIndoor/Outdoor PresetsPDF Report
EMC

EMC Calculator

Equilibrium Moisture Content Predictor

Environment Conditions
deg C

Ambient air temperature around the wood.

% RH

Relative humidity of surrounding air (1-99%).

Quick Environment Presets
%

For drying direction analysis.

OK
Equilibrium Moisture Content Results
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% EMC
Equilibrium MC
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direction
Drying Direction
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% MC change
Expected Change
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rating
Use Suitability
ConditionTemperatureRHResulting EMC
Hailwood-Horrobin Calculation
About EMC Calculator
Equilibrium Moisture Content (EMC) is the moisture level at which wood neither gains nor loses moisture when exposed to a constant temperature and relative humidity. Every piece of wood constantly moves toward its EMC. Knowing the target EMC for your climate is the single most important factor in preventing warping, cracking and joint failure. It replaces the printed EMC chart for wood that hangs in most kiln rooms — enter temperature and relative humidity and the equilibrium value appears instantly.
Where Is This Used?
Kiln Drying OperationsFurniture ManufacturingFlooring InstallationMusical InstrumentsWood Workshops IndiaQuality Control Labs
Formulas Used
Hailwood-Horrobin (USDA FPL standard):W = 330 + 0.452xT + 0.00415xT^2 (T in deg F)k = 0.791 + 0.000463xT - 0.000000844xT^2k1 = 6.34 + 0.000775xT - 0.0000935xT^2k2 = 1.09 + 0.0284xT - 0.0000904xT^2EMC = (1800/W) x [kh/(1-kh) + (k1kh + 2k1k2k^2h^2)/(1 + k1kh + k1k2k^2h^2)]where h = RH/100
How the calculation works
Temperature and humidity are fed into a published sorption model that describes how much moisture wood holds at equilibrium. The equation looks intimidating, but it does one thing: it converts the air’s condition into the moisture content the wood will settle at and stay at, given enough time.
Show the formula
Hailwood-Horrobin two-hydrate model, as published in USDA FPL Wood Handbook GTR-190 Ch. 4. With temperature in degrees F and h = RH/100: W = 330 + 0.452T + 0.00415T^2; K = 0.791 + 0.000463T - 0.000000844T^2; K1 = 6.34 + 0.000775T - 0.0000000935T^2; K2 = 1.09 + 0.0284T - 0.0000000904T^2. EMC = (1800/W) x [ Kh/(1-Kh) + (K1Kh + 2K1K2K^2h^2) / (1 + K1Kh + K1K2K^2h^2) ].
The model describes equilibrium, not the time taken to reach it. Species effects on EMC are small and are not modelled.
References and what each one provides
  • Hailwood–Horrobin sorption modelPublished in USDA FPL Wood Handbook, Ch. 4The two-hydrate equation relating temperature and relative humidity to equilibrium moisture content.
  • USDA Wood Handbook (FPL GTR-190)USDA Forest Products LaboratoryThe published Hailwood–Horrobin coefficients and their temperature terms.
Unverified values are marked as such rather than presented as sourced.
Limitations of this calculation
Predicts the equilibrium value for the conditions entered. It does not predict how long equilibrium takes, and does not account for species differences, surface coatings, vapour barriers, or fluctuating rather than steady conditions.
Do not use this for Kiln process sign-off, or acceptance testing of a delivered consignment.
Figures are engineering estimates from the inputs and assumptions shown. Verify against the actual material and, where the result affects structure or safety, against a qualified professional.
Sources & verification
  • Hailwood–Horrobin model
  • USDA FPL Wood Handbook GTR-190, Ch. 4, Table 4-2
Formula checked against the sources above by an automated regression test (tests/test-calculators.js) that derives each expected value independently of this page. Last reviewed .
How to use this tool
  1. Enter your wood and climate details below, then press Calculate.
  2. Pick your apply preset and target use from the dropdown.
  3. Fill in air temperature, relative humidity, current mc. 
  4. Press Calculate. 
Worked example
With the tool's starting values — air temperature = 25, relative humidity = 65, current mc = 12 — pressing Calculate gives: 11.8 % EMC (equilibrium mc); STABLE direction (drying direction); -0.2 % MC change (expected change). 
Common mistakes to avoid
  • Using outdoor humidity for indoor furniture. Indoor RH — especially with AC or heating — is what the wood will see.
  • Reading EMC as the wood's current moisture. EMC is the target it will drift toward, not what it is today.
  • Ignoring seasonal swing. Design for the yearly EMC range, not one day's number.
Frequently Asked Questions
Why is EMC important for woodworking in India?
India has extreme seasonal humidity swings - from 30% RH in summer to 85%+ during monsoon. Wood dried to 12% MC for AC interiors can swell badly during monsoon. The EMC tells you the correct target moisture for each region and season, preventing costly failures.
What EMC should furniture be dried to?
For air-conditioned interiors: 8-10% MC. For non-AC homes in most of India: 10-12% MC. For coastal/monsoon regions: 12-14%. Always match the EMC of the final service environment, not the workshop.
Does EMC depend on wood species?
EMC is primarily a function of temperature and humidity, nearly identical across species. However, the rate at which wood reaches EMC, and how much it moves dimensionally, varies greatly by species - which is why this tool lets you select a reference species.
ResourcesView the formulasWood Species DatabaseFormula Library
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Full listing in the moisture content reference.