Enthalpy Calculator

Calculate moist air enthalpy from dry bulb and humidity ratio or RH. Compare two air states to find the total heat transfer for coil sizing.

⚡ Air State 1

⚡ Air State 2 (optional — for ΔH)

📊 Results

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State 1 Enthalpy (BTU/lb)
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State 2 Enthalpy (BTU/lb)
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ΔH (BTU/lb)
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State 1 W (gr/lb)
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State 2 W (gr/lb)
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ΔW (gr/lb)
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About This Calculator

This enthalpy calculator returns moist-air enthalpy and humidity ratio from dry-bulb temperature and relative humidity. Enter a second air state to read the enthalpy difference (ΔH) and humidity-ratio difference (ΔW) between the two points — the total-heat and moisture change a coil or process produces.

Enthalpy is the single number that captures both sensible and latent heat, so it is the basis for cooling- and heating-coil load, energy-recovery effectiveness, and economizer enthalpy changeover. The tool also shows the worked formula for each state so you can trace exactly how the result was obtained.

Formula & Method

Enthalpyh = 0.240 · T + W · (1061 + 0.444 · T)  [BTU/lb dry air]
Saturation pressurePws = 0.6112 · exp(17.67 · T ÷ (T + 243.5)) kPa (T in °C)
Humidity ratioW = 0.622 · Pw ÷ (P − Pw)
DifferenceΔH = h1 − h2,   ΔW = W1 − W2

Enthalpy uses the standard IP moist-air relation h = 0.240 · T + W · (1061 + 0.444 · T), where 0.240 BTU/lb·°F is the specific heat of dry air and 1061 + 0.444 · T is the enthalpy of the water vapor. Humidity ratio comes from W = 0.622 · Pw ÷ (P − Pw) with vapor pressure Pw = RH · Pws and a Magnus-type saturation pressure, at standard sea-level pressure (P ≈ 101.325 kPa, 14.696 psia). These relations follow ASHRAE Fundamentals Chapter 1 psychrometrics.

Frequently Asked Questions

What is moist-air enthalpy?
Enthalpy is the total heat content of the air per pound of dry air, combining the sensible heat of the air and the latent heat of the water vapor it carries. It is the quantity that stays constant during an ideal adiabatic process such as evaporative cooling, which is why constant-enthalpy lines run diagonally across the psychrometric chart.
How is enthalpy calculated here?
The tool uses the standard IP relation h = 0.240 times T plus W times (1061 plus 0.444 times T), where T is dry bulb in degrees Fahrenheit and W is humidity ratio in pounds of water per pound of dry air. The 0.240 term is the sensible heat of dry air, and 1061 plus 0.444 times T is the enthalpy of the water vapor. This follows ASHRAE Fundamentals Chapter 1.
Why compare two air states with delta H?
The total heat a coil must add or remove is proportional to the enthalpy change between entering and leaving air. Delta H captures both sensible and latent heat in one number, so multiplying it by mass airflow gives total coil load. Entering a second state lets you read the enthalpy difference directly for quick coil-load and energy-recovery checks.
How do I turn delta H into a coil load in BTU/h?
Total load in BTU per hour is approximately 4.5 times CFM times delta H, where 4.5 equals 60 minutes per hour divided by the roughly 13.33 cubic feet per pound specific volume of standard air. This calculator gives you delta H in BTU per pound of dry air; combine it with your airflow to size the coil. At altitude, adjust the 4.5 factor for the lower air density.

Related Tools

Psychrometric Calculator Humidity Ratio Wet Bulb / RH Dew Point Calculator Coil Leaving Air
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Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.