How to Calculate Enthalpy from Temperature and Relative Humidity

Your HVAC contractor quotes you a cooling system that needs to handle a 45 kJ/kg load. Your engineer specifies an air handling unit rated for a 12 kJ/kg enthalpy difference. Your energy model shows an enthalpy of 67 kJ/kg on a summer design day. If you do not know what enthalpy means or how to calculate it from temperature and relative humidity, you cannot check if any of those numbers are right.

Enthalpy is not complicated once you understand what it is counting. And our calculator gives you the answer instantly:

What Is Enthalpy of Moist Air?

Enthalpy (symbol h, unit kJ/kg) is the total heat energy content of a kilogram of moist air. It has two components:

1. Sensible heat — the energy needed to raise the temperature of the air itself
2. Latent heat — the energy stored in the water vapor suspended in the air

The latent heat component is the reason that removing moisture (dehumidification) requires energy even when you are not changing the air temperature. Condensing water vapor out of air releases 2,501 kJ per kilogram of water — a massive energy flux that dominates HVAC system design in humid climates.

h = 1.006 × T + W × (2501 + 1.86 × T)
Where:
h = enthalpy of moist air [kJ per kg of dry air]
T = dry-bulb temperature [°C]
W = humidity ratio (mixing ratio) [kg water / kg dry air]
First, calculate humidity ratio:
e_s(T) = 6.112 × exp(17.625 × T / (243.04 + T))   [hPa]
e = e_s(T) × (RH / 100)
W = 0.62198 × e / (1013.25 − e)   [kg/kg — note: NOT g/kg here]

The constants in the formula are:

• 1.006 — specific heat of dry air at constant pressure (kJ/kg·°C)
• 2501 — latent heat of vaporization of water at 0°C (kJ/kg)
• 1.86 — specific heat of water vapor (kJ/kg·°C)

Step-by-Step Enthalpy Calculation

Given: Air temperature = 30°C, Relative humidity = 60%

Step 1 — Saturation vapor pressure:

e_s(30) = 6.112 × exp(17.625 × 30 / 273.04)
        = 6.112 × exp(1.937)
        ≈ 6.112 × 6.933
        ≈ 42.36 hPa

Step 2 — Actual vapor pressure:

e = 42.36 × 0.60 = 25.42 hPa

Step 3 — Humidity ratio (in kg/kg for the formula):

W = 0.62198 × 25.42 / (1013.25 − 25.42)
  = 15.81 / 987.83
  ≈ 0.01600 kg/kg  (= 16.0 g/kg)

Step 4 — Enthalpy:

h = 1.006 × 30 + 0.01600 × (2501 + 1.86 × 30)
  = 30.18 + 0.01600 × (2501 + 55.8)
  = 30.18 + 0.01600 × 2556.8
  = 30.18 + 40.91
  = 71.09 kJ/kg

At 30°C and 60% RH: Enthalpy ≈ 71.1 kJ/kg

Notice the split: 30.18 kJ/kg is sensible heat (temperature), and 40.91 kJ/kg is latent heat (moisture). The moisture contributes 57% of the total energy content. This is why dehumidification is so energy-intensive in warm, humid climates.

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Enthalpy Reference Table

Pre-calculated enthalpy values (kJ/kg dry air) at sea level:

Note: 30°C / 60% RH matches our worked example above: 71.1 kJ/kg

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What Is Grains Per Pound (GPP) and How to Calculate It

Grains per pound (GPP) is the US HVAC industry's traditional unit for humidity ratio — common in residential and commercial dehumidifier sizing, moisture damage assessments, and duct system design.

GPP = Mixing Ratio (g/kg) × 7
Because: 1 pound = 453.6 grams, 1 grain = 0.0648 grams
So 1 g/kg = 453.6/0.0648 ÷ 1000 ≈ 7.0 grains/pound

Using our 30°C / 60% RH example where mixing ratio = 16.0 g/kg:

GPP = 16.0 × 7 = 112 grains per pound

GPP Reference Table

*Note: 30°C / 60% RH GPP = 112, matching our formula above

Typical indoor target: 50–70 GPP (roughly 40–50% RH at 72°F/22°C). Above 90 GPP, mold spore activation risk becomes significant. Above 110 GPP, dust mite reproduction accelerates.

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Why Enthalpy Matters for HVAC Design

Cooling load calculation: The cooling capacity of an air handler is determined by the mass flow rate of air multiplied by the enthalpy difference between supply and return air: Cooling Load (kW) = ṁ × (h_return − h_supply). Without enthalpy, you cannot correctly size the system.

Economizer control: Many commercial HVAC systems use enthalpy-based economizer controls. Instead of opening outdoor air dampers whenever the outdoor temperature is below a setpoint, they open when the outdoor enthalpy is below the return air enthalpy. This avoids the trap of bringing in cool but very humid outdoor air that would actually increase the building's dehumidification load.

Energy benchmarking: Building energy models like EnergyPlus and eQUEST use hourly enthalpy data from weather files (TMY3/TMY4) to calculate annual heating and cooling energy. The enthalpy values in those files are derived from temperature and relative humidity using exactly this formula.

For a visual tool that shows all these properties simultaneously, see our psychrometric chart guide.

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Frequently Asked Questions

What is a typical enthalpy value for comfortable indoor air? At 22°C and 50% RH, moist air enthalpy is approximately 43.5 kJ/kg — a common indoor comfort target. In contrast, a hot humid summer day at 35°C / 70% RH has an enthalpy of 113.5 kJ/kg, meaning an HVAC system must remove about 70 kJ/kg of energy to bring outdoor air to comfort conditions.

Is enthalpy the same as the "feels like" temperature? No. Heat index ("feels like") is a perception index calibrated to human thermal comfort. Enthalpy is an absolute physical measurement of total heat energy per unit mass of air. They correlate — high enthalpy air always feels hotter — but they are measured differently and used for different purposes.

How do I calculate enthalpy in BTU/lb instead of kJ/kg? Multiply kJ/kg by 0.4299 to convert to BTU/lb. For the traditional US formula: h = 0.240 × T_dry + W × (1061 + 0.444 × T_dry) where temperature is in °F and W is in lb/lb (or lb moisture per lb dry air).

Why does enthalpy increase so much faster at high temperatures? Because saturation vapor pressure increases exponentially with temperature (the Magnus formula). At higher temperatures, the same relative humidity represents a far greater absolute quantity of water vapor — and each gram of that vapor carries 2,501 joules of latent heat. The latent heat term dominates at warm, humid conditions, which is why tropical climates have disproportionately high cooling energy demands.

What is enthalpy used for in dehumidifier sizing? Residential dehumidifiers are rated in pints per day of water removal. The enthalpy of the air entering the dehumidifier versus the air leaving determines the energy removed as latent heat. More precisely, the water removal rate equals (W_entering − W_leaving) × airflow_rate. Our calculator outputs humidity ratio (g/kg) directly — compare inlet and outlet values to size dehumidification requirements. For detailed sizing guidance, see our guide to ideal indoor humidity.