What is specific enthalpy of moist air?

It is the total heat content per kilogram of dry air, measured in kJ/kg or BTU/lb. It includes sensible heat (from temperature) and latent heat (from water vapor). HVAC systems do work by changing the enthalpy of an air stream.

What atmospheric pressure does this calculator assume?

Standard sea-level pressure of 101.325 kPa (14.696 psi). At higher elevations, saturation pressure and humidity ratio differ slightly. For sites above 1,500 m, adjust with a psychrometric correction factor.

Why does humidity matter so much for enthalpy?

Water vapor carries roughly 2,501 kJ/kg of latent heat at 0 °C—about 2,500 times the sensible heat per degree. Small changes in humidity ratio cause large enthalpy swings, which is why latent loads often dominate in humid climates.

How do I use enthalpy to size a cooling coil?

Multiply the mass flow rate of dry air (kg/s) by the enthalpy difference between entering and leaving air conditions (kJ/kg). The result is coil capacity in kW. Convert to tons of refrigeration by dividing kW by 3.517.

What is the humidity ratio (W)?

Humidity ratio (also called mixing ratio or specific humidity) is the mass of water vapor per kilogram of dry air, expressed in kg/kg or g/kg. It determines the latent heat component of enthalpy and is constant during sensible heating or cooling.

Enthalpy of Air Calculator

Enter dry-bulb temperature and relative humidity to get the specific enthalpy of moist air in both SI and imperial units.

Air Conditions

Unit system:

Dry-Bulb Temperature (°C)

Typical range: −40 to 60 °C

Relative Humidity (%)

Specific Enthalpy (SI)

—

kJ / kg dry air

Specific Enthalpy (IP)

—

BTU / lb dry air

Psychrometric Breakdown

Dry-Bulb Temperature	—
Relative Humidity	—
Saturation Pressure	—
Partial Vapor Pressure	—
Humidity Ratio (W)	—
Sensible Heat Component	—
Latent Heat Component	—

Dew Point

—

Wet-Bulb Temperature

—

Summary

Specific enthalpy of moist air is the total heat content per unit mass of dry air, combining sensible heat (dry-bulb temperature) and latent heat (water vapor). It is the key quantity in HVAC load calculations, allowing engineers to determine how much energy a system must add or remove. This calculator uses the standard psychrometric equations from ASHRAE Fundamentals to compute enthalpy at standard atmospheric pressure (101.325 kPa).

How it works

Enter the dry-bulb temperature in °C or °F.
Enter the relative humidity as a percentage (0–100%).
The calculator finds saturation pressure using the Antoine equation.
Humidity ratio W (kg water/kg dry air) is derived from relative humidity and saturation pressure.
Specific enthalpy h = 1.006·T + W·(2501 + 1.86·T) kJ/kg is applied.
Results are displayed in both SI (kJ/kg) and imperial (BTU/lb) units.

Use cases

Size cooling coils by computing enthalpy difference across the coil.
Calculate HVAC sensible and latent heat loads for a space.
Verify psychrometric chart readings for air handling unit design.
Determine energy content of outdoor air for economizer cycle analysis.
Estimate chiller or heat pump capacity requirements.
Compare supply and return air conditions to assess dehumidification duty.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu