What is a psychrometric calculator?

A psychrometric calculator determines all thermodynamic properties of moist air from any two known properties. Given dry-bulb temperature and relative humidity, for example, it calculates wet-bulb temperature, dew point, humidity ratio, enthalpy, specific volume, density, and vapor pressure using ASHRAE equations.

Why does altitude matter for psychrometric calculations?

Psychrometric properties depend on barometric pressure, which decreases with altitude. At 5,000 ft elevation, barometric pressure is about 17% lower than sea level, which changes humidity ratios, specific volumes, and enthalpy values. Using sea-level values at high elevation leads to undersized equipment and incorrect humidity control.

What is the difference between humidity ratio and relative humidity?

Relative humidity (RH) is the ratio of actual water vapor pressure to saturation pressure at that temperature, expressed as a percentage. Humidity ratio (W) is the actual mass of water vapor per unit mass of dry air, measured in grains/lb or lb/lb. RH changes with temperature even if no moisture is added or removed; humidity ratio only changes when moisture is actually added or removed from the air.

What is the sensible heat ratio (SHR)?

The sensible heat ratio is the fraction of total heat change that is sensible (temperature change) versus latent (moisture change). SHR = sensible heat / total heat. For comfort cooling, SHR is typically 0.7-0.8, meaning 70-80% of the cooling load is sensible. A lower SHR indicates more moisture removal is needed, which affects coil selection and airflow design.

Psychrometric Calculator

Calculate all moist air properties from any two inputs using ASHRAE Hyland-Wexler equations. Altitude-corrected. Includes process calculator for heating, cooling, and dehumidification analysis.

Atmospheric Conditions

Altitude / Pressure

Input Properties

Select Two Properties

Dry-Bulb Temperature(°F)

Relative Humidity(%)

Understanding Psychrometric Properties

What Is a Psychrometric Chart?

A psychrometric chart maps the thermodynamic properties of moist air at a given barometric pressure. Given any two independent properties, all other properties are fixed. This calculator replaces chart reading with precise ASHRAE equations, giving you exact values instead of interpolated readings.

Dry-Bulb vs. Wet-Bulb Temperature

Dry-bulb temperature is the standard air temperature measured by a thermometer. Wet-bulb temperature is measured with a wetted wick and reflects the cooling effect of evaporation. The difference between them (wet-bulb depression) indicates how much moisture the air can absorb — critical for cooling tower sizing and evaporative cooling.

Altitude and Barometric Pressure

Psychrometric properties depend on barometric pressure. At higher elevations, lower air pressure changes humidity ratios, specific volumes, and enthalpy values. A 5,000 ft elevation reduces pressure by about 17%. Always use altitude-corrected values for accurate HVAC design above 2,000 ft.

Using the Process Calculator

The process calculator shows what happens when air moves through HVAC equipment. Enter the entering and leaving conditions to see sensible heat (temperature change), latent heat (moisture change), and total heat. The sensible heat ratio (SHR) is critical for coil selection — typical comfort cooling has an SHR of 0.7-0.8.

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