Psychrometric Calculator
Calculate moist air psychrometric properties: vapor pressure, humidity ratio, dew point, enthalpy, and specific volume from temperature and relative humidity.
About This Calculator
The Psychrometric Calculator computes the thermodynamic properties of moist air using ASHRAE standard formulas. It is an essential tool for HVAC engineers, architects, environmental scientists, and building services professionals who need to analyze air conditioning systems, industrial drying processes, and indoor air quality.
The calculator uses the Magnus formula for saturation vapor pressure (Ps = 611.2 × exp(17.67 × T / (T + 243.5)) Pa), from which it derives partial vapor pressure (Pv = RH × Ps / 100), humidity ratio (ω = 0.6219 × Pv / (Pa − Pv)), dew point temperature (Tdp = 243.5 × ln(Pv/611.2) / (17.67 − ln(Pv/611.2))), enthalpy (h = 1.006 × Tdb + ω × (2499.86 + 1.84 × Tdb) kJ/kg), and specific volume (v). Atmospheric pressure is corrected for altitude using the barometric formula.
Regional Notes
India (IN): Tropical climate with high humidity — typical summer dry bulb 35-45 °C with 60-90% RH in coastal regions. ASHRAE standard for Indian HVAC uses 35 °C DB / 28 °C WB for Mumbai and 43 °C DB / 24 °C WB for Delhi summer design conditions. NBCC and ECBC reference psychrometric charts for building energy compliance.
United States (US): ASHRAE Handbook of Fundamentals provides psychrometric charts for altitudes from sea level to 3,000 m. Typical summer design: 35 °C DB / 26 °C WB for southern US. US energy codes (ASHRAE 90.1, IECC) require psychrometric analysis for HVAC system design and commissioning.
United Kingdom (UK): CIBSE Guide A provides psychrometric data for UK climate. Typical summer design: 28 °C DB / 20 °C WB for London. UK Building Regulations Part L references psychrometric analysis for ventilation and air conditioning energy performance assessment.
Frequently Asked Questions
What is psychrometrics?
Psychrometrics is the science of moist air properties. It studies the thermodynamic behavior of dry air and water vapor mixtures, essential for HVAC design, industrial drying, and environmental control. Key parameters include dry bulb temperature, wet bulb temperature, dew point, relative humidity, humidity ratio, enthalpy, and specific volume.
How is saturation vapor pressure calculated?
Saturation vapor pressure is calculated using the Magnus formula: Ps = 611.2 × exp(17.67 × T / (T + 243.5)) where T is the dry bulb temperature in °C. At 25 °C, Ps = 3,168 Pa (31.68 mbar). At 30 °C, Ps = 4,246 Pa (42.46 mbar).
What is the difference between dew point and wet bulb temperature?
Dew point temperature (Tdp) is the temperature at which water vapor condenses at constant pressure — it depends only on the actual moisture content. Wet bulb temperature (Twb) is the lowest temperature air can reach through evaporative cooling — it depends on both temperature and humidity. For saturated air, Tdp = Twb = Tdb.
How does altitude affect psychrometric properties?
At higher altitudes, atmospheric pressure decreases, which reduces the partial pressure of water vapor the air can hold. For the same dry bulb temperature and relative humidity, higher altitude means lower humidity ratio and lower dew point temperature. ASHRAE standard psychrometric charts are typically for sea level (101,325 Pa).
What is enthalpy of moist air?
Enthalpy of moist air combines the sensible heat of dry air and the latent heat of water vapor. It is calculated as h = 1.006 × Tdb + ω × (2499.86 + 1.84 × Tdb) kJ/kg dry air. This property is essential for sizing cooling coils, heating coils, and humidification equipment in HVAC systems.
What is a healthy indoor relative humidity range?
ASHRAE recommends indoor relative humidity between 30-60% for human comfort and health. Below 30% can cause dry eyes, skin irritation, and respiratory discomfort, especially in colder climates with indoor heating. Above 60% promotes mold growth, dust mites, and structural dampness. Absolute humidity above 12 g/m³ indicates excessive moisture.
How do I use this psychrometric calculator?
Enter the dry bulb temperature (°C) and relative humidity (%). Optionally enter altitude in meters for non-sea-level conditions. Click Calculate to see saturation vapor pressure, partial vapor pressure, humidity ratio, dew point temperature, enthalpy, and specific volume. The chart shows vapor pressure comparisons and air composition breakdown.
What is specific volume in psychrometrics?
Specific volume of moist air is the volume occupied per unit mass of dry air, typically in m³/kg. It increases with temperature and decreases with higher atmospheric pressure. The formula is v = 0.287 × (Tdb + 273.15) / (Pa — Pv) × (1 + 1.6078 × ω). It is critical for fan sizing and duct design in HVAC applications.