Psychrometric Properties Calculator
Compute moist-air properties — humidity ratio, enthalpy, and dew point — from dry-bulb temperature, relative humidity, and barometric pressure. Essential for HVAC engineers sizing equipment and diagnosing comfort problems.
Last updated: September 2026
Formula below · 2 sources (NIST, Wikipedia) · Updated Sep 2026
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About this calculator
Psychrometrics describes the thermodynamic properties of moist air. Saturation vapour pressure uses the Magnus formula e_s = 0.61078 × exp(17.27 × T / (T + 237.3)) kPa, and the actual vapour pressure is p_w = φ × e_s. Humidity ratio W = 0.622 × p_w / (P − p_w) is shown in g of water per kg of dry air. Specific enthalpy h = 1.006·T + W·(2501 + 1.86·T) kJ/kg dry air (with W in kg/kg). Dew point inverts the Magnus formula exactly: T_dp = 237.3 × a / (17.27 − a), with a = ln(p_w / 0.61078). Wet-bulb temperature uses Stull's (2011) empirical formula, accurate to about ±0.3 °C between 5% and 99% RH at sea-level pressure. These relationships underpin psychrometric charts and are used in cooling-load calculations, dehumidification design and air-handling unit selection.
How to use
Given: dry-bulb T = 25 °C, relative humidity φ = 60 %, pressure P = 101.325 kPa. Step 1 — saturation vapour pressure (Magnus): e_s = 0.61078 × exp(17.27 × 25 / 262.3) = 3.168 kPa. Step 2 — vapour pressure p_w = 0.60 × 3.168 = 1.901 kPa. Step 3 — humidity ratio W = 0.622 × 1.901 / (101.325 − 1.901) = 0.01189 kg/kg = 11.89 g/kg. Step 4 — enthalpy h = 1.006 × 25 + 0.01189 × (2501 + 1.86 × 25) = 25.15 + 30.29 = 55.4 kJ/kg dry air. Step 5 — dew point: a = ln(1.901 / 0.61078) = 1.1353, T_dp = 237.3 × 1.1353 / (17.27 − 1.1353) = 16.7 °C. Step 6 — wet bulb (Stull): 19.5 °C. Try 30 °C and 60 % as well: W = 16.03 g/kg, h = 71.2 kJ/kg and T_dp = 21.4 °C.
Frequently asked questions
What is humidity ratio and how does it differ from relative humidity?
Relative humidity (RH) expresses how close the air is to saturation as a percentage, so it changes with temperature even when the actual moisture content stays constant. Humidity ratio (W), also called specific humidity or mixing ratio, measures the actual mass of water vapor in grams per kilogram of dry air. Because W is independent of temperature, it is the preferred variable for HVAC load calculations and psychrometric chart plotting. For example, air at 30 °C and 60 % RH has W ≈ 16 g/kg, but if that air is cooled to 20 °C its RH rises to roughly 87 % while W remains the same.
Why does barometric pressure matter for psychrometric calculations?
Water vapor pressure is a fraction of total atmospheric pressure, so the ratio of moisture to dry air depends on total pressure. At higher altitudes where barometric pressure is lower (e.g., 85 kPa in Denver vs. 101.3 kPa at sea level), the same relative humidity corresponds to a higher humidity ratio and different enthalpy. HVAC equipment sized using sea-level psychrometrics can be significantly undersized for high-altitude installations. Always enter the local barometric pressure for accurate results.
How do I use dew-point temperature to prevent condensation on ductwork?
Condensation forms on any surface whose temperature falls below the dew point of surrounding air. If your dew-point calculation returns 18 °C, any duct surface colder than 18 °C will sweat. Engineers use this to set minimum supply-air temperatures, specify duct insulation thickness, and position vapor barriers. In humid climates, the dew point can exceed 24 °C during summer, making uninsulated cold-water pipes and ducts prone to heavy condensation and mold risk.