Psychrometric Calculator
Compute dew point, wet-bulb temperature, humidity ratio, or enthalpy from dry-bulb temperature, relative humidity, and pressure. Essential for HVAC engineers, meteorologists, and building scientists.
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 air-conditioning load calculations, moisture control and weather analysis.
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. Select the property you want and read the result directly.
Frequently asked questions
What is the difference between dry-bulb and wet-bulb temperature in psychrometrics?
The dry-bulb temperature is the ordinary air temperature measured by a thermometer shielded from radiation. The wet-bulb temperature is measured with a thermometer whose bulb is wrapped in a water-moistened wick; evaporative cooling lowers its reading relative to the dry-bulb. The gap between the two values indicates how much moisture the air can still absorb — a smaller gap means higher humidity. HVAC engineers use both readings together on a psychrometric chart to determine all other air properties.
How does relative humidity affect the humidity ratio calculated by this tool?
Relative humidity (RH) expresses how close the air is to saturation at a given temperature, while the humidity ratio W is the actual mass of water vapor per kilogram of dry air. As RH increases, the partial pressure of water vapor rises proportionally, so W increases nonlinearly because saturation vapor pressure grows exponentially with temperature. At 30 °C and 100 % RH, air holds roughly four times more water vapor than at 0 °C and 100 % RH. This calculator converts RH to W using the Magnus formula for saturation vapor pressure.
When should I use a psychrometric calculator instead of a simple humidity sensor reading?
A basic humidity sensor gives only relative humidity, which is insufficient for HVAC load calculations, dehumidifier sizing, or drying-process design. A psychrometric calculator converts that single reading into actionable engineering quantities — humidity ratio, enthalpy, dew point, and wet-bulb temperature — that are needed to size equipment and predict moisture condensation. It is especially valuable when atmospheric pressure differs from standard (e.g., at high elevation), because saturation conditions shift with total pressure. Engineers and technicians routinely use these calculations during commissioning, troubleshooting, and energy audits.