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.
About this calculator
Psychrometrics describes the thermodynamic properties of moist air. The dew point is approximated as T_dp ≈ T_dry − (100 − RH) / 5, and the wet-bulb temperature as T_wb ≈ T_dry − (100 − RH) / 3. The humidity ratio (specific humidity) uses the Magnus saturation vapor pressure formula: e_s = 6.112 × exp(17.67 × T / (T + 243.5)) hPa, giving W = 0.622 × (RH/100 × e_s) / (P − RH/100 × e_s) in kg/kg. Enthalpy of moist air is h = 1.006 × T + (RH/100) × 0.622 × (2501 + 1.805 × T) in kJ/kg. These relationships underpin air-conditioning load calculations, moisture control, and weather analysis.
How to use
Suppose dry-bulb temperature T = 30 °C, relative humidity RH = 60 %, and pressure P = 101.325 kPa. Step 1 — Saturation vapor pressure: e_s = 6.112 × exp(17.67 × 30 / 253.5) ≈ 42.43 hPa. Step 2 — Humidity ratio: W = 0.622 × (0.60 × 42.43) / (101.325 − 0.60 × 42.43) ≈ 0.622 × 25.46 / 75.87 ≈ 0.0161 kg/kg. Step 3 — Dew point: T_dp ≈ 30 − (100 − 60) / 5 = 30 − 8 = 22 °C. Step 4 — Enthalpy: h ≈ 1.006 × 30 + 0.60 × 0.622 × (2501 + 54.15) ≈ 30.18 + 953.7 ≈ 983.9 kJ/kg. Select your desired property 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.