Psychrometric Properties Calculator
Determine humidity ratio, enthalpy, dew point, or wet-bulb temperature from dry-bulb temperature, relative humidity, and barometric pressure. Ideal for HVAC design, indoor air-quality analysis, and meteorological work.
About this calculator
This calculator derives moist-air properties from the Antoine-type saturation vapor pressure equation: p_ws = 610.78 × exp(17.2694 × T / (T + 238.3)) Pa, where T is in °C. The actual vapor pressure is p_w = RH × p_ws. Humidity ratio W = 0.622 × p_w / (p − p_w) kg/kg. Specific enthalpy h = 1.006 × T + W × (2501 + 1.86 × T) kJ/kg. Dew point T_dp = 238.3 × ln(p_w / 610.78) / (17.2694 − ln(p_w / 610.78)) °C. Wet-bulb temperature is estimated using Stull's empirical formula involving arctangent terms of T and RH. All properties are interrelated through these equations, forming the basis of the psychrometric chart used in building services engineering.
How to use
Given: T = 25 °C, RH = 50 %, barometric pressure p = 101.325 kPa (101 325 Pa). Step 1 — Saturation pressure: p_ws = 610.78 × exp(17.2694 × 25 / 263.3) ≈ 3 167 Pa. Step 2 — Vapor pressure: p_w = 0.50 × 3 167 ≈ 1 584 Pa. Step 3 — Humidity ratio: W = 0.622 × 1 584 / (101 325 − 1 584) ≈ 0.622 × 1 584 / 99 741 ≈ 0.00988 kg/kg. Step 4 — Enthalpy: h = 1.006 × 25 + 0.00988 × (2501 + 46.5) ≈ 25.15 + 25.16 ≈ 50.3 kJ/kg. Step 5 — Dew point: T_dp = 238.3 × ln(1584/610.78) / (17.2694 − ln(1584/610.78)) ≈ 13.9 °C. Select the desired output property from the dropdown to display the corresponding result.
Frequently asked questions
What is humidity ratio and how is it different from relative humidity?
Relative humidity (RH) is the ratio of actual vapor pressure to saturation vapor pressure at the same temperature, expressed as a percentage; it tells you how 'full' the air is relative to its capacity. Humidity ratio W, also called specific humidity or mixing ratio, is the mass of water vapor per kilogram of dry air in kg/kg and is independent of temperature changes at constant moisture content. When you heat air without adding moisture, RH drops but W stays constant — a critical distinction for HVAC supply-air calculations. This calculator converts between these two representations using the barometric pressure you provide.
How does barometric pressure affect psychrometric property calculations?
Barometric pressure sets the total pressure of the air-vapor mixture and therefore directly influences humidity ratio: at lower pressures (high altitudes), the same RH corresponds to a higher W because the denominator (p − p_w) is smaller. Enthalpy is also affected because W appears in the enthalpy equation. In contrast, dew point and wet-bulb temperature depend primarily on vapor pressure rather than total pressure, so they shift only modestly with elevation. HVAC engineers designing systems for Denver (≈ 84 kPa) versus Miami (≈ 101 kPa) must account for this difference to correctly size equipment.
Why is the wet-bulb temperature approximation used here different from the psychrometric chart value?
This calculator uses Stull's (2011) empirical formula, which fits measured wet-bulb data across a wide range of conditions using arctangent functions and is accurate to within about 0.3 °C for temperatures 0–60 °C and RH 5–99 %. Traditional psychrometric charts and sling psychrometers use the psychrometric equation T_wb = T − (p − p_w) / (A × γ), requiring knowledge of the psychrometric constant. Stull's approach avoids iterative steam-table lookups while maintaining engineering accuracy. For extreme conditions — very low humidity or temperatures near freezing — a full iterative solution against steam tables is preferable.