Vapor Pressure Calculator

Vapor pressure fundamentals for HVAC psychrometric analysis

Vapor pressure is the partial pressure exerted by water molecules in moist air. Two quantities matter: saturation vapor pressure (P_sat), which depends only on temperature and represents the maximum moisture the air can hold without condensation, and partial vapor pressure (P_w), which is the actual vapor pressure of the moist air sample. Their ratio is relative humidity: RH = P_w / P_sat × 100%. ASHRAE uses the Antoine-type equation to compute P_sat with high accuracy: above 0°C, the six-constant polynomial in Kelvin gives values accurate to within 0.02% across the range −40 to 200°C, consistent with the water thermophysical reference data NIST maintains, which is more than adequate for any HVAC application.

Vapor pressure is the starting point for all psychrometric calculations. Humidity ratio follows from Dalton's law: W = 0.62198 × P_w / (P_total − P_w). Enthalpy, specific volume, and dew point all trace back to P_w and P_sat. When you need to convert between any pair of psychrometric properties, vapor pressure is usually the intermediate step. This tool shows you both pressures and the vapor pressure deficit so you can verify calculations from the humidity ratio calculator and the dew point calculator.

Vapor pressure deficit and moisture-driven processes in buildings

Vapor pressure deficit (VPD = P_sat − P_w) is the driving force for moisture evaporation and condensation. A high VPD (dry air far from saturation) accelerates moisture evaporation from surfaces, occupant skin, and building materials. A VPD near zero (near-saturated air) reduces evaporation and can trigger condensation on slightly cooler surfaces. In Canadian buildings, winter outdoor air arriving at temperatures of −20°C has a saturation pressure of only about 0.10 kPa. After mixing with and heating to 21°C indoor temperature, P_sat rises to 2.50 kPa while P_w remains close to 0.10 kPa, giving a VPD near 2.40 kPa, very high, and the reason heated winter air feels and acts so dry and typically needs a properly sized humidifier to compensate. For indoor air at 21°C and 35% RH, VPD is about 1.63 kPa, still in the range that causes dry skin and static discharge.

Vapor pressure units and conversion across Canadian HVAC practice

ASHRAE and Canadian standards express vapor pressure in kPa. Weather services report pressure in hPa in raw meteorological data, as defined in Environment and Climate Change Canada's glossary, though ECCC's public current-conditions displays show pressure in kPa. Older psychrometric charts and US equipment data sheets often use inHg. Laboratory and medical equipment uses mmHg (Torr). The conversions are fixed: 1 kPa = 7.501 mmHg = 0.2953 inHg = 10 mbar = 1000 Pa. Atmospheric pressure context matters: at Calgary (1045 m, P_total ≈ 89.4 kPa), the same P_w corresponds to a higher humidity ratio W than at sea level because the dry-air partial pressure P_da = P_total − P_w is smaller. Saturation pressure itself doesn't change with elevation, but all W and RH-derived quantities do. Use the relative humidity calculator to verify RH at different elevations from a known P_w.