Vapor Pressure of Water Calculator

Water vapor pressure is the pressure produced by water molecules in a saturated vapor that is in equilibrium with liquid water. At any fixed temperature, molecules continually evaporate from the surface and condense back into the liquid. In a closed space, equilibrium is reached when those rates balance. The corresponding partial pressure depends strongly on temperature and is often called saturation vapor pressure. This calculator uses the Antoine equation with coefficients fitted for liquid water between 1 °C and 100 °C. The correlation calculates pressure in millimeters of mercury and then converts it to kilopascals, bar, or pounds per square inch when requested. At 25 °C, saturation pressure is only a small fraction of standard atmospheric pressure. Near 100 °C it approaches one standard atmosphere, which explains why water boils near that temperature at sea-level pressure. The exact normal boiling point is where saturation pressure equals the local external pressure. Saturation pressure appears in humidity, drying, meteorology, HVAC design, distillation, vacuum systems, and laboratory calculations. Relative humidity compares the actual water-vapor partial pressure with saturation pressure at the same temperature. Dew point reverses that idea: it is the temperature at which the existing partial pressure becomes saturated. A warm air parcel can therefore contain more water vapor before reaching saturation than a cool parcel, although vapor pressure is fundamentally a property of equilibrium at the water surface rather than a capacity of air. The Antoine coefficients are empirical and must only be used over their fitted range. This implementation accepts temperatures from 1 °C through 100 °C and describes equilibrium over liquid water. Values below freezing require a distinction between supercooled liquid water and ice, and values above the normal boiling point may require another coefficient set or a higher external pressure. Dissolved salts and other solutes lower water activity and can reduce vapor pressure. For precision engineering, calibration, weather standards, or safety work, use a recognized steam-table formulation and account for measurement uncertainty, composition, and local pressure.