Water Saturation Vapor Pressure vs Temperature Charts Tables Calculator

Water tends to evaporate by projecting molecules into the space above its surface. If that space is confined, the partial pressure exerted by those molecules increases until the rate at which molecules re-enter the liquid equals the rate at which they leave. The vapour pressure of water is the pressure at which water vapour is in thermodynamic equilibrium with its condensed (liquid) state. At higher pressures, water would condense. At this equilibrium condition the vapour pressure is called the saturation pressure.

Saturation pressure increases rapidly and non-linearly with temperature, spanning more than four orders of magnitude from 0 °C to 370 °C. This relationship is described by the Clausius-Clapeyron equation, which links the slope of the saturation curve to the heat of vaporization and the temperature:

d(ln P) / dT = ΔH_vap / (R · T²)

At 0 °C, the saturation pressure is only 0.61 kPa (0.089 psi). At 100 °C it reaches 101.4 kPa (14.7 psi) — standard atmospheric pressure, which is why water boils at 100 °C at sea level. At 370 °C it reaches approximately 21 MPa (3050 psi), approaching the critical point.

Saturation pressure determines at what temperature water boils for a given system pressure — and vice versa. For steam systems, boilers, heat exchangers, and autoclave processes, knowing the saturation pressure is essential for safe design. Liquid water above 100 °C / 212 °F exists only under pressure equal to or greater than the saturation pressure at that temperature. If the pressure drops below the saturation pressure, flash vaporization (cavitation in pumps) occurs.