Article — Vapor Pressure of Water Calculator
Vapor Pressure of Water Calculator
Vapor pressure of water is the pressure exerted by water vapor in equilibrium with liquid water at a given temperature. At 25 °C it is 3.17 kPa; at 100 °C exactly 101.3 kPa — equal to standard atmospheric pressure. The Antoine equation predicts this curve to within 0.5% over the 1 to 100 °C range.
What water vapor pressure means
Put liquid water in a sealed bottle, give it time, and a steady-state forms: some water molecules leave the surface as vapor, an equal number return from vapor to liquid. The vapor's partial pressure at this dynamic equilibrium is the saturation vapor pressure. It depends only on temperature — not on the amount of water in the bottle, not on the bottle's volume, not on the presence of other gases.
This temperature dependence is steep. Doubling roughly every 11 °C in the lab range, water vapor pressure goes from about 0.6 kPa at 0 °C to 101.3 kPa at 100 °C — more than 165-fold. The shape of this curve drives meteorology, drying processes, distillation, and physiology.
Saturation vapor pressure at human body temperature (37 °C) is 6.28 kPa. That is why exhaled breath fogs up cold air: warm saturated vapor cools below its dew point. The same physics drives anesthesia ventilator humidification — supply air must be at body-temperature vapor pressure to avoid drying the airway.
The Antoine equation for water
Antoine's empirical form is log₁₀ P = A − B / (C + T). For water in the 1–100 °C range the constants are A = 8.07131, B = 1730.63, C = 233.426, giving P in mmHg with T in °C. Above 100 °C, a different parameter set (A = 8.14019, B = 1810.94, C = 244.485) extends usable accuracy through about 150 °C.
The calculator handles both ranges automatically. The Antoine constants embed the physical chemistry of water (latent heat of vaporization, molecular interactions) in a compact fit. Antoine equations exist for thousands of pure substances; only the A, B, C values change.
1-100 °C A=8.07131 B=1730.63 C=233.426100-150 °C A=8.14019 B=1810.94 C=244.485Output unit P in mmHgConvert to kPa P × 0.13332Water vapor pressure by temperature
A table is the fastest way to see the curve. Refrigerator-temperature water (0 °C) has vapor pressure of 0.611 kPa. Room temperature (20 °C) is 2.34 kPa. Body temperature (37 °C) is 6.28 kPa. The atmospheric reference point (100 °C) is exactly 101.325 kPa — that pressure-temperature pair was the original definition of atmospheric pressure before SI redefinitions.
The exponential rise of vapor pressure with temperature means small temperature changes have large effects on evaporation and humidity behavior. A 5 °C rise from 20 to 25 °C boosts P_sat from 2.34 to 3.17 kPa — a 35% jump. That is why summer afternoons feel so much more humid than mornings even when the absolute moisture in air is constant.
Vapor pressure and boiling point
A liquid boils when its saturation vapor pressure equals the external pressure. At standard atmospheric pressure (101.325 kPa), water's saturation vapor pressure hits this value at exactly 100 °C — the definition of the normal boiling point. Apply more pressure and the boiling point rises; reduce pressure and it falls.
This is the principle behind pressure cookers and autoclaves. A pressure cooker holds about 200 kPa (2 atm) inside, raising water's boiling temperature to about 121 °C. Food cooks faster because thermal reactions accelerate with temperature. Autoclaves use the same effect for sterilization — 121 °C steam kills bacterial spores in a few minutes whereas 100 °C steam would need much longer.
Vapor pressure and altitude
Atmospheric pressure drops with elevation by about 12% per 1000 meters. Water still has the same saturation vapor pressure at any given temperature (vapor pressure depends only on T), but the temperature at which P_sat equals local atmospheric pressure shifts downward.
The high-altitude rule of thumb: boiling temperature drops about 1 °C per 300 m of elevation gain. At Mexico City (2240 m) water boils at 92 °C; at Lhasa (3650 m) at 87 °C; at Mount Everest summit (8848 m) at 68 °C. Recipes must be adjusted accordingly.
This altitude effect makes cooking eggs, pasta, and dried beans noticeably slower in mountain kitchens. Pressure cookers compensate by sealing in steam to recreate sea-level (or higher) conditions inside the pot.
Vapor pressure in humidity calculations
Saturation vapor pressure is the denominator in relative humidity: RH = (P_vapor / P_sat) × 100%. P_vapor is the actual partial pressure of water in air; P_sat is what the air could hold if fully saturated at the same temperature.
Air at 25 °C with RH 60% holds water vapor at 0.6 × 3.17 = 1.90 kPa partial pressure. Cool that same air without removing water and the partial pressure stays at 1.90 kPa. At the temperature where P_sat drops to 1.90 kPa — about 16.7 °C — RH reaches 100% and condensation starts. That 16.7 °C is the dew point.
Dew-point measurement is how high-end hygrometers work: cool a small surface until water condenses on it; the temperature at first condensation is the dew point, from which you compute P_vapor via the Antoine equation. This is far more accurate than capacitive humidity sensors.
Common water vapor pressure mistakes
Three errors recur in homework, lab work, and engineering specs.
Saturation vapor pressure is the maximum that water vapor can reach at a given T. Actual vapor pressure (in unsaturated air) is lower. Mixing these up turns a humidity calculation into nonsense. The Antoine equation always gives P_sat, never P_actual.
Second mistake: ignoring unit conventions. Antoine's standard form gives P in mmHg. Many engineering tables use kPa or bar. A typo of a factor of 7.5 (mmHg-to-kPa ratio) shows up as a 7.5-fold error in derived quantities. Third mistake: applying water Antoine constants to other substances. Each substance needs its own A, B, C. Ethanol, acetone, methanol, ammonia each have very different vapor-pressure curves and require their own Antoine sets.