Water evaporates into air until the air holds all it can — and “all it can” is a precise number at every temperature: the saturation vapor pressure. It drives humidity, drying, distillation, weather and the behaviour of every vacuum pump ever plumbed. This vapor pressure of water calculator evaluates the Antoine equation — the standard empirical fit — at any temperature in its validated range, and reports the pressure in the four units working people actually use.
The Antoine Equation
The relation fitted to a century of measurements is log10 P = A − B ÷ (C + T), with P in mmHg, T in °C, and water’s constants A = 8.07131, B = 1730.63, C = 233.426 for the 1–100 °C range. At the page’s default 25 °C: 8.07131 − 1730.63 ÷ 258.426 = 1.374, so P = 10^1.374 ≈ 23.69 mmHg (3.16 kPa) — the textbook saturation pressure of a warm room. The equation’s finest party trick is its endpoint: at 100 °C it returns ≈ 760 mmHg, one full atmosphere. That is not a coincidence of the fit; it is the definition of the normal boiling point asserting itself through the data.
Why the Curve Bends Upward
Vapor pressure does not grow linearly with temperature — it accelerates, roughly doubling for every 10–11 °C in the everyday range (17.5 mmHg at 20 °C, 31.8 at 30 °C, 55.3 at 40 °C, and a full 760 at 100 °C). The molecular story: escaping the liquid requires beating intermolecular attraction, and the fraction of molecules with enough energy grows exponentially with temperature — the same Boltzmann-tail mathematics behind the Arrhenius equation in kinetics. The engineering consequences are everywhere: pressure cookers, autoclaves, evaporative cooling limits, and why a “small” temperature rise in a sealed solvent container is not small at all.
For quick reference, the landmarks the equation reproduces: 10 °C → 9.2 mmHg; 20 °C → 17.5; 30 °C → 31.8; 40 °C → 55.3; 60 °C → 149; 80 °C → 355. Notice the atmosphere fraction: at 30 °C water vapor alone can claim about 4 percent of the air in a saturated space, which is why tropical humidity is a physical presence and why gas collected over water needs its dry-gas correction (Dalton’s law again) before any stoichiometry touches it.
Where This Number Works
Humidity: relative humidity is actual vapor pressure as a percentage of this saturation value — the dew point is the temperature at which the two meet, and every weather report silently runs the Antoine relation backward. Wet-gas corrections: gas collected over water is part water vapor; subtract the saturation pressure at the bath temperature to get the dry gas pressure for PV = nRT. Drying and vacuum work: a vacuum pump cannot pull below the vapor pressure of any water present — wet loads stall evaporators at exactly this floor. Distillation: boiling begins when the mixture’s total vapor pressure meets the ambient pressure; this is the pure-water benchmark the mixture departs from. Food and canning: the same curve sets the pressure inside a sealed jar or retort at temperature, which is why canning schedules are written as pressure-and-time pairs rather than temperatures alone — the pressure gauge is, through this relation, a thermometer for saturated steam.
The Limits of the Fit
Antoine constants are fits, not laws, and each published set owns a temperature territory: the constants here are validated for 1–100 °C, and the calculator declines outside that range rather than extrapolate politely. Other sets cover −20 to 1 °C (over ice, a physically different surface with lower pressures) and 100–374 °C up to the critical point. Within range, agreement with steam tables is within a fraction of a percent — good enough for humidity, corrections and design estimates; custody-transfer engineering still reaches for the steam tables themselves.
Common Mistakes
Applying Celsius constants to a Fahrenheit input (the calculator converts first — imitate it). Using vapor pressure where partial pressure belongs: in unsaturated air, water’s partial pressure is the RH fraction of this saturation number, not this number. Forgetting the wet-gas correction and reporting mole counts a few percent fat. Confusing saturation pressure with boiling: water at 25 °C under 23.69 mmHg of total pressure does boil — in a vacuum chamber — because boiling compares vapor pressure to whatever pressure surrounds the liquid, not to one atmosphere specifically.
Related Tools
The physical-chemistry shelf continues in the Chemistry Calculators hub — the TDS Calculator covers water’s dissolved load, and the Chemical Equation Balancer handles the equations of the reactions water so often hosts.
Frequently Asked Questions
What is the vapor pressure of water at 25 °C?
About 23.7 mmHg (3.17 kPa, 0.0312 atm). The Antoine equation with water’s standard constants returns 23.69 mmHg — the saturation pressure that defines 100 percent relative humidity at room temperature.
How is vapor pressure calculated?
With the Antoine equation: log10 P = A − B/(C + T), using A = 8.07131, B = 1730.63 and C = 233.426 for water between 1 and 100 °C, P in mmHg. At 100 °C it correctly returns one atmosphere, 760 mmHg.
Why does vapor pressure rise so fast with temperature?
Escaping the liquid is an energy threshold, and the share of molecules above any energy threshold grows exponentially with temperature. The practical effect: saturation pressure roughly doubles every 10 to 11 °C around room temperature.
What is the difference between vapor pressure and partial pressure?
Vapor pressure (saturation) is the maximum water vapor pressure possible at a temperature. Partial pressure is what is actually present: relative humidity × the saturation value. They coincide only in saturated air.
How is vapor pressure used in gas calculations?
Gas collected over water is mixed with water vapor at the saturation pressure for that temperature. Subtract it from the total pressure (Dalton’s law) to get the dry gas pressure before using PV = nRT.
Does altitude change the vapor pressure of water?
No — saturation vapor pressure depends on temperature alone. Altitude changes the surrounding pressure, which changes the boiling point; the vapor pressure at a given temperature is the same on a mountain as at the sea.