This vapor pressure calculator uses the Antoine equation, log₁₀ P = A − B/(C + T), to find a pure liquid’s vapor pressure at any temperature — with published Antoine constants for water, ethanol and acetone built in, plus custom-constant entry for any other liquid.

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The Antoine Equation: an Honest Empirical Fit

Vapor pressure — the pressure at which a liquid’s molecules escape into the gas phase as fast as they return — climbs steeply and non-linearly with temperature, and the Antoine equation is the three-constant fit engineers actually use: log₁₀ P = A − B/(C + T), with constants tabulated per liquid per temperature range in the NIST Chemistry WebBook. The defaults validate the whole apparatus: water at 25 °C returns 23.7 mmHg (the textbook value), and at 100 °C the same constants return 760.1 mmHg — atmospheric pressure, as the definition of boiling demands. Ethanol at 25 °C returns 58.8 mmHg (measured ≈ 59) and acetone 230.9 mmHg (measured ≈ 230): three independent liquids, three hits — the fit earns its keep inside its fitted ranges.

Respect the caveats the constants carry. Antoine parameters are range-bound: the water set printed here serves 1–100 °C; a different published set serves 99–374 °C, and swapping them matters at the seams. The equation is also a PURE-liquid statement — dissolve salt or sugar and Raoult’s law lowers the pressure (the Raoult’s Law page on this hub computes exactly that mole-fraction scaling), while volatile mixtures need each component’s partial pressure summed (Dalton). And where Antoine data don’t exist, the Clausius–Clapeyron relation — ln(P₂/P₁) = −(ΔHvap/R)(1/T₂ − 1/T₁) — estimates pressure changes from the enthalpy of vaporization, the theoretically-rooted backup every physical chemist keeps beside the empirical fit.

Worked Examples (Verified)

  • Water, 25 °C: 10^(8.07131 − 1730.63/258.426) = 23.7 mmHg (3.16 kPa).
  • Water, 100 °C: 760.1 mmHg — boils at sea level, by construction.
  • Acetone, 25 °C: 230.9 mmHg — nearly 10× water’s; why the bottle chills your skin and the fumes find you fast.

Vapor Pressure Discipline

  • Boiling happens when P_vapor = P_external: at altitude (lower external pressure) water boils cooler — the same equation read from the other side.
  • Constants are unit-specific: the sets here give mmHg with T in °C. Other tables publish bar or kPa versions — never mix sets across unit systems.
  • Stay inside the fitted range; Antoine extrapolation beyond it degrades quietly and then badly near the critical point, where the equation has no validity at all.
  • Humidity is this number in disguise: relative humidity = actual water partial pressure ÷ this saturation value at the air temperature.
  • Reid vapor pressure (gasoline spec) is a standardized measurement at 37.8 °C — related concept, regulated method; don’t substitute one for the other in compliance contexts.

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Frequently Asked Questions

What is vapor pressure?

The pressure exerted by a vapor in equilibrium with its liquid at a given temperature — the point where evaporation and condensation rates balance in a closed space. It rises steeply with temperature (water: 23.7 mmHg at 25 °C, 760 at 100 °C) and is a pure-substance property: it depends on how strongly the liquid’s molecules hold each other (intermolecular forces), not on how much liquid or vapor is present. This calculator evaluates it with the Antoine equation.

What is the Antoine equation?

log₁₀ P = A − B/(C + T): a three-constant empirical fit of vapor pressure (here mmHg) against temperature (°C), with A, B, C tabulated per substance and per temperature range (NIST Chemistry WebBook is the standard source). It is astonishingly accurate inside its fitted range — the water constants here reproduce both the 25 °C table value and the 100 °C boiling definition — and should not be extrapolated far outside it. Custom constants can be entered for any liquid.

What is the vapor pressure of water at 25 °C?

23.7 mmHg (3.16 kPa) — computed above and matching the standard tables. This single number runs weather (dew point and humidity), physiology (alveolar gas math) and vacuum practice (the floor for water-aspirator vacuum). At 37 °C (body temperature) it is 47 mmHg — the correction applied in blood-gas analysis.

How does vapor pressure explain boiling at altitude?

A liquid boils when its vapor pressure reaches the surrounding pressure. On a mountain the surrounding pressure is lower (say 630 mmHg at ~1,500 m), so water reaches that mark at a lower temperature — the Antoine equation with P set to 630 returns ≈ 95 °C. Food cooks slower in that “boiling” water because the temperature, not the bubbling, does the cooking — hence pressure cookers, which raise the external pressure and with it the boiling point.

Vapor pressure vs partial pressure — same thing?

Related, different: partial pressure is the share of a gas mixture’s total pressure due to one component (Dalton’s law). Vapor pressure is the SPECIFIC partial pressure at saturation — the maximum water (or solvent) vapor a space can hold at that temperature. Air at 50% relative humidity has water partial pressure = 0.5 × the vapor pressure this calculator returns.

Why do different tables list different Antoine constants for the same liquid?

Because the constants are fitted per temperature RANGE (and per unit system). Water has a low-range set (1–100 °C, used here) and a high-range set (99–374 °C) that differ meaningfully; both are “correct” in their ranges. Always check a set’s stated range and units (mmHg/°C here) before use — the NIST WebBook prints them alongside the constants, and this page’s presets inherit their ranges.

Can I get vapor pressure of a solution here?

Indirectly: Raoult’s law scales the pure-solvent value by the solvent’s mole fraction (P = X_solvent × P°) for non-volatile solutes — compute P° here, then apply the mole fraction (our Raoult’s Law page handles it directly). Volatile solutes contribute their own partial pressures on top. Concentrated or ionic solutions deviate from Raoult (activity corrections), which is why seawater’s vapor pressure sits slightly below even the Raoult prediction.

Is this vapor pressure calculator free?

Yes — free, instant, no sign-up. Pick a preset liquid or enter custom Antoine constants and a temperature for P in mmHg, kPa and atm.