Estimate the saturation vapor pressure of a pure liquid from temperature, or invert the relation to find the boiling temperature at a given pressure, using the three-parameter Antoine equation. Default constants are NIST values for water (valid 1–100 °C); swap in A, B, and C for any other species.
Vapor pressure governs evaporation, boiling, distillation, flash calculations, and the sizing of relief and vent systems. The Antoine equation is the workhorse correlation for the saturation vapor pressure of a pure liquid as a function of temperature, using just three fitted constants. This calculator evaluates it in both directions — pressure from temperature, and boiling temperature from pressure — and converts the result between mmHg, kPa, and bar.
In its common form, log₁₀(P) = A − B/(C + T), where P is the saturation vapor pressure, T the temperature, and A, B, C are substance-specific constants fitted to experimental data. Solving for temperature gives T = B/(A − log₁₀P) − C. The equation is a semi-empirical refinement of the Clausius-Clapeyron relation: the extra constant C lets a single set of parameters fit data accurately over a useful temperature span.
Antoine constants are only valid for the units they were fitted in. The defaults here use P in mmHg and T in °C (the classic form for water, A = 8.07131, B = 1730.63, C = 233.426). Other tabulations use P in bar or kPa and T in K, with different A, B, C. Mixing a kPa-based A with a mmHg pressure, or °C constants with a kelvin temperature, produces nonsense — always confirm the reference units before entering coefficients.
Antoine parameters are regressed from measured vapor-pressure data and published in sources such as the NIST Chemistry WebBook, the DIPPR database, and Perry's Handbook. Each fit is valid only over a stated temperature range; the water constants above apply from about 1 to 100 °C. Many species need two or three sets of constants to span from the triple point to the critical point. Extrapolating beyond the fitted range can give large errors, especially near the critical point.
Vapor pressures appear in mmHg (torr), kPa, bar, atm, and psi depending on the source. The exact conversions are 1 mmHg = 0.133322 kPa, 1 kPa = 0.01 bar, 1 atm = 760 mmHg = 101.325 kPa = 1.01325 bar. This calculator reports the pressure in mmHg, kPa, and bar simultaneously so you can drop the result straight into a flash or relief calculation without a unit mistake.
The default form returns pressure in millimetres of mercury (mmHg, equivalent to torr) with temperature in degrees Celsius. The supplied A, B, and C must be the set fitted for those units. If your constants are tabulated for kPa or bar and kelvin, the numbers will differ — use a matching constant set or convert.
The NIST Chemistry WebBook lists Antoine parameters and their valid temperature ranges for thousands of compounds. Other standard sources are the DIPPR database, Perry's Chemical Engineers' Handbook, and Yaws' handbooks. Always copy the temperature range alongside the constants so you know where the fit is valid.
A, B, and C are regressed from data over a limited temperature span — vapor pressure varies over many orders of magnitude from the triple point to the critical point, and a single three-parameter fit cannot capture all of it. Outside the fitted range, especially near the critical temperature, the predicted pressure can be substantially wrong. Use the constant set whose range covers your conditions.
Switch to the "Boiling T from P" mode and enter 760 mmHg (1 atm). The Antoine equation then returns the normal boiling point. For water with the default constants this gives close to 100 °C, confirming the fit. For vacuum distillation, enter the reduced operating pressure to see how much the boiling point drops.
The Antoine equation describes the vapor pressure of a single pure component. For mixtures you combine each component's vapor pressure with a phase-equilibrium model — Raoult's law for near-ideal solutions, or activity coefficients (NRTL, UNIQUAC, Wilson) for non-ideal ones. The pure-component Antoine pressures computed here are the starting point for those bubble- and dew-point calculations.