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Antoine Equation (Vapour Pressure)

Also known as antoine constants · vapor pressure equation · vapour pressure correlation · boiling point from pressure

log⁡10P=A−BC+T\log_{10} P = A - \frac{B}{C + T}

Worked example: Water at 100 degC → 760.09 mmHg (101.337 kPa) — press Try an example to run it live, then adjust anything.

Enter your known values, leave one input blank, and solves for the missing one. Tap a variable’s symbol to see what it means, with a typical value. Try different units for next level excitement!

Constant used — built into this formula, no need to enter
T0=273.15 KT_0 = 273.15\ \text{K}Ice point (0 °C in kelvin) · exact
See also
Clausius–Clapeyron Equation (Two-Point Form) — the same curve from a heat of vaporisation instead of fitted constantsRaoult's Law — what to do with the pure-component vapour pressure once you have it
Here the solver did the work — could you?

Vapour pressure curves →

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Antoine Equation (Vapour Pressure) explained

TPA, B, C

Antoine's 1888 fit is three fitted constants standing in for a whole thermodynamic derivation, and it works startlingly well. With the water constants A=8.07131A = 8.07131, B=1730.63B = 1730.63, C=233.426C = 233.426, the equation says that at 100 °C the vapour pressure is 108.07131−1730.63/333.426=760.1 mmHg10^{8.07131 - 1730.63/333.426} = 760.1\ \text{mmHg}. That is one atmosphere to within a tenth of a millimetre of mercury, recovered from a curve fit rather than from steam tables.

The trap is the units, and it swallows more students than any other feature of the equation. Published constants are overwhelmingly quoted for pressure in mmHg and temperature in degrees Celsius. There is a second convention in kPa and kelvin, and one in bar, and the constants are not interchangeable: use a mmHg set with a temperature in kelvin and you will land several orders of magnitude out with no warning. This page assumes the mmHg and Celsius convention and converts your entered pressure and temperature to match, so enter the constants exactly as the handbook prints them.

The other thing worth knowing is that every constant set has a stated temperature range, and it is not a suggestion. Water has one set for 1-100 °C and a different set for 99-374 °C, and the two disagree by several percent where they overlap. Extrapolate past the range and the error grows without any hint on the page that anything is wrong, because a smooth curve keeps returning smooth numbers. Check the range before you trust the answer.

Antoine Equation (Vapour Pressure)

log⁡10P=A−BC+T\log_{10} P = A - \frac{B}{C + T}
Where
  • PP= Vapour pressure (kPa)
  • TT= Temperature (°C)
  • AA= Antoine constant A
  • BB= Antoine constant B (°C)
  • CC= Antoine constant C (°C)

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