Vapor Pressure Calculator
Antoine constants are inputs, because ranges matter.
Work out Vapor Pressure. Antoine constants are inputs, because ranges matter. States the assumption instead of hiding it.
Defaults are water in mmHg, fitted over roughly 1–100 °C
760 mmHg is one atmosphere
Vapour pressure at 25 °C
23.69 mmHg
3.158 kPa · 0.0312 atm
Antoine constants are fitted to one substance over a stated temperature range, and a set used outside that range returns a confident wrong answer. That is why the constants are inputs here rather than a hidden built-in table. Check both the range and the pressure unit a table quotes before using its values — some are in mmHg and some in bar, and the constants differ between them.
How the Vapor Pressure Calculator works
Vapour pressure at any temperature from the Antoine equation, and the boiling point at any pressure by rearranging it. The constants stay visible as inputs rather than hiding in a lookup table, because a set used outside its fitted temperature range returns a confident wrong answer.
Also known as: vapour pressure calculator · antoine equation calculator · boiling point under vacuum · boiling point at reduced pressure
What the Antoine equation is and is not
log₁₀P = A − B/(C + T) is an empirical fit, not a derivation. It is a modification of the Clausius–Clapeyron relation with a third constant added to absorb the temperature dependence of the enthalpy of vaporisation, and the three constants come from regression against measured data.
That makes it accurate and local. Within the range it was fitted over it typically reproduces measured vapour pressures to a fraction of a percent, which is better than most physical property correlations manage. Outside that range it has no basis at all and fails without warning — the equation still returns a number, and the number is wrong.
This is why the constants are inputs on this page rather than a built-in table. A hidden lookup would give an answer without saying which set produced it or whether the temperature was inside the fitted range, and both of those are more important than the answer.
Reading a table of constants without being caught
Published Antoine constants vary in three ways that all break a calculation silently. Pressure units differ — mmHg, bar and kPa are all in use, and the constant A shifts by log₁₀ of the conversion factor between them. Temperature units differ, Celsius and kelvin, which moves C by 273.15. And some sources use natural logarithms rather than base 10, which rescales A and B by ln(10).
Water's own constants illustrate the range problem. A = 8.07131, B = 1730.63, C = 233.426 is the standard set for mmHg over roughly 1 to 100 °C. Above 100 °C a different set takes over — A = 8.14019, B = 1810.94, C = 244.485 — and using the first set at 150 °C returns a plausible-looking pressure that is wrong.
The check worth running: put the substance's known normal boiling point into the equation and see whether it returns 760 mmHg. If it does not, either the constants are for different units or you are outside their range, and either way the answer you wanted is not trustworthy.
Boiling under reduced pressure
A liquid boils when its vapour pressure equals the pressure above it. That is why boiling point is a function of ambient pressure rather than a property of the liquid alone — water boils at 100 °C at sea level, around 93 °C in Denver, and near 71 °C on the summit of Mont Blanc.
Rearranging Antoine for temperature, T = B/(A − log₁₀P) − C, gives the boiling point at any pressure. Water at 250 mmHg boils near 72 °C; at 25 mmHg, near 26 °C. That is what rotary evaporation exploits — solvent is removed at a temperature low enough to leave a heat-sensitive product intact.
Vacuum distillation applies the same principle to separations that would otherwise be impossible. Many organic compounds decompose before reaching their atmospheric boiling point, so distilling them at all requires lowering the pressure until the boiling point drops below the decomposition temperature. The Antoine equation is how that pressure is chosen in advance rather than by trial.
Where to go next
The Vapor Pressure question rarely arrives on its own. These are the ones that usually come with it:
- Boiling Point Calculator — Water's boiling point at altitude, from the pressure.
- Boiling Point Elevation Calculator — ΔT = i·Kb·b, with the van 't Hoff factor made explicit.
- Ideal Gas Law Calculator — PV = nRT, solving for whichever term is missing.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
What is the Antoine equation?
log₁₀P = A − B/(C + T), an empirical three-constant fit relating vapour pressure to temperature. It is accurate within its fitted range and unreliable outside it.
Why are the constants inputs rather than built in?
Because Antoine constants are specific to one substance over one stated temperature range, and published sets use different pressure units. A hidden table would give an answer without telling you which set produced it or whether the range applied.
What are the Antoine constants for water?
A = 8.07131, B = 1730.63, C = 233.426 for pressure in mmHg over roughly 1 to 100 °C. Above 100 °C a second set takes over, which is exactly why the range needs checking.
How do I find a boiling point at reduced pressure?
Rearrange the equation for temperature: T = B/(A − log₁₀P) − C. Water boils at 100 °C at 760 mmHg but near 72 °C at 250 mmHg, which is what makes rotary evaporation work.
Why does a liquid boil when vapour pressure equals atmospheric pressure?
Because vapour bubbles can only form inside the liquid once their internal pressure matches the pressure holding them closed. Below that point evaporation happens only at the surface.
Do different tables give different constants?
Yes, and often in different units — some in mmHg, some in bar, some using natural rather than base-10 logarithms. Check the units and the range before using any published set.
How does the Antoine equation relate to Clausius–Clapeyron?
It is a modification of it. Clausius–Clapeyron gives ln P = −ΔHvap/RT + C, assuming the enthalpy of vaporisation is constant. Antoine adds a third fitted constant that absorbs the temperature dependence ΔHvap actually has, which is why it fits real data better over a wider range.
Where do I find Antoine constants?
The NIST WebBook is the usual first stop, and it states the temperature range and units with each set. Perry's Chemical Engineers' Handbook and the DIPPR database are the standard engineering sources.
What is the normal boiling point?
The temperature at which vapour pressure reaches exactly 1 atm, or 760 mmHg. It is quoted for every liquid as a reference point, and it is the value the Antoine equation should reproduce if the constants and units are right.
What is the critical point?
The temperature and pressure above which liquid and vapour become indistinguishable, so there is no boiling and no vapour pressure curve to extend. Water's critical point is 374 °C and 218 atm, and the Antoine equation is meaningless beyond it.
How does vapour pressure relate to relative humidity?
Relative humidity is the partial pressure of water vapour in air divided by the saturated vapour pressure at that temperature, as a percentage. That is why the same absolute water content gives a higher relative humidity on a cold day.
Does a mixture have a single vapour pressure?
No. Raoult's law gives the vapour pressure of an ideal mixture as the sum of each component's pure vapour pressure times its mole fraction. Real mixtures deviate, sometimes far enough to form an azeotrope that distils at a fixed composition and cannot be separated by simple distillation.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/vapor-pressure-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Vapor Pressure Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/vapor-pressure-calculator" style="color:#64748b">Vapor Pressure Calculator</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.
Related calculators
Boiling Point Calculator
Water's boiling point at altitude, from the pressure.
OpenBoiling Point Elevation Calculator
ΔT = i·Kb·b, with the van 't Hoff factor made explicit.
OpenIdeal Gas Law Calculator
PV = nRT, solving for whichever term is missing.
OpenSpeed Distance Time Calculator
Any one of the three from the other two, in four units.
Open