Water Vapor Pressure Calculator (Antoine Equation)

Enter a temperature (°C or °F) to calculate the vapor pressure of water in mmHg, kPa, atm, and bar using the widely used Antoine equation. Change the ambient pressure to see how altitude lowers the boiling point.

Last updated: 2026-09-16

What Water Vapor Pressure Means, via the Antoine Equation

Vapor pressure is the pressure exerted by a vapor once it reaches equilibrium with its liquid in a sealed container — the point where evaporation and condensation happen at exactly the same rate (vapor-liquid equilibrium). Because warmer molecules move more energetically and escape the liquid more readily, vapor pressure rises sharply as temperature increases. This tool uses the Antoine equation, log10(P) = A − B / (C + T), a formula widely taught in chemical engineering, to calculate water's vapor pressure between 0 and 100°C in four units at once: mmHg, kPa, atm, and bar.

Boiling is defined as the moment a liquid's vapor pressure equals the surrounding ambient pressure. By changing the ambient pressure away from the standard sea-level value, you can see numerically how the boiling point drops as altitude increases. It's a practical companion for high school and college chemistry coursework, and for real-world questions that come up while hiking or cooking at altitude.

How to Calculate Water Vapor Pressure

Choose the temperature unit Pick Celsius (°C) or Fahrenheit (°F). Fahrenheit input is automatically converted to Celsius before the calculation runs.
Enter the temperature Type in the temperature you want to check. The supported range is 0 to 100°C (32 to 212°F).
Read the vapor pressure in four units The results panel shows the vapor pressure simultaneously in mmHg, kPa, atm, and bar.
Adjust the ambient pressure Use the sea-level or Mt. Fuji summit presets, or type your own kPa value, to see whether boiling occurs at that pressure.

Tips for getting more out of it

The Antoine coefficients used here are valid from 0 to 100°C. Outside that range, no result is shown since a simple extrapolation would be unreliable.
Change the ambient pressure to match an altitude to see at what temperature water would boil there.
A pressure cooker artificially raises the internal pressure above atmospheric, which pushes the boiling point above 100°C and shortens cooking time.
If you only know the temperature in °F, just switch the unit button — it's converted to °C automatically before calculating.
It helps to remember the definition: boiling starts exactly when a liquid's vapor pressure equals the surrounding (ambient) pressure.

Ways to Use This Vapor Pressure Calculator

Checking chemistry homework and lab reports

After solving an Antoine-equation vapor pressure problem by hand, quickly verify whether your own calculation matches this tool's result.

Understanding why water struggles to boil at altitude

If you've noticed water boiling while still lukewarm on a mountain trip, this tool shows numerically how ambient pressure and vapor pressure explain that experience.

Estimating cooking times at high elevation

Before cooking somewhere at high altitude, check how far the boiling point drops so you can plan longer heating times accordingly.

Reviewing the basics behind HVAC and industrial humidity calculations

The concept of saturated vapor pressure also underlies humidity calculations and drying-process design, so this is a handy refresher on the fundamentals.

Vapor Pressure Glossary

Vapor pressure: The pressure exerted by a vapor once it reaches equilibrium with its liquid in a sealed container. It rises sharply as temperature increases.
Antoine equation: An empirical formula, log10(P) = A − B / (C + T), that approximates the relationship between vapor pressure and temperature using substance-specific constants A, B, and C.
Boiling point: The temperature at which a liquid's vapor pressure equals the surrounding ambient pressure. Lower ambient pressure means that condition is met at a lower temperature.
Atmospheric pressure: The pressure exerted on a location by the air above it. It decreases with altitude as the air thins out.
Vapor-liquid equilibrium: A state inside a sealed container where the rate of evaporation equals the rate of condensation, so the system appears unchanging from the outside.
Standard atmosphere: The reference value for atmospheric pressure near sea level, defined as 101.325 kPa (1 atm). This tool uses it as the default ambient pressure.

Frequently Asked Questions

Boiling occurs when a liquid's vapor pressure equals the surrounding ambient pressure. Since atmospheric pressure drops with altitude, water's vapor pressure reaches that lower ambient pressure at a lower temperature, so the boiling point drops. Near the summit of Mt. Fuji, water is known to boil at around 87°C.

In a sealed container, a liquid and the vapor it produces reach an equilibrium where evaporation and condensation happen at the same rate; the pressure of the vapor at that point is the vapor pressure. Because higher temperatures mean more energetic molecules, vapor pressure rises sharply as temperature increases.

It's an empirical formula that approximates the relationship between vapor pressure and temperature as log10(P) = A − B / (C + T). Using substance-specific constants A, B, and C, it lets you compute vapor pressure with practical accuracy without resorting to the more complex Clausius-Clapeyron equation.

A pressure cooker seals its lid to keep the internal pressure above atmospheric. Since boiling requires the vapor pressure to match the surrounding pressure, a higher surrounding pressure means a higher temperature is needed before boiling starts. That lets the water inside stay well above 100°C without boiling, transferring more heat to the food.

The Antoine coefficients used here are the standard values fitted to match experimental data for water between 0 and 100°C. Outside that range — such as sub-zero temperatures or superheated steam well above 100°C — a naive extrapolation would introduce significant error, so this tool intentionally withholds a result.

Side Note — "Water boils at 100°C" is a conditional fact

When a liquid is kept in a sealed container, the rates of evaporation and condensation eventually balance out, reaching a state called vapor-liquid equilibrium where gas and liquid coexist. The pressure of that gas is the vapor pressure. It isn't a fixed property of a substance — as temperature rises, molecules move more energetically and evaporate more readily, so vapor pressure increases sharply with temperature. In the 19th century, the French engineer Louis Charles Antoine showed that this temperature–vapor pressure relationship could be approximated by a simple empirical equation, and the Antoine equation remains one of the most widely used vapor pressure approximations in chemical engineering textbooks today.

Boiling occurs when bubbles forming inside a liquid can rise to the surface without collapsing — in other words, when the liquid's vapor pressure equals the surrounding ambient pressure. Under standard atmospheric pressure at sea level (about 101.325 kPa), water's vapor pressure reaches that value at exactly 100°C, which is why we learn that "water boils at 100°C." But that's only true under the specific condition of being near sea level.

As altitude increases, the surrounding atmospheric pressure drops. Near the summit of Mt. Fuji (about 3,776 m), pressure falls to roughly 60% of its sea-level value, so the temperature at which water's vapor pressure reaches that lower pressure — its boiling point — also drops into the 80s (°C). This is why rice cooked during a climb often doesn't fully cook through. Conversely, a pressure cooker seals in steam to artificially raise internal pressure, pushing the boiling point above 100°C so food cooks thoroughly in less time.

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