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.
Tips
- 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.
Frequently Asked Questions
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.