Vapor pressure governs evaporation, boiling, and distillation. This reference computes the saturation vapor pressure of water and common solvents at any temperature from the Antoine equation and shows the result in mmHg, kPa, and atm, alongside a quick reference table.
How it works
The saturation vapor pressure comes from the Antoine equation, an empirical fit to experimental data:
log10(P) = A − B / (C + T)Here P is the vapor pressure in mmHg, T is the temperature in degrees
Celsius, and A, B, and C are constants specific to each substance,
determined by regression against measured data. Because the temperature sits in
the denominator and the whole expression is a base-10 logarithm, pressure rises
exponentially with temperature. The tool converts the mmHg result to kPa (×
0.13332) and atm (÷ 760).
Reading the result
A liquid boils when its vapor pressure reaches the surrounding pressure. At sea level that is 760 mmHg (1 atm), so the temperature at which a solvent’s vapor pressure hits 760 mmHg is its normal boiling point — about 100°C for water and 78°C for ethanol. Each set of Antoine constants is valid only over a stated temperature window; outside it the equation extrapolates and the tool flags the value as approximate.
Example values
At 25°C the tool gives water a vapor pressure near 23.8 mmHg (about 3.2 kPa), matching standard tables. Heat it to 100°C and the value reaches 760 mmHg, the boiling point at sea level. Ethanol at 25°C comes in around 59 mmHg — considerably higher than water at the same temperature, which is why ethanol evaporates noticeably faster. Acetone, used widely as a lab solvent, has a vapor pressure roughly three times that of ethanol at room temperature, reflecting its fast evaporation and the need for good ventilation when handling it.
Practical applications
Distillation design. The relative vapor pressures of two liquids at a given temperature determine how efficiently they can be separated. A large difference means good separation in few theoretical stages; similar vapor pressures require many stages or a different approach.
Safety and ventilation. High vapor pressure means a liquid evaporates quickly into the surrounding air. Knowing the vapor pressure at room temperature helps estimate concentration buildup in an enclosed space, which is relevant for flammability limits and occupational exposure thresholds.
Boiling point at altitude. At high altitude the atmospheric pressure is lower than 760 mmHg. Find the temperature at which the solvent’s vapor pressure equals the local atmospheric pressure to predict the actual boiling point. For reference, Denver (about 5,280 ft) has an atmospheric pressure near 630 mmHg, so water boils at roughly 95°C rather than 100°C.
Dew point and humidity. The saturation vapor pressure of water at ambient temperature sets the upper limit of how much moisture the air can hold. Relative humidity is the ratio of actual water vapor pressure to the saturation value.
The greyed rows in the reference table mark temperatures outside each solvent’s Antoine validity range — treat those as rough extrapolations rather than precise figures, and consult NIST data for high-accuracy work.