Vapor Pressure of Water Calculator

Vapor Pressure of Water Calculator

The Vapor Pressure of Water Calculator computes the equilibrium vapor pressure of liquid water (or ice) at any temperature. Water vapor pressure increases with temperature as more molecules gain enough energy to escape the liquid phase.

This calculator supports five empirical formulas, each tuned for different accuracy ranges:

• Buck equation: Most accurate from 0 to 100 °C (within 0.1% of reference values).
• Antoine equation: Industry standard, with separate coefficients below and above 100 °C.
• Tetens equation: Best for 0 to 50 °C.
• Magnus equation: Common in meteorology for humidity calculations.
• Simplified formula: Quick approximation using exponential form.

The Antoine equation for water below 100 °C is:

$$P = 10^{A - \frac{B}{C + T}} \quad (\mathrm{mmHg})$$

with $A = 8.07131$, $B = 1730.63$, $C = 233.426$, and $T$ in °C. At 30 °C, water vapor pressure is about 4.25 kPa (31.8 torr). At 100 °C it reaches 101.32 kPa (1 atm), the normal boiling point.

Related tools: Vapor Pressure Calculator, Relative Humidity Calculator, and Vapor Pressure Deficit Calculator.