Vapor Pressure of Water Calculator

Calculate the saturation vapor pressure of water at any temperature using your choice of empirical formulas. Compare results from Antoine, Buck, Magnus, and Tetens equations across multiple pressure units.

Temperature of water (-100°C to 374°C)

Water Vapor Pressure Formulas: 1. Antoine Equation (0-100°C): log₁₀(P) = 8.07131 - 1730.63/(233.426 + T) P in mmHg, T in °C 2. Buck Equation (-40°C to +50°C): P = 0.61121 × exp((18.678 - T/234.5) × T/(257.14 + T)) P in kPa 3. Magnus Formula (-40°C to +50°C): P = 0.61094 × exp(17.625 × T/(T + 243.04)) P in kPa 4. Tetens Formula (0-100°C): P = 0.61078 × exp(17.27 × T/(T + 237.3)) P in kPa Key Reference Values: 0°C: 4.58 mmHg (0.611 kPa) 25°C: 23.76 mmHg (3.17 kPa) 50°C: 92.5 mmHg (12.33 kPa) 100°C: 760 mmHg (101.33 kPa) Boiling occurs when vapor pressure equals atmospheric pressure.
Example 1: Room Temperature (25°C) Temperature: 25°C Antoine: 23.756 mmHg Buck: 23.754 mmHg Magnus: 23.760 mmHg Tetens: 23.756 mmHg (All formulas agree within 0.003%) At 25°C, the saturation vapor pressure is ≈ 23.76 mmHg (3.17 kPa). This means air at 25°C with 50% RH has water vapor pressure of 11.88 mmHg. Example 2: Body Temperature (37°C) Temperature: 37°C Average VP ≈ 47.07 mmHg (6.28 kPa) This is why sweat evaporates - the air can hold much more moisture at body temperature than at room temp. Example 3: Boiling Point Check (100°C) Temperature: 100°C All formulas give ≈ 760 mmHg (101.33 kPa) At this temperature, vapor pressure equals standard atmospheric pressure, so water boils freely. Example 4: Near Freezing (0°C) Temperature: 0°C VP ≈ 4.58 mmHg (0.611 kPa) At 0°C, the saturation vapor pressure is very low, which is why cold air feels dry and frost forms. Example 5: Hot Day (40°C) Temperature: 40°C VP ≈ 55.3 mmHg (7.37 kPa) On a hot day, the air can hold over 12 times more moisture than at freezing temperatures!

What is the vapor pressure of water and why does it matter?

Vapor pressure is the pressure exerted by water vapor in equilibrium with liquid water at a given temperature. It determines evaporation rates, boiling points, and humidity levels. At 100°C, water's vapor pressure reaches 1 atm (760 mmHg), which is why water boils at 100°C at sea level. This property is critical in meteorology, distillation, HVAC design, and chemical engineering.

Which vapor pressure formula is most accurate?

The Antoine equation is widely used for the 1-100°C range with high accuracy (error <0.1%). The Buck equation is preferred in meteorology for its simplicity and good accuracy from -40°C to +50°C. The Goff-Gratch formula is the most accurate over the widest range (-100°C to 200°C) and is adopted by the World Meteorological Organization, but it is more complex to compute.

How does altitude affect the vapor pressure of water?

Altitude does not directly change water's vapor pressure at a given temperature, but it lowers the atmospheric pressure. Water boils when its vapor pressure equals the surrounding atmospheric pressure. At 3000 m elevation (atmospheric pressure ≈ 525 mmHg), water boils at about 90°C because its vapor pressure reaches atmospheric pressure at a lower temperature.

What is the relationship between vapor pressure and relative humidity?

Relative humidity (RH) is the ratio of the actual water vapor pressure to the saturation vapor pressure at that temperature, expressed as a percentage. RH = (actual VP / saturation VP) × 100%. When RH reaches 100%, the air is saturated and condensation occurs (dew point). The saturation vapor pressure increases exponentially with temperature, which is why warm air can hold more moisture.

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