23.686
mmHg
3.1579
kPa
31.579
mbar
0.458
psi
23.686
mmHg
3.1579
kPa
31.579
mbar
0.458
psi
The Vapor Pressure of Water Calculator computes the saturation vapor pressure of water at any temperature between 0°C and 100°C using the Antoine equation with established constants. Results are provided in four common pressure units: mmHg (Torr), kPa, mbar (hPa), and psi. This specialized tool serves meteorologists calculating humidity, HVAC engineers designing climate systems, and chemistry professionals working with aqueous solutions.
Water vapor pressure is one of the most frequently looked-up thermodynamic values in science and engineering. It governs evaporation rates, humidity calculations, drying processes, and the behavior of steam systems.
This calculator uses the Antoine equation optimized for water in the 1-100°C range:
$$\log_{10}(P_{mmHg}) = 8.07131 - \frac{1730.63}{233.426 + T}$$
where \(T\) is the temperature in °C and \(P\) is in mmHg. The constants (A=8.07131, B=1730.63, C=233.426) are derived from the Stull reference data and are widely used in the NIST database.
The conversions to other pressure units are:
$$P_{kPa} = P_{mmHg} \times 0.133322$$
$$P_{mbar} = P_{mmHg} \times 1.33322$$
$$P_{psi} = P_{mmHg} \times 0.0193368$$
For meteorological applications, mbar (equivalent to hPa) is the standard unit. The Buck equation (1981) is an alternative that provides even higher accuracy over a wider range but uses a different functional form. The Antoine equation remains popular due to its simplicity and excellent accuracy within its validated range.
The vapor pressure of water at a given temperature represents the maximum pressure of water vapor that can exist in equilibrium with liquid water. When the partial pressure of water vapor in the air equals this value, the air is said to be saturated (100% relative humidity). Vapor pressure increases steeply with temperature — at 100°C it reaches 760 mmHg (1 atm), which is why water boils at that temperature under standard conditions.
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At 25°C, water's vapor pressure is about 23.8 mmHg (3.17 kPa). This means in a closed container at 25°C, the air above water would contain water vapor exerting about 3.2 kPa of pressure.
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At body temperature (37°C), the vapor pressure of water is about 47 mmHg. This is important in respiratory physiology — inspired air becomes fully saturated with water vapor in the lungs, and this vapor pressure must be accounted for in blood gas calculations.
At 20°C, the vapor pressure of water is approximately 17.5 mmHg (2.34 kPa, 23.4 mbar). This value is commonly used as a reference in humidity calculations and environmental science. It means that at 20°C, saturated air contains water vapor contributing 17.5 mmHg to the total atmospheric pressure.
Relative humidity (RH) is defined as RH = (e/e_s) × 100%, where e is the actual partial pressure of water vapor and e_s is the saturation vapor pressure at the current temperature. When RH = 100%, the air is saturated and e = e_s.
Water vapor pressure determines cloud formation, precipitation, and atmospheric stability. When air rises and cools, its saturation vapor pressure decreases. If the actual water vapor pressure exceeds the saturation value, condensation occurs, forming clouds and potentially rain.
The dew point is the temperature at which the current amount of water vapor in the air would become saturated. In other words, it is the temperature at which the actual vapor pressure equals the saturation vapor pressure. Cooling air to its dew point causes condensation (dew or fog).
Water boils when its vapor pressure equals the atmospheric pressure. At standard pressure (760 mmHg), this occurs at 100°C. Under vacuum (lower pressure), water boils at lower temperatures. In a pressure cooker (higher pressure), it boils above 100°C.
When collecting gases over water, the measured gas pressure includes water vapor. You must subtract the vapor pressure of water at the collection temperature to find the partial pressure of the dry gas: P_gas = P_total - P_water. This is essential for accurate gas law calculations.
The Antoine equation with the constants used here (A=8.07131, B=1730.63, C=233.426) is accurate to within about 0.1% for temperatures between 1°C and 100°C. For higher precision or temperatures outside this range, the Wagner equation or IAPWS-IF97 formulation is recommended.
At 0°C, water's vapor pressure is about 4.58 mmHg. Below 0°C, ice has its own (lower) vapor pressure. The difference in vapor pressure between supercooled water and ice at the same sub-zero temperature drives the Bergeron process in clouds, where ice crystals grow at the expense of water droplets.
The rate of evaporation depends on the difference between the water's vapor pressure and the partial pressure of water in the surrounding air. Higher temperature (higher vapor pressure) and lower humidity (lower partial pressure) both increase the driving force for evaporation and speed up drying.
At 100°C, the vapor pressure of water is approximately 760 mmHg (101.325 kPa, 1 atm). This is precisely why water's normal boiling point is defined as 100°C — it is the temperature at which the vapor pressure equals standard atmospheric pressure.
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