Vapor Pressure of Water Calculator
Enter a temperature to get the saturation vapor pressure of water instantly. The calculator runs five established formulas side by side - Buck (most accurate), Magnus, Tetens, Antoine, and a simple exponential - so you can compare methods. Switch between Celsius and Fahrenheit for the temperature input and choose your preferred pressure unit for the output. A reference table and vapor pressure curve are included below.
What is vapor pressure of water?
Vapor pressure is the pressure exerted by water vapor in thermodynamic equilibrium with liquid water (or ice) in a closed system. It depends on temperature only: the higher the temperature, the more molecules have enough kinetic energy to escape the liquid surface, so the higher the vapor pressure. At the boiling point - 100 C at sea level - the vapor pressure equals atmospheric pressure (101.325 kPa), and bubbles can form throughout the liquid. Above sea level, atmospheric pressure is lower, so water boils below 100 C; in a pressure cooker, atmospheric pressure is raised, so the boiling point increases above 100 C.
The five formulas compared
This calculator implements five widely used empirical formulas. The Buck equation (1981) is the most accurate in the 0-100 C range, matching NIST tabulated data to within 0.04% at all checked temperatures. The Magnus formula (also called the Magnus-Tetens or Alduchov-Eskridge version) is simpler and very common in meteorology; it is accurate to within about 1% for the 0-100 C range but drifts near 100 C. The Tetens formula is slightly less accurate than Magnus but is sometimes preferred in numerical weather models. The Antoine equation, from 1888, uses three empirical constants that change depending on the temperature range; the two-range version here covers -20 C to 374 C. The simple exponential is the least accurate of the five but is useful for quick hand calculations. For most scientific and engineering purposes in the 0-100 C range, the Buck formula is the recommended choice.
How vapor pressure affects everyday life
Vapor pressure drives evaporation, boiling, humidity, and atmospheric science. Relative humidity is the ratio of actual water vapor pressure to the saturation vapor pressure at the current temperature: when they are equal (100% RH), dew forms or rain falls. The vapor pressure deficit (VPD), used in plant science and HVAC engineering, is the difference between saturation pressure and actual vapor pressure - a high VPD means dry air that draws moisture rapidly from leaves and skin. Altitude cooking is another direct application: at 3,000 m elevation the atmospheric pressure is roughly 70 kPa, which equals the vapor pressure of water near 90 C, so water boils at about 90 C rather than 100 C, requiring longer cooking times. Autoclave sterilization uses elevated pressure to raise the boiling point above 120 C, killing heat-resistant spores.
Using this calculator for humidity and dew point
To find the dew point, calculate the temperature at which the actual vapor pressure (partial pressure of water vapor in the air) equals the saturation vapor pressure. For example, if a room at 25 C has a relative humidity of 60%, the actual vapor pressure is 0.60 * 3.169 kPa = 1.901 kPa. The dew point is the temperature where the saturation pressure equals 1.901 kPa - you can find that by adjusting the temperature input until the output matches 1.901 kPa, or use a dedicated dew-point calculator. Meteorologists use the Magnus formula for this purpose because it can be analytically inverted to give dew point directly from RH and temperature.
Saturation vapor pressure of water at key temperatures
| Temperature (C) | Temperature (F) | Pressure (kPa) | Pressure (torr) | Pressure (atm) |
|---|---|---|---|---|
| 0 | 32 | 0.6113 | 4.585 | 0.006 |
| 5 | 41 | 0.8726 | 6.545 | 0.0086 |
| 10 | 50 | 1.2281 | 9.212 | 0.0121 |
| 15 | 59 | 1.7056 | 12.793 | 0.0168 |
| 20 | 68 | 2.3388 | 17.542 | 0.0231 |
| 25 | 77 | 3.169 | 23.77 | 0.0313 |
| 30 | 86 | 4.2455 | 31.844 | 0.0419 |
| 35 | 95 | 5.6267 | 42.204 | 0.0555 |
| 40 | 104 | 7.3814 | 55.365 | 0.0728 |
| 45 | 113 | 9.5898 | 71.929 | 0.0946 |
| 50 | 122 | 12.344 | 92.588 | 0.1218 |
| 60 | 140 | 19.932 | 149.502 | 0.1967 |
| 70 | 158 | 31.176 | 233.839 | 0.3077 |
| 80 | 176 | 47.373 | 355.327 | 0.4675 |
| 90 | 194 | 70.117 | 525.921 | 0.692 |
| 100 | 212 | 101.32 | 759.963 | 1 |
Values are from NIST/Lide tabulated data. Pressures in kPa, torr (= mmHg), and atm. The Buck formula reproduces these to within 0.04% in the 0-100 C range.
Frequently asked questions
What is the vapor pressure of water at 25 C?
At 25 C the saturation vapor pressure of water is approximately 3.169 kPa (23.77 mmHg or 0.0313 atm) according to NIST tabulated data. The Buck formula gives 3.1683 kPa, within 0.02% of the tabulated value. This is the pressure used as a standard reference in many thermochemical calculations.
What is the vapor pressure of water at 100 C?
At 100 C the vapor pressure is 101.325 kPa (1 atm, 760 mmHg). This is by definition the normal boiling point: when vapor pressure equals atmospheric pressure, liquid water throughout the bulk can form bubbles and boil. Above sea level, where atmospheric pressure is lower, the boiling point falls below 100 C.
Which formula should I use for meteorology?
The Magnus formula (Alduchov-Eskridge 1996 revision) is the most common in meteorology and humidity calculations because it can be analytically inverted to solve for dew point. The Buck formula is slightly more accurate but requires numerical inversion. For relative humidity and VPD work, either is suitable for the 0-40 C range typical of weather applications.
Does vapor pressure depend on anything besides temperature?
For pure water, vapor pressure depends only on temperature. Dissolved salts and solutes lower vapor pressure slightly (Raoult's law), which is why seawater evaporates a little more slowly than fresh water at the same temperature. Pressure of the gas above the liquid has essentially no effect on vapor pressure for incompressible liquids.
What happens to vapor pressure below 0 C?
Below 0 C, liquid water is unstable (or supercooled), and the equilibrium vapor pressure is the sublimation pressure of ice - vapor in equilibrium with solid water. This is always lower than the vapor pressure of supercooled liquid water at the same temperature, which is why ice tends to grow at the expense of supercooled droplets in mixed-phase clouds (the Bergeron-Findeisen process). The Buck and Tetens formulas have separate coefficient sets for ice.
How do I convert between pressure units?
1 kPa = 1000 Pa = 10 hPa = 7.5006 mmHg = 7.5006 torr = 0.009869 atm = 0.01 bar = 0.14504 psi. This calculator converts automatically when you select a different pressure unit from the dropdown.