What this calculator does
Water vapour pressure is the pressure exerted by water vapour when air above a water surface is holding as much moisture as it can at a given temperature, a state called saturation. It rises steeply with temperature: air can hold far more water vapour at 30°C than at 0°C, which is why saturation vapour pressure is the starting point for humidity, evaporation and dew point calculations.
This calculator returns the saturation vapour pressure at any single temperature, using the Arden Buck equation, a close, widely used approximation to the exact thermodynamic relationship for water. It is closely related to but distinct from dew point, which works the other direction, solving for the temperature at which a given amount of moisture in the air would saturate.
The formula
The Arden Buck equation estimates saturation vapour pressure directly from temperature in degrees Celsius, using an exponential fit tuned to match measured values for water. The result in kilopascals is converted to hectopascals (equal to millibars), millimetres of mercury and psi for convenience, since different fields report vapour pressure in different units.
| Term | Meaning |
|---|---|
| Saturation vapour pressure | The maximum vapour pressure water can exert at a given temperature before condensation begins. |
| kPa / hPa | Kilopascals and hectopascals (millibars), the standard SI and meteorological pressure units. |
| mmHg | Millimetres of mercury, a pressure unit still common in some scientific and medical contexts. |
The inputs explained
| Field | What to enter |
|---|---|
| Water/air temperature (°C) | The air or water temperature. Valid from -40°C to 60°C, the range the Arden Buck correlation is fitted over. |
When to use it
Finding the starting point for a humidity calculation
Relative humidity is actual vapour pressure divided by saturation vapour pressure at the same temperature, so this figure is the denominator any humidity calculation needs first.
Estimating evaporation potential
A higher saturation vapour pressure at a given temperature means air can absorb more moisture before it is saturated, which is part of why evaporation speeds up sharply on hot days.
Checking a meteorology or physics textbook figure
Saturation vapour pressure values are commonly quoted in reference tables; this calculator reproduces those figures directly from temperature using the same underlying physics, for any temperature rather than only the ones a table lists.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How saturation vapour pressure rises with temperature
Saturation vapour pressure across a range of everyday temperatures.
| Temperature | Saturation vapour pressure | In hectopascals (millibars) |
|---|---|---|
| 0°C | 0.6112 kPa | 6.112 hPa |
| 10°C | 1.228 kPa | 12.279 hPa |
| 20°C | 2.338 kPa | 23.383 hPa |
| 30°C | 4.245 kPa | 42.451 hPa |
| 40°C | 7.382 kPa | 73.824 hPa |
| 50°C | 12.349 kPa | 123.494 hPa |
Questions
What is the difference between water vapour pressure and dew point?
Saturation vapour pressure is a function of temperature alone: it says how much pressure water vapour would exert if the air were fully saturated at that temperature. Dew point works in the other direction, taking the actual amount of moisture in the air and finding the temperature at which that amount would be enough to saturate it.
Why does the calculator reject temperatures above 60°C?
The Arden Buck equation used here is fitted to match measured water vapour pressure data over the range -40°C to 60°C. Outside that range the approximation is no longer reliable, so the calculator declines to return a figure rather than show a misleading one.
Is this the same as atmospheric pressure?
No. Atmospheric pressure is the total pressure of the whole air column, of which water vapour pressure is only one part, alongside the partial pressures of nitrogen, oxygen and other gases.
How accurate is the Arden Buck equation?
It matches published reference values for water vapour pressure to within about 0.1% across its valid range, which is why it is widely used in meteorology in preference to older, less accurate approximations.
To find the dew point temperature from air temperature and relative humidity instead, see the dew point calculator. For the viscosity of water itself at a given temperature, see the water viscosity calculator.