# Vapour pressure of water

The vapor pressure of water is the pressure at which water vapor is in thermodynamic equilibrium with its condensed state. At higher pressures water would condense. The water vapor pressure is the partial pressure of water vapor in any gas mixture in equilibrium with solid or liquid water. As for other substances, water vapor pressure is a function of temperature and can be determined with Clausius–Clapeyron relation.

Vapour pressure of water (0–100 Celsius)[1]
T(C) T(F) P(kPa) P(torr) P(atm)
0 32 0.6113 4.5851 0.0060
5 41 0.8726 6.5450 0.0086
10 50 1.2281 9.2115 0.0121
15 59 1.7056 12.7931 0.0168
20 68 2.3388 17.5424 0.0231
25 77 3.1690 23.7695 0.0313
30 86 4.2455 31.8439 0.0419
35 95 5.6267 42.2037 0.0555
40 104 7.3814 55.3651 0.0728
45 113 9.5898 71.9294 0.0946
50 122 12.3440 92.5876 0.1218
55 131 15.7520 118.1497 0.1555
60 140 19.9320 149.5023 0.1967
65 149 25.0220 187.6804 0.2469
70 158 31.1760 233.8392 0.3077
75 167 38.5630 289.2463 0.3806
80 176 47.3730 355.3267 0.4675
85 185 57.8150 433.6482 0.5706
90 194 70.1170 525.9208 0.6920
95 203 84.5290 634.0196 0.8342
100 212 101.3200 759.9625 1.0000

## Approximation formula

The vapor pressure of water may be approximated by the following relations (in order of increasing accuracy):

• $P = \exp\left(20.386-\frac{5132}{T}\right)\,\mathrm{mmHg}$
where P is the vapor pressure (mmHg) and T is the temperature in Kelvin.
• Using the Antoine equation
$\log_{10}P = A - \frac{B}{C + T}$
where the temperature T is in degrees Celsius and the vapor pressure P is in mmHg. The constants are given as
A B C Tmin, °C Tmax, °C
Water 8.07131 1730.63 233.426 1 99
Water 8.14019 1810.94 244.485 100 374

## Graphical pressure dependency on temperature

Vapour pressure diagrams of water; data taken from Dortmund Data Bank. Graphics shows triple point, critical point and boiling point of water.