# Vapour pressure of water

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

## Approximation formula

The vapour pressure of water may be approximated by the following formula:

• $P = \exp\left(20.386-\frac{5132}{T}\right)\,\mathrm{mmHg}$

where P is the vapour pressure (mmHg) and T is the temperature in Kelvin. A more accurate formula, using the Antoine equation, is:

• $\log_{10}P = A - \frac{B}{C + T}$

where the temperature T is in degrees Celsius and the vapour pressure P is in mmHg. The constants are given (over two ranges of temperature) by

A B C Tmin, °C Tmax, °C
Water 8.07131 1730.63 233.426 1 100
Water 8.14019 1810.94 244.485 99 374

## 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.

## Tables of water vapour pressures

To convert kPa to mmHg multiply by 760/101.325. Many of the values in the table below are wrong due to vandalism.

Water vapour pressure in units of kPa and mmHg.[1]
Temperature
(°C)
Vapour
pressure
(kPa)
Vapour
pressure
(mmHg)
0 0.6 4.5
3 0.8 6.0
5 0.9 6.8
8 1.1 8.3
10 1.2 9.0
12 1.4 10.5
14 1.6 12.0
16 1.8 13.5
18 2.1 15.8
19 2.2 16.5
20 2.3 17.3
21 2.5 18.8
22 2.6 19.5
23 2.8 21.0
24 3.0 22.5
25 3.2 24.0
26 3.4 25.0
27 3.6 27.0
28 3.8 28.5
29 4.0 30.0
30 4.2 31.5
32 4.8 36.0
35 5.6 42.0
40 7.4 55.5
50 12.3 92.3
60 19.9 149.3
70 31.2 234.0
80 47.3 354.9
90 70.1 525.8
100 101.3 760.0
From ITS-90 formulations of water vapour pressure.[2]
ITS-90 Temperature
(°C)
Vapour
pressure
(Pa)
0 611.213
0.01 611.657
2 705.973
4 813.522
6 935.316
8 1072.944
10 1228.139
12 1402.782
14 1598.914
16 1818.747
18 2064.670
20 2339.262
22 2645.302
24 2985.777
26 3363.893
28 3783.090
30 4247.046
32 4759.694
34 5325.230
36 5948.122
38 6633.128
40 7385.299
42 8209.997
44 9112.902
46 10100.025
48 11177.719
50 12352.690
52 13632.007
54 15023.115
56 16533.844
58 18172.420
60 19947.476
62 21868.064
64 23943.660
66 26184.180
68 28599.985
70 31201.894
72 34001.191
74 37009.635
76 40239.470
78 43703.430
80 47414.751
82 51387.177
84 55634.969
86 60172.910
88 65016.317
90 70181.042
92 75683.482
94 81540.588
96 87769.863
98 94389.378
99.97436 101325.000
100 101417.770