Vapour Pressure of Water at 20 °C
| Value | 2,339 Pa |
| Status | Measured: ± 3 Pa (0.0013 relative) |
| Source | IAPWS-IF97 / Crane TP-410 |
| Categories | Material PropertiesEngineering & TradeFluid Properties |
| microbar | 23,390 μbar |
| barye | 23,390 Ba |
| millitorr | 17,543.943 mTorr |
| pascal | 2,339 Pa |
| millimeter of water column | 238.51162 mmH₂O |
| pound per square foot | 48.851031 lb/ft² |
| centimeter of water column | 23.851162 cmH₂O |
| hectopascal | 23.39 hPa |
| millibar | 23.39 mbar |
| torr | 17.543943 Torr |
| millimeter of mercury | 17.54394 mmHg |
| inch of water column | 9.3902214 in w.g. |
| kilopascal | 2.339 kPa |
| foot of water column | 0.78251845 ft H₂O |
| inch of mercury | 0.69070631 inHg |
| pound per square inch | 0.33924327 psi |
| meter of water column | 0.23851162 m H₂O |
| decibar | 0.2339 dbar |
| kilogram-force per square centimeter | 0.023851162 kgf/cm² |
| bar | 0.02339 bar |
| atmosphere | 0.023084135 atm |
| megapascal | 0.002339 MPa |
| kip per square inch | 0.00033924327 ksi |
| gigapascal | 0.000002339 GPa |
| million pounds per square inch | 3.3924327e-07 Mpsi |
Water at any temperature
0 to 100 °CThe value above is one point on this curve. Liquid water from 0 to 100 °C at 1 atm — density, viscosity, specific heat, thermal conductivity and vapour pressure, all at the same state.
Learning zone
Water boils whenever the local absolute pressure falls to its saturation vapour pressure, and that pressure is a steep function of temperature: 872 Pa at 5 °C, 2339 at 20 °C, 7384 at 40 °C, 47.4 kPa at 80 °C and 101.325 kPa at 100 °C. Roughly, it doubles every 10 °C in the ordinary range. Nothing about boiling requires 100 °C — that is simply where the curve crosses one atmosphere.
This is the term that decides whether a pump cavitates. NPSH available subtracts vapour pressure from the absolute pressure at the suction, so hot water eats the margin fast: pumping 80 °C condensate leaves only about 54 kPa of atmospheric pressure to work with before the suction lift and friction are even counted, which is why boiler feed pumps are flooded-suction and why condensate pumps have such generous inlet piping. Vapour pressure also governs the vacuum side of siphons and priming lines, and sets the practical suction-lift limit of any pump well below the theoretical 10.3 m of water column.