Vapor pressure of H2O at various temperatures (celsius)
| Temp (C) | Pressure (mm Hg) |
| -10 | 2.1 |
| -5 | 3.2 |
| 0 | 4.6 |
| 5 | 6.5 |
| 10 | 7.0 |
| 15 | 9.2 |
| 20 | 17.5 |
| 22 | 19.8 |
| 25 | 23.8 |
| 30 | 31.8 |
| 35 | 42.2 |
| 40 | 55.3 |
| 50 | 92.5 |
| 60 | 149.4 |
| 70 | 233.7 |
| 80 | 355.1 |
| 90 | 525.8 |
| 95 | 633.9 |
| 100 | 760 |
Modified from Nebergall et. al 1976
Note that when water vapor pressure equals atmospheric pressure, then the water molecules are free to jump into the gas state. The pressure "up" cancels the pressure "down" and boiling begins. You can boil at any temperature if you drop the pressure.
To find a value not on the chart you must interpolate. This can introduce some error since water vapor pressure in not a straight line graph.
Example: What is the water vapor pressure at 32C Use the chart entry closest to the number you want. 32C is closer to 30 than to 35. So set up a proportionality and solve for X.
__30C = __35C
31.8mm Hg X
X = 37.1 mm Hg (This is still an approximation!)