How to Determine Boiling Points with Pressure

By Athena Hessong; Updated April 24, 2017
Boiling point changes with pressure.

The boiling point of water decreases as you go higher in elevation. This is because there is less pressure from the atmosphere bearing down on top of the water, which allows the water to bubble and boil at lower temperatures. Cooking times are changed because of this decrease in boiling points since water boiling at a lower temperature means the food will cook at a lower temperature, and an increase in the time for cooking is needed. Determine the temperature at which water will boil based on the pressure the water is under.

Subtract the pressure at sea level in millimeters of mercury (mm Hg), 760 mm Hg, from the current pressure . For instance, if your pressure is 816 mm Hg: 816 - 760 = 56 mm Hg.

Divide this number by 28 mm Hg since each change of 28 mm Hg results in a roughly 1 degree C change in pressure. For this example: 56 mm Hg/28 mm Hg = 2.

Add this number to 100 degrees C to find the boiling point. For this example: 100 degrees C + 2 = 102 degrees C.


A current pressure lower than 760 mm Hg results in a negative number. Adding a negative number to 100 C leaves you with a number lower that 100 C.