How to Convert Units of Gas Pressure

Pressure is measured in many different units, including millimeters mercury (mm Hg), atmospheres (atm), Torr, Pascals , pounds per square inch (psi) and Bar. The conversion factors among these units are as follows:


A Torr is equal to mm Hg, named in order of Evangelista Torricelli, the creator of the first barometer.

Let’s do some sample problems converting among units.

Sample Problem

The weather news gives the atmospheric pressure as 1.07 atm. What is this atmospheric pressure in mm Hg?

Wanted: ? mm Hg

Given: 1.07 atm

To convert from atmospheres (atm) to mm Hg, we need the conversion factor, 1 atm = 760 mm Hg.

Conversion Factor: 1 atm = 760 mm Hg


1.07 atm is equivalent to 813 mm Hg. Notice that the answer is in the same number of significant figures as the given value of 1.07 atm.

Sample Problem

The atmospheric pressure in a certain location is 761.3 mm Hg. What is this pressure in Pa?

Wanted: ? Pa (pascals)

Given: 761.3 mm Hg

Conversion Factor: We need to go from mm Hg to Pa, so we use the conversion factor 760 mm Hg = 101325 Pa.


761.3 mm Hg is equivalent to 1.015 x 105 Pa.

Sample Problem

The air pressure inside a submarine is 0.62 atm. What would be the height of a column of mercury balanced by this pressure?

The height of a column of mercury is measured in mm Hg (millimeters is a measure of height). Therefore, we need to convert pressure from atm to mm Hg.

Wanted: ? mm Hg

Given: 0.62 atm

Conversion Factor: 1 atm = 760 mm Hg


0.62 atm is equivalent to 470 mm Hg.

Sample Problem

Determine the gas pressure in atm in the open-end manometer below.


Recall that, to read a manometer, when the side open to the air is lower than the side open to the gas, the air pressure exceeds the gas pressure. The difference in height between the two columns tells the difference in pressure between the two.

For more help on how to read a manometer, click here.

Since the air pressure is 0.960 atm, and the air pressure is greater than the gas pressure by 45 mm Hg, the gas pressure will be 0.960 atm – 45 mm Hg. Since the problem asks for the gas pressure in units of atmospheres, we need to first convert 45 mm Hg to atm.

Wanted: ? atm

Given: 45 mm Hg

Conversion Factor: 1 atm = 760 mm Hg


Now that we’ve converted 45 mm Hg to 0.059 atm, we can finish answering the problem.

Gas Pressure = Air Pressure – 0.059 atm.

Gas Pressure = 0.960 atm – 0.059 atm = 0.901 atm.

The pressure of the gas is 0.901 atm.