Pressure is the amount of force acting on a specific amount of surface area. The force is typically measured in pounds and the surface area in square inches, making the units of pressure pounds per square inch or psi. If a 100-lb weight was placed on top of a block with a surface area of 10 in^{2}, the average weight distribution would be 10 lb for each of the square inches (100 ÷ 10), or 10 psi.

When atmospheric pressure is being measured, in addition to psi, other means of pressure measurement can be used. These include inches or millimeters of mercury, and millibars. Standard day atmospheric pressure is equal to 14.7 psi, 29.92 inches of mercury (“Hg), 760 millimeters of mercury (mm Hg), or 1013.2 millibars. The relationship between these units of measure is as follows:

1 psi = 2.04 “Hg

1 psi = 51.7 mm Hg

1 psi = 68.9 millibars

The concept behind measuring pressure in inches of mercury involves filling a test tube with the liquid mercury and then covering the top. The test tube is then turned upside down and placed in an open container of mercury, and the top is uncovered. Gravity acting on the mercury in the test tube will try to make the mercury run out. Atmospheric pressure pushing down on the mercury in the open container tries to make the mercury stay in the test tube. At some point these two forces (gravity and atmospheric pressure) will equal out and the mercury will stabilize at a certain height in the test tube. Under standard day atmospheric conditions, the air in a 1-in^{2} column extending all the way to the top of the atmosphere would weigh 14.7 lb. A 1 in^{2} column of mercury, 29.92 inches tall, would also weigh 14.7 lb. That is why 14.7 psi is equal to 29.92 “Hg. Figure 3-33 demonstrates this point.