When measuring pressure in objects like car tires, it's often useful to know by how much it exceeds atmospheric pressure. This difference between absolute pressure and atmospheric pressure is called gauge pressure. Unlike absolute pressure, gauge pressure can actually be negative. Some instruments like tire pressure gauges automatically measure gauge pressure; others like manometers can be used to calculate gauge pressure using a few simple rules.

Read the height of the two mercury columns on the manometer if you're measuring gauge pressure using this instrument. If you're working this problem as a quiz question for a physics class, these values will be given to you.

Subtract the height of the column on the left from the column on the right.

Calculate gauge pressure using the following equation: P = pgh, where p is the density of mercury (13500 kilograms per cubic meter), g is acceleration due to gravity (9.80 meters per second squared) and h is the difference in height in meters.

#### Tip

If you already know or have measured the absolute pressure, you can calculate gauge pressure by just subtracting atmospheric pressure. Atmospheric pressure is 101.3 kilopascals at sea level.