# Pounds per square inch

Pounds per square inch or PSI is an imperial unit of pressure. Using the imperial units of pounds and square inches, it is a measure of force per unit area. Therefore, 1 PSI is measured as one pound of force applied per one square inch.[1] Since it is acting on a much smaller area than the pascal, 1 PSI is much more pressure than 1 pascal (metric unit). PSI is a non-[SI] unit, British unit that is now obsolete.[1]

Sometimes, PSI is specified on whether it is measuring gauge, absolute, or differential pressure.[2]

• Psid: when the difference of pressure is measured between two points in the environment. This is known as differential pressure.

PSI is often used for tire pressure and other situations where gauge pressure is being measured instead of absolute pressure.