pounds per square inch (PSI)

What is pounds per square inch (PSI)?

Pounds per square inch (PSI) is the pressure that results when a 1-pound force is applied to a unit area of 1 square inch. It is the measurement of pressure used in the imperial unit system of measurement.

PSI is commonly used to measure the pressure of gases, known as pneumatic pressure, or liquids, also called hydraulic pressure. PSI is also used as a measure of tensile strength, defined as resistance to pulling forces, and elastic modulus strength, defined as resistance to deformation, which controls the stiffness of materials.

In pneumatic and hydraulic pressure, PSI expresses the force the gas or liquid exerts on its containing vessel. In fluid pressure measurement, the use of PSI is relative to atmosphere. This is because PSI as measured by a PSI gauge is inherently measured as a differential balance against atmospheric pressure, generally being immersed in it. PSI measurement can also be absolute: relative to a vacuum or PSI absolute.

How is PSI used?

PSI is most commonly associated with tire pressure. It's also used to measure tensile strength in materials where thousands of PSI, or KPSI, are common. It's also used to measure elastic modulus of materials where millions of PSI, or MPSI, are common.

Gauges measuring pressure are used in all manner of instrumentation for vehicles, pneumatic and hydraulic machines, and industrial and safety systems. In compressed air power, a PSI gauge pressure tool is the equivalent to a fuel gauge.

Pascal and other metrics

The term pascal is used globally to describe a unit of pressure or force applied to an area. PSI is a measure of pressure commonly used in the U.S. Pa is the abbreviation for pascal.

A pascal is defined as 1 newton of force applied to 1 square meter of area with the force approaching the area at a right angle.

Mathematically, pressure is expressed with this formula:

Pressure (P) = Force (F) ÷ Area (A)

Translating this into pascals, the formula becomes the following:

1 pascal (Pa) = 1 newton (N) ÷ 1 square meter (m²)

A kilopascal (kPa) is calculated as follows:

1 kPa = 1,000 N ÷ 1 m²

As can be seen in the above formulas, pressure and surface area are the inverse of each other. This means objects with a smaller surface area, such as a pin or drill bit, can penetrate solid surfaces, such as a wall or concrete, with much less pressure than a hand or finger.

Standard temperature and pressure is another important metric that incorporates pressure. It refers to atmospheric conditions when at sea level and is usually measured as 14.7 PSI. It is typically used for measuring properties of gases. By contrast, when normal atmospheric conditions are not being used, a metric called normal temperature and pressure is used.

In chemistry, standard ambient temperature and pressure is the metric of choice. Two other standards for pressure and temperature include the International Standard Atmosphere and U.S. Standard Atmosphere.

PSI in other measurement systems

Other terms and units of measurement related to PSI and calculating pressure include the following:

• Pa = 6894.76 × PSI.
• Atmospheric pressure = PSI ÷ 14.6959.
• Technical atmospheres: 1 kilogram of force per centimeter squared = PSI ÷ 14.2233.
• Torr, the measurement used by the inventor of the barometer = PSI × 51.71493.
• Bar, a rounding-off of atmosphere based on 100,000 Pa = PSI ÷ 14.5038.

Standards organizations that define PSI

Numerous national and international standards organizations define PSI and other metrics for measuring pressure and stress. The following is a partial list of these standards groups:

United States

• National Institute of Standards and Technology.
• Environmental Protection Agency.
• Occupational Safety and Health Administration.
• Federal Aviation Administration.

International

• International Organization for Standardization.
• Organization of the Petroleum Exporting Countries.

Standards bodies define PSI and related terms differently and with different values. Because of that, it's important to define the application where PSI is being used and then check for the appropriate standard measurement. Calculators and conversion tables are available to make comparisons easier.

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