Pounds per square inch, or psi, is the English unit of measure for pressure. The pound serves as the measure of weight or force, and the square inch is a measure of area. Therefore, psi measures the amount of force in a given area, and force and area have an inverse relationship---as the force increases, pressure increases, whereas as the area increases, pressure decreases.
How PSI Is Used
Pounds per square inch indicates the amount of pressure imposed on an object, as well as the maximum amount of pressure an object can withstand. For example, a tire may be inflated to 26 psi, but the maximum pressure the tire may withstand is 40 psi. Furthermore, a concrete footing may carry a load of 2,500 psi, but the maximum pressure the concrete can hold is 3,000 psi.
English Units Related to PSI
Pounds per square inch is used in combination with other English units. For example, psig, or pounds per square inch gauge, is a measurement of pressure relative to the ambient atmosphere. The pressure exerted by the atmosphere at sea level is 14.7 psi. So, a tank pressure reading 200 psi at sea level is actually 214.7 psig. Another related unit is kilopounds per square inch, or ksi: 1 ksi equals 1,000 psi. Architects and engineers use this unit is used to calculate material stiffness versus stress.
Weight Versus Mass
A pound is often equated with a kilogram, which is a unit of mass; however, the pound measures weight. Weight is a force composed of mass times the acceleration of gravity.
Metric and Other Equivalent Units of Pressure
1 psi is equal to 6,894 pascals (Pa).
1 psi is equal to 0.06894 bars (bar).
1 psi is equal to 0.06805 atmospheres (atm).
1 psi is equal to 51.72 millimeters of Mercury (mmHg) or Torrs (Torr).
Real World Examples
Pounds per square inch may sound like an obscure engineering or scientific measurement, but it crops up in everday life. For example, a bike tire should usually be pumped to 80 psi; storms drop the atmospheric pressure below 14.7 psi. Scuba tanks are commonly filled to 2,500 psi. Furthermore, oil gauges measure the pressure of the oil in an automobile engine. If the oil pressure gets too low or high, it damages the engine.
- "Mechanical and Electrical Equipment for Buildings;" Stein, Benjamin; 1992
- Photo Credit pressure gauges image by laurent dambies from Fotolia.com
Definition of Concrete MPa
Concrete is a building material made from a mixture of cement, water, sand and gravel. Known for its strength and durability, concrete...
What Do PSA Numbers Mean?
Prostate specific antigen is produced in small amounts by cells in the prostate. This antigen, known as PSA, is often monitored by...
What Is the Definition of Differential Pressure?
Differential pressure is defined as the difference of pressure measurements between two points in a system. This measurement is significant in applications...
What Does PSI Mean in Court?
Most states have sentencing guidelines – specific frameworks within which the court must order punishment – that determine exactly what happens after...
What Is the Maximum Cold PSI for Tires?
Tire pressure is measured in pounds per square inch (PSI). When measuring air pressure in your tires, the tires should be cold,...
What is a PSI for Probation?
A pre-sentencing investigation, or PSI, helps the judge determine whether a person convicted of a crime goes to jail or is allowed...
How to Make 4000 PSI Concrete
Most people outside of the construction industry do not realize that concrete comes in various strengths. These strengths are gauged according to...
How to Know What PSI is Needed for an Air Compressor
Air compressors produce pounds per square inch, or PSI. The PSI a particular air tool requires is labeled on the packaging. The...
How to Convert Tire Pressure BAR to PSI
Tire pressures are stated by manufacturers in bars or pounds per square inch, or psi. Knowing the equivalent pressure of a bar...
How to Calculate PSI
Pounds per square inch (psi) is a unit of pressure most commonly associated with tire pressure for an automobile or bike tire....
How to Calculate Concrete PSI
The load-bearing capabilities of concrete mixtures are expressed in terms of how many pounds per square inch (PSI) it can support before...
What Is the Meaning of Retailing?
Retail companies make up a significant portion of American businesses. The companies not only create jobs, but also make a direct connection...