|Pound per square inch|
|Unit system||Imperial units, US customary units|
|Unit of||Pressure, stress|
|1 psi in ...||... is equal to ...|
|SI units||6.894757 kPa|
The pound per square inch (abbreviation: psi) or, more accurately, pound-force per square inch (symbol: lbf/in2), is a unit of measurement of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force with magnitude of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 Pa.
The pound per square inch absolute (psia) is used to make it clear that the pressure is relative to a vacuum rather than the ambient atmospheric pressure. Since atmospheric pressure at sea level is around 14.7 psi (101 kilopascals), this will be added to any pressure reading made in air at sea level. The converse is pound per square inch gauge (psig), indicating that the pressure is relative to atmospheric pressure. For example, a bicycle tire pumped up to 65 psig in a local atmospheric pressure at sea level (14.7 psi) will have a pressure of 79.7 psia (14.7 psi + 65 psi). When gauge pressure is referenced to something other than ambient atmospheric pressure, then the unit is pound per square inch differential (psid).
The kilopound per square inch (ksi) is a scaled unit derived from psi, equivalent to a thousand psi (1000 lbf/in2).
ksi are not widely used for gas pressures. They are mostly used in materials science, where the tensile strength of a material is measured as a large number of psi.
The conversion in SI units is 1 ksi = 6.895 MPa, or 1 MPa = 0.145 ksi.
The megapound per square inch (Mpsi) is another multiple equal to a million psi. It is used in mechanics for the elastic modulus of materials, especially for metals.
The conversion in SI units is 1 Mpsi = 6.895 GPa, or 1 GPa = 0.145 Mpsi.
Main article: Orders of magnitude (pressure)
The conversions to and from SI are computed from exact definitions but result in a repeating decimal. 
As the pascal is a very small unit relative to industrial pressures, the kilopascal is commonly used. 1000 kPa ≈ 145 lbf/in2.
Approximate conversions (rounded to some arbitrary number of digits, except when denoted by "≡") are shown in the following table.
|Pascal||Bar||Technical atmosphere||Standard atmosphere||Torr||Pound per square inch|
|1 Pa||—||1 Pa = 10−5 bar||1 Pa = 1.0197×10−5 at||1 Pa = 9.8692×10−6 atm||1 Pa = 7.5006×10−3 Torr||1 Pa = 0.000145037737730 lbf/in2|
|1 bar||105||—||= 1.0197||= 0.98692||= 750.06||= 14.503773773022|
|1 atm||≡ 101325||≡ 1.01325||1.0332||—||760||14.6959487755142|
|1 Torr||133.322368421||0.001333224||0.00135951||1/760 ≈ 0.001315789||—||0.019336775|
A metal's yield strength and ultimate tensile strength values are expressed in tons per square inch, pounds per square inch or thousand pounds (KSI) per square inch. For example, a tensile strength of a steel that can withstand 40,000 pounds of force per square inch may be expressed as 40,000 PSI or 40 KSI (with K being the denominator for thousands of pounds). The tensile strength of steel may also be shown in MPa, or megapascal.