Pounds per square inch
Pound per square inch  

A pressure gauge reading in psi (red scale) and kPa (black scale)
 
Unit information  
Unit system  Imperial units, US customary units 
Unit of  Pressure, Stress 
Symbol  psi or lbf/in^{2} 
Named after  what doin 
Unit conversions  
1 psi in ...  ... is equal to ... 
SI units  6.894757 kPa 
The pound per square inch or, more accurately, poundforce per square inch (symbol: lbf/in^{2};^{[1]} abbreviation: psi) is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one poundforce applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 N/m^{2}.
Pounds 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, this will be added to any pressure reading made in air at sea level. The converse is pounds 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 psia) will have a pressure of 79.7 psia (14.7 psi + 65 psi).^{[2]}^{[3]} When gauge pressure is referenced to something other than ambient atmospheric pressure, then the units would be pounds per square inch differential (psid).
Multiples
The kilopound per square inch (ksi) is a scaled unit derived from psi, equivalent to a thousand psi (1000 lbf/in^{2}).
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.^{[4]}
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.^{[5]}
The conversion in SI Units is 1 Mpsi = 6.895 GPa, or 1 GPa = 0.145 Mpsi.
Magnitude
 Inch of water: 0.036 psid
 Blood pressure – clinically normal human blood pressure (120/80 mmHg): 2.32 psig/1.55 psig
 Natural gas residential piped in for consumer appliance; 4–6 psig.
 Boost pressure provided by an automotive turbocharger (common): 6–15 psig
 NFL football: 12.5–13.5 psig
 Atmospheric pressure at sea level (standard): 14.7 psia
 Automobile tire overpressure (common): 32 psig
 Bicycle tire overpressure (common): 65 psig
 Workshop or garage air tools: 90 psig
 Air brake (rail) or air brake (road vehicle) reservoir overpressure (common): 90–120 psig
 Road racing bicycle tire overpressure: 120 psig
 Steam locomotive fire tube boiler (UK, 20th century): 150–280 psig
 Union Pacific Big Boy steam locomotive boiler: 300 psig
 Natural gas pipelines: 800–1000 psig
 Full SCBA (selfcontained breathing apparatus) for IDLH (nonfire) atmospheres: 2216 psig
 Full SCUBA (selfcontained underwater breathing apparatus) tank overpressure (common): 3000 psig
 Full SCBA (selfcontained breathing apparatus) for interior firefighting operations: 4500 psig
 Airbus A380 hydraulic system: 5000 psig
 Ultimate strength of ASTM A36 steel: 58,000 psi
 Water jet cutter: 40,000–100,000 psig
Conversions
The exact conversions to and from SI are, by definition^{[citation needed]}:
1 psi = (4.4482216152605 / 0.00064516) Pa
1 Pa = (0.00064516 / 4.4482216152605) psi
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  Pounds per square inch  

(Pa)  (bar)  (at)  (atm)  (Torr)  (lbf/in^{2})  
1 Pa  ≡ 1 N/m^{2}  10^{−5}  ×10^{−5} 1.0197  ×10^{−6} 9.8692  ×10^{−3} 7.5006  0.000 145 037 737 730 
1 bar  10^{5}  ≡ 100 kPa
≡ 10^{6} dyn/cm^{2} 
1.0197  92 0.986  750.06  14.503 773 773 022 
1 at  066.5 98  665 0.980  ≡ 1 kgf/cm^{2}  0.967 841 105 354 1  735.559 240 1  14.223 343 307 120 3 
1 atm  325 101  25 1.013  1.0332  1  760  14.695 948 775 514 2 
1 Torr  133.322 368 421  0.001 333 224  0.001 359 51  1/760 ≈ 0.001 315 789  1 Torr
≈ 1 mmHg 
0.019 336 775 
1 lbf/in^{2}  6894.757 293 168  0.068 947 573  0.070 306 958  0.068 045 964  51.714 932 572  ≡ 1 lbf/in^{2} 
See also
References
 ^ IEEE Standard Letter Symbols for Units of Measurement (SI Units, Customary InchPound Units, and Certain Other Units), IEEE Std 260.1™2004 (Revision of IEEE Std 260.11993)
 ^ "Glossary of Industrial Air Cleaning Technology". United Air Specialists, Inc. Archived from the original on August 1, 2011.
 ^ "Gage v. Sealed v. Absolute pressure" (PDF). Dynisco.

^ "Tensile Strength of Steel and Other Metals". All Metals & Forge Group. Retrieved 20160726.
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 [multiplier] for thousands of pounds). The tensile strength of steel may also be shown in MPa, or megapascal.
 ^ An example of the use of Mpsi in mechanics for the elastic moduli of several materials
External links
 Pressure measurement primer
 Online pressure conversions
 ksi to psi conversions