{{short description|Unit of pressure or stress}} {{Infobox unit | bgcolor = | name = Pound per square inch | image = Psidial.jpg | caption = A pressure gauge reading in psi (red scale) and kPa (black scale) | standard = Imperial units, US customary units | quantity = Pressure, stress | symbol = psi | symbol2 = lbf/in<sup>2</sup> | namedafter = | extralabel = | extradata = | units1 = SI units | inunits1 = ≈ {{convert|1|psi|Pa|sigfig=7|disp=out|comma=gaps}} }} The '''pound per square inch''' (abbreviation: '''psi''') or, more accurately, '''pound-force per square inch''' (symbol: '''lbf/in<sup>2</sup>'''),<ref>IEEE Standard Letter Symbols for Units of Measurement (SI Units, Customary Inch-Pound Units, and Certain Other Units), IEEE Std 260.1™-2004 (Revision of IEEE Std 260.1-1993)</ref> is a unit of measurement of pressure or of stress based on avoirdupois units and used primarily in the United States. 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 {{convert|1|psi|kPa|sigfig=4|disp=out|abbr=off|lk=on}}.

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 {{convert|14.7|psi|kPa|lk=out|abbr=in}}, 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&nbsp;psig in a local atmospheric pressure at sea level (14.7&nbsp;psi) will have a pressure of 79.7&nbsp;psia (14.7 psi + 65 psi).<ref>{{cite web|url=http://www.uasinc.com/InfoGlossaryP.aspx |title=Glossary of Industrial Air Cleaning Technology |work=United Air Specialists, Inc |url-status=dead |archive-url=https://web.archive.org/web/20110801114928/http://www.uasinc.com/InfoGlossaryP.aspx |archive-date=August 1, 2011 }}</ref><ref>{{cite web | url = http://www.dynisco.com/stuff/contentmgr/files/1/162fe0347e667528bd874845e66a40ef/pdf/gageabspress.pdf | title = Gage v. Sealed v. Absolute pressure | work = Dynisco }}</ref> When gauge pressure is referenced to something other than ambient atmospheric pressure, then the unit is '''pound 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<sup>2</sup>).

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.<ref>{{cite web |url=http://www.steelforge.com/literature/metal-tidbits/tensile-strength/ |title=Tensile Strength of Steel and Other Metals |date=18 April 2013 |publisher=All Metals & Forge Group |access-date=2016-07-26 |quote=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.}}</ref>

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.<ref>{{Cite web |url=http://www.eitexam.com/Search2/Mechanics/PropertiesEq.asp |title=An example of the use of Mpsi in mechanics for the elastic moduli of several materials |access-date=2015-06-06 |archive-date=2016-12-16 |archive-url=https://web.archive.org/web/20161216151259/http://www.eitexam.com/Search2/Mechanics/PropertiesEq.asp |url-status=dead }}</ref>

The conversion in SI units is 1 Mpsi = 6.895 GPa, or 1 GPa = 0.145 Mpsi.

== Magnitude == {{main|Orders of magnitude (pressure)}}

* Inch of water: 0.036&nbsp;psid * Blood pressure – clinically normal human blood pressure (120/80 millimetre of mercury (mmHg)): 2.32&nbsp;psig/1.55&nbsp;psig * Natural gas residential piped in for consumer appliance; 4–6 psig. * Boost pressure provided by an automotive turbocharger (common): 6–15&nbsp;psig * NFL football: 12.5–13.5&nbsp;psig * Atmospheric pressure at sea level (standard): 14.7&nbsp;psia * Automobile tire overpressure (common): 32&nbsp;psig * Bicycle tire overpressure (common): 65&nbsp;psig * Workshop or garage air tools: 90&nbsp;psig * Railway air brakes or road brakes reservoir overpressure (common): 90–120&nbsp;psig * Road racing bicycle tire overpressure: 120&nbsp;psig * Steam locomotive fire tube boiler (UK, 20th century): 150–280&nbsp;psig * Union Pacific Big Boy steam locomotive boiler: 300&nbsp;psig * US Navy steam boiler pressure: 800 psi * Natural gas pipelines: 800–1,000&nbsp;psig * Full SCBA (self-contained breathing apparatus) for IDLH (non-fire) atmospheres: 2,216&nbsp;psig * Nuclear reactor primary loop: 2300 psi * Full SCUBA (self-contained underwater breathing apparatus) tank overpressure (common): 3,000&nbsp;psig * Full SCBA (self-contained breathing apparatus) for interior firefighting operations: 4,500 psig * Airbus A380 hydraulic system: 5,000&nbsp;psig * Land Rover Td5 diesel engine fuel injection pressure: 22,500&nbsp;psi * Ultimate tensile strength of ASTM A36 steel: 58,000&nbsp;psi<!--No suffix!--> * Water jet cutter: 40,000–100,000&nbsp;psig

== Conversions == The conversions to and from SI are computed from exact definitions but result in a repeating decimal.<ref>{{Cite book |title=BS 350: Part 1: 1974 – Conversion factors and tables |date=1974 |publisher=British Standards Institution |isbn=0-580-08471-X |page=49 }}</ref><ref>{{Cite book |url=https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication811e2008.pdf |title=NIST Special Publication 811 – Guide for the Use of the International System of Units (SI) |date=2008 |publisher=National Institute of Standards and Technology |page=66 }}</ref>

<math display=block>\begin{align} 1 \, \mathrm{lbf / in ^ 2} & = \frac{(0.453\,592\,37 \, \mathrm{kg} \times 9.806\,65 \, \mathrm{m / s ^ 2}) / \mathrm{lbf}}{(0.0254 \, \mathrm{m / in}) ^ 2} \, \mathrm{lbf / in ^ 2} \\ & = \frac{8896\,443\,230\,521}{1290\,320\,000} \, \mathrm{Pa} \\ & \approx 6894.757 \, \mathrm{Pa} \\ 1 \, \mathrm{Pa} & = \frac{1290\,320\,000}{8896\,443\,230\,521} \, \mathrm{lbf / in ^ 2} \\ & \approx 0.000\,145\,0377 \, \mathrm{lbf / in ^ 2} \\ 1 \, \mathrm{kPa} & \approx 0.\,145\,0377 \, \mathrm{lbf / in ^ 2} \\ \end{align}</math>

Approximate conversions (rounded to some arbitrary number of digits, except when denoted by "≡") are shown in the following table.

{{Pressure Units}}

== See also == * Conversion of units: Pressure or mechanical stress * Pressure: Units

== References == {{reflist}}

== External links == * [http://www.aeroconsystems.com/electronics/Pressure_transducer_basics/Transducer_primer.htm Pressure measurement primer] * [http://www.imperialtometric.com/conversion_en.htm Online pressure conversions]

{{Imperial units}} {{United States Customary Units}}

Category:Units of pressure Category:Customary units of measurement in the United States Category:Imperial units