# Pound per square inch

> Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Pound_per_square_inch
> Markdown URL: https://mediated.wiki/source/Pound_per_square_inch.md
> Source: https://en.wikipedia.org/wiki/Pound_per_square_inch
> Source revision: 1322513661
> License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/)

{{short description|Unit of pressure or stress}}
{{Infobox unit
| bgcolor       = 
| name          = Pound per square inch
| image         = Psidial.jpg
| caption       = A [pressure gauge](/source/pressure_gauge) reading in psi (red scale) and [kPa](/source/kPa) (black scale)
| standard      = [Imperial units](/source/Imperial_units), [US customary units](/source/US_customary_units)
| quantity      = [Pressure](/source/Pressure), [stress](/source/stress_(physics))
| symbol        = psi
| symbol2       = lbf/in<sup>2</sup>
| namedafter    = 
| extralabel    = 
| extradata     = 
| units1        = [SI units](/source/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](/source/unit_of_measurement) of [pressure](/source/pressure) or of [stress](/source/stress_(mechanics)) based on [avoirdupois](/source/avoirdupois) units and used primarily in the United States. It is the pressure resulting from a [force](/source/force_(physics)) with magnitude of one [pound-force](/source/pound-force) applied to an area of one [square inch](/source/square_inch). In [SI units](/source/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](/source/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](/source/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](/source/materials_science), where the [tensile strength](/source/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](/source/mechanics) for the [elastic modulus](/source/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](/source/Inch_of_water): 0.036&nbsp;psid
* [Blood pressure](/source/Blood_pressure) – clinically normal human blood pressure (120/80 [millimetre of mercury](/source/millimetre_of_mercury) (mmHg)): 2.32&nbsp;psig/1.55&nbsp;psig
* [Natural gas](/source/Natural_gas) residential piped in for consumer appliance; 4–6 psig.
* Boost pressure provided by an automotive [turbocharger](/source/turbocharger) (common): 6–15&nbsp;psig
* [NFL football](/source/American_football_ball): 12.5–13.5&nbsp;psig
* [Atmospheric pressure](/source/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 brake](/source/Railway_air_brake)s or [road brakes](/source/air_brake_(road_vehicle)) reservoir overpressure (common): 90–120&nbsp;psig
* [Road racing bicycle](/source/racing_bicycle) tire overpressure: 120&nbsp;psig
* Steam locomotive [fire tube boiler](/source/fire_tube_boiler) (UK, 20th century): 150–280&nbsp;psig
* [Union Pacific Big Boy](/source/Union_Pacific_Big_Boy) [steam locomotive boiler](/source/Steam_locomotive): 300&nbsp;psig
* [US Navy](/source/United_States_Navy) steam boiler pressure: 800 psi
* [Natural gas](/source/Natural_gas) pipelines: 800–1,000&nbsp;psig
* Full [SCBA](/source/SCBA) (self-contained breathing apparatus) for [IDLH](/source/IDLH) (non-fire) atmospheres: 2,216&nbsp;psig
* [Nuclear reactor](/source/Nuclear_reactor) primary loop: 2300 psi
* Full [SCUBA](/source/Scuba_set) (self-contained underwater breathing apparatus) tank overpressure (common): 3,000&nbsp;psig
* Full [SCBA](/source/SCBA) (self-contained breathing apparatus) for interior firefighting operations: 4,500 psig
* [Airbus A380](/source/Airbus_A380) hydraulic system: 5,000&nbsp;psig
* [Land Rover](/source/Land_Rover) Td5 diesel engine fuel injection pressure: 22,500&nbsp;psi
* [Ultimate tensile strength](/source/Ultimate_tensile_strength) of [ASTM](/source/ASTM_International) [A36 steel](/source/A36_steel): 58,000&nbsp;psi<!--No suffix!-->
* [Water jet cutter](/source/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](/source/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](/source/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](/source/Conversion_of_units)
* [Pressure: Units](/source/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

---
Adapted from the Wikipedia article [Pound per square inch](https://en.wikipedia.org/wiki/Pound_per_square_inch) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Pound_per_square_inch?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.