Pounds per square inch

Pound per square inch
	A pressure gauge reading in psi (red scale) and kPa (black scale)
General information
Unit system	Imperial units, US customary units
Unit of	Pressure, Stress
Symbol	psi or lbf/in2
Conversions
1 psi in ...	... is equal to ...
SI units	6.894757 kPa

The pound per square inch or, more accurately, pound-force per square inch (symbol: lbf/in²;[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 pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 N/m².

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 (101 kilopascals), 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/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.[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
US Navy steam boiler pressure 800 psi
Natural gas pipelines: 800–1000 psig
Full SCBA (self-contained breathing apparatus) for IDLH (non-fire) atmospheres: 2216 psig
Nuclear reactor primary loop 2300 psi
Full SCUBA (self-contained underwater breathing apparatus) tank overpressure (common): 3000 psig
Full SCBA (self-contained 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 conversions to and from SI are computed from exact definitions but result in an inexact number that must be rounded at some point. [6][7]

$P_{\text{Pa}}=P_{\text{PSI}}\times {\frac {(0.45359237~{\text{kg}}\times 9.80665~{\text{m}}/{\text{s}}^{2})/{\text{lbf}}}{(0.0254~{\text{m}}/{\text{in}})^{2}}}$

$P_{\text{PSI}}=P_{\text{Pa}}\times {\frac {(0.0254~{\text{m}}/{\text{in}})^{2}}{(0.45359237~{\text{kg}}\times 9.80665~{\text{m}}/{\text{s}}^{2})/{\text{lbf}}}}$

As the pascal is 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.

Pressure units
	Pascal	Bar	Technical atmosphere	Standard atmosphere	Torr	Pound per square inch
	(Pa)	(bar)	(at)	(atm)	(Torr)	(lbf/in²)
1 Pa	≡ 1 N/m²	10⁻⁵	1.0197×10⁻⁵	9.8692×10⁻⁶	7.5006×10⁻³	0.000 145 037 737 730
1 bar	10⁵	≡ 100 kPa ≡ 10⁶ dyn/cm²	1.0197	0.98692	750.06	14.503 773 773 022
1 at	98066.5	0.980665	≡ 1 kgf/cm²	0.967 841 105 354 1	735.559 240 1	14.223 343 307 120 3
1 atm	≡ 101325	≡ 1.01325	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/in²	6894.757 293 168	0.068 947 573	0.070 306 958	0.068 045 964	51.714 932 572	≡ 1 lbf/in²

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References

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)
"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 2016-07-26. 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
BS 350: Part 1: 1974 – Conversion factors and tables. British Standards Institution. 1974. p. 49. ISBN 0 580 08471 X.
NIST Special Publication 811 – Guide for the Use of the International System of Units (SI) (PDF). National Institute of Standards and Technology. 2008. p. 66.

External links

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[1] 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)

[2] "Glossary of Industrial Air Cleaning Technology". United Air Specialists, Inc. Archived from the original on August 1, 2011.

[3] "Gage v. Sealed v. Absolute pressure" (PDF). Dynisco.

[4] "Tensile Strength of Steel and Other Metals". All Metals & Forge Group. Retrieved 2016-07-26. 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.

[5] An example of the use of Mpsi in mechanics for the elastic moduli of several materials

[6] BS 350: Part 1: 1974 – Conversion factors and tables. British Standards Institution. 1974. p. 49. ISBN 0 580 08471 X.

[7] NIST Special Publication 811 – Guide for the Use of the International System of Units (SI) (PDF). National Institute of Standards and Technology. 2008. p. 66.