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A pressure gauge reading in psi (red scale) and kPa (black scale)

The pound per square inch or, more accurately, pound-force per square inch (symbol: psi or lbf/in² or lb_f/in² or lbf/sq in or lb_f/sq in)/ pressure per square inch is a unit of pressure or of stress based on avoirdupois units. It has been largely replaced by the pascal. It is the pressure resulting from a force of one pound-force applied to an area of one square inch:

Using unit conversion:

1 psi = ${\frac {1{\text{ lbf}}}{(1{\text{ in}})^{2}}}={\frac {4.4482216152605{\text{ N}}}{(0.0254{\text{ m}})^{2}}}$ ≈ 6894.757293168 N/m²

Therefore, one pound per square inch is approximately 6894.757 Pa.

Now converting the psi to standard atmospheres:

${\frac {1{\text{ psi}}}{6894.757293{\text{ Pa}}}}\cdot {\frac {1.01325{\text{ x}}\ 10^{5}{\text{ Pa}}}{1{\text{ atm}}}}$ ≈ 14.69594877551 psi/atm

Therefore, 1 atmosphere is approximately 14.696 pounds per square inch.

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 or pounds per square inch gage (psig), indicating that the pressure is relative to atmospheric pressure. For example a bicycle tire pumped up to 65 psi above atmospheric pressure, will have a pressure of 65 + 14.7 = 79.7 psia or 65 psig.

ksi

The kilopound per square inch is a scaled unit derived from psi, equivalent to a thousand psi.

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.

Magnitude

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In liquid: pressure per square inch (psi)

• Blood Pressure – Average human blood pressure (120/80): 2.32 psi/1.55 psi
• Water jet cutter: 40,000–100,000 psi
• Commercial jet airliner hydraulic pressure: 3000 psi
• Boost Pressure Provided By an Automotive Turbocharger(common):P_g = 6–15 psi
• Atmospheric pressure at sea level (standard): P_a = 14.7 psi
• Automobile tyre overpressure (common): P_g = 32 psi
• Bicycle tyre overpressure (common): P_g = 65 psi
• Workshop or garage air tools: P_g = 90 psi
• Air brake (rail) or Air brake (road vehicle) reservoir overpressure (common): 90 psi ≤ P_g ≤ 120 psi
• Road racing bicycle tire overpressure: P_g = 120 psi
• Steam locomotive fire tube boiler (UK, 20th century): 150 psi ≤ P_g ≤ 280 psi
• Union Pacific Big Boy steam locomotive boiler: 300 psi
• Natural gas pipelines: 800 to 1000 psi
• Full SCBA Self Contained Breathing Apparatus for toxic atmospheres: 2216 psi
• Full scuba tank overpressure (common): P_g = 3000 psi
• Airbus A380 hydraulic system: 5000 psi
• Ultimate strength of ASTM A36 steel: 58,000 psi
• Inch of water: 0.036 psi

See also

• Conversion of units: Pressure or mechanical stress
• Pressure: Units

References

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External links

• Pressure measurement primer
• Online pressure conversions