The pound per square inch (abbreviation: psi) or, more accurately, pound-force per square inch (symbol: lbf/in²),^[1] is a unit of measurement of pressure or of stress based on avoirdupois units. 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 6,895 pascals.

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 14.7 psi (101 kilopascals), 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 psig in a local atmospheric pressure at sea level (14.7 psi) 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 unit is pound per square inch differential (psid).

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.

• Inch of water: 0.036 psid
• Blood pressure – clinically normal human blood pressure (120/80 millimetre of mercury (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
• Railway air brakes or road brakes 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: 800–1,000 psig
• Full SCBA (self-contained breathing apparatus) for IDLH (non-fire) atmospheres: 2,216 psig
• Nuclear reactor primary loop: 2300 psi
• Full SCUBA (self-contained underwater breathing apparatus) tank overpressure (common): 3,000 psig
• Full SCBA (self-contained breathing apparatus) for interior firefighting operations: 4,500 psig
• Airbus A380 hydraulic system: 5,000 psig
• Land Rover Td5 diesel engine fuel injection pressure: 22,500 psi
• Ultimate tensile strength of ASTM A36 steel: 58,000 psi
• Water jet cutter: 40,000–100,000 psig

The conversions to and from SI are computed from exact definitions but result in a repeating decimal. ^[6][7]

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

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

As the pascal is a 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.

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