Pounds per square inch

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 (abbreviations: psi, lbf/in², lb_f/in², lbf/sq in, lb_f/sq in) 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:

1 psi = $\frac{1\text{ lbf}}{(1\text{ in})^2}$ ≈ $\frac{4.4482\text{ N}}{(0.0254\text{ m})^2}$ ≈ 6894.757 N/m²

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

Now converting the psi to standard atmospheres:

1 atm = $101325\text{ Pa}$ = $\frac{101325}{6894.757293168}$ ≈ 14.70 psi

Therefore, 1 atmosphere is approximately 14.7 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 local atmospheric pressure (say, 14.7 psia locally), will have a pressure of 65 + 14.7 = 79.7 psia or 65 psig. 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.

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.

The conversion in SI Units is 1 ksi = 6.895 MPa

The megapound per square inch (Mpsi) is an other multiple equivalent to a million of psi. It is used in Mechanics for the elastic modulus of the materials, especially for metals.

The conversion in SI Units is 1 Mpsi = 6.895 GPa

Magnitude

Main article: Orders of magnitude (pressure)

• Inch of water: 0.036 psi
• Blood pressure – clinically normal human blood pressure (120/80 mmHg): 2.32 psi/1.55 psi
• Natural gas residential piped in for consumer appliance; 4-6 psi.
• Boost pressure provided by an automotive turbocharger (common): P_g = 6–15 psi
• NFL football: P_g = 12.5–13.5 psi
• Atmospheric pressure at sea level (standard): P_a = 14.7 psi
• Automobile tire overpressure (common): P_g = 32 psi
• Bicycle tire 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 IDLH (non-fire) atmospheres: 2216 psi
• Full SCUBA (self-contained underwater breathing apparatus) tank overpressure (common): P_g = 3000 psi
• Full SCBA (self-contained breathing apparatus) for interior firefighting operations: 4500 psi
• Airbus A380 hydraulic system: 5000 psi
• Ultimate strength of ASTM A36 steel: 58,000 psi
• Water jet cutter: 40,000–100,000 psi

Conversions

Pressure units

	Pascal	Bar	Technical atmosphere	Standard atmosphere	Torr	Pounds per square inch
	(Pa)	(bar)	(at)	(atm)	(Torr)	(psi)
1 Pa	≡ 1 N/m2	10−5	1.0197×10−5	9.8692×10−6	7.5006×10−3	1.450377×10−4
1 bar	105	≡ 100 kPa ≡ 106 dyn/cm2	1.0197	0.98692	750.06	14.50377
1 at	9.80665×104	0.980665	≡ 1 kp/cm2	0.9678411	735.5592	14.22334
1 atm	1.01325×105	1.01325	1.0332	1	≡ 760	14.69595
1 Torr	133.3224	1.333224×10−3	1.359551×10−3	1.315789×10−3	≡ 1/760 atm ≈ 1 mmHg	1.933678×10−2
1 psi	6.8948×103	6.8948×10−2	7.03069×10−2	6.8046×10−2	51.71493	≡ 1 lbF /in2

See also

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

External links

• Pressure measurement primer
• Online pressure conversions