Pounds per square inch to Megapascals (psi to MPa)

One pound per square inch (psi) is the pressure exerted by a force of one pound-force (lbf) acting on an area of one square inch. By substitution from the 1959 International Yard and Pound Agreement values for the pound and the inch, and using standard gravity (9.80665 m/s²) for the conversion of pound-mass to pound-force, one psi equals exactly 6,894.757293168 pascals — typically rounded to 6,894.76 Pa or 6.895 kPa in engineering tables. The conversion to bar is 1 bar = 14.5037738 psi (or, going the other way, 1 psi ≈ 0.0689476 bar); to standard atmospheres 1 atm = 14.6959488 psi; to kilopascals 1 psi = 6.89476 kPa. Three closely related variants demand careful disambiguation in engineering writing: psia (pounds per square inch absolute) measures pressure relative to a perfect vacuum; psig (pounds per square inch gauge) measures pressure relative to local atmospheric pressure, so psig + ~14.696 = psia at standard sea-level conditions; and psid (pounds per square inch differential) measures the pressure difference between two points in a system. A tyre gauge reading "30 psi" is reporting psig — the actual absolute pressure inside the tyre is closer to 44.7 psia. Conflating absolute and gauge readings is one of the most common sources of engineering error when using the unit, particularly in thermodynamic calculations where the perfect-gas equation requires absolute pressure.

The pound per square inch is a compound unit, not a primitively defined one — it inherits its magnitude from the avoirdupois pound and the international inch via the 1959 International Yard and Pound Agreement, which fixed the pound at exactly 0.45359237 kilograms and the inch at exactly 0.0254 metres. No single treaty, statute or weights-and-measures act defines psi independently; the unit emerged from nineteenth-century engineering practice as steam power, hydraulics and pneumatics needed a working measure of force per area in the imperial system already standard in British and American workshops. The Bourdon-tube pressure gauge, patented in France in 1849 by Eugène Bourdon and rapidly adopted across Anglo-American steam engineering, was the instrument that put psi readings on the workshop wall; James Watt's earlier indicator diagrams had already established pressure-times-volume thinking in pounds and inches a century before. Through the late nineteenth and early twentieth centuries the American Society of Mechanical Engineers (founded 1880), the Society of Automotive Engineers (founded 1905) and the American Petroleum Institute consolidated psi as the working pressure unit across US industrial standards, and the unit was reinforced in practice by every industry that grew up around imperial fasteners, fittings and gauge faces. Capitalisation is conventional rather than rule-bound: engineering style guides and ASME publications write the unit lower-case ("psi"), reflecting that the abbreviation stands for a descriptive phrase rather than a proper noun. Consumer-facing tyre gauges, air-compressor labels and hardware-store signage render it upper-case ("PSI"), reflecting the unit's split life as both a precision engineering quantity and a piece of everyday American vocabulary.

US automotive engineering is the consumer-facing centerpiece of psi. The Federal Motor Vehicle Safety Standard 138, promulgated by NHTSA and effective for all light vehicles sold in the United States since model year 2008, mandates tyre-pressure monitoring systems (TPMS) and specifies recommended cold-tyre inflation pressures in psi on the vehicle's door-jamb placard — typically in the 30–35 psi range for passenger cars and 35–40 psi for light trucks. The Society of Automotive Engineers (SAE) standards for hydraulic brake-fluid working pressures, fuel-system pressures and engine oil pressures are all denominated in psi, and every gas-station air pump in the United States reads in psi. US compressed-gas and pressure-vessel engineering: the Compressed Gas Association (CGA) cylinder standards, the Department of Transportation (DOT) cylinder specifications (DOT-3AA for steel high-pressure cylinders, DOT-4B for low-pressure refrigerant cylinders), and the ASME Boiler and Pressure Vessel Code all specify pressures in psi for the US market. A standard medical oxygen E-cylinder is rated at 2,200 psi service pressure; an industrial nitrogen K-cylinder runs at 2,640 psi; a typical home propane tank fills to about 200 psi at summer temperatures. US industrial hydraulics: Parker Hannifin, Eaton and Bosch Rexroth (in their North American product lines), together with the National Fluid Power Association, spec hydraulic pumps, valves, hoses and cylinders in psi for US-market documentation, with mobile-equipment hydraulics running 2,500–4,000 psi and aerospace hydraulic systems at 3,000 psi or 5,000 psi (the latter on newer fly-by-wire airframes for weight savings). The identical Parker product sold into Europe is catalogued in bar. US plumbing and water systems: the Uniform Plumbing Code and the International Plumbing Code, both adopted by US states and municipalities, specify residential water-supply pressures in psi (40–80 psi typical, with code-mandated pressure-reducing valves required above 80 psi). US HVAC refrigerant pressures — R-410A at about 118 psi suction and 418 psi discharge in a typical air-conditioning operating cycle — are specified in psi on every US-market refrigeration gauge manifold. Firearms: the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI), the US industry standards body, publishes maximum chamber pressures in psi for US-market cartridges (.308 Winchester at 62,000 psi, 9mm Luger at 35,000 psi, .223 Remington at 55,000 psi). The Permanent International Commission for the Proof of Small Arms (CIP), the European counterpart, publishes the corresponding pressures in bar or megapascals — which is why a SAAMI 9mm and a CIP 9×19 Parabellum are nominally the same cartridge with subtly different pressure specifications and slightly different proof-test methodologies. International scope: psi is essentially a US and US-influenced industrial unit. The UK retains psi alongside bar for tyre pressure on gas-station gauges and on the printed cards that come with bicycle pumps, but European automotive specifications, EU industrial machinery directives under the Pressure Equipment Directive 2014/68/EU, and most of the rest of the world denominate pressure in bar or kilopascals. The United States is the only major economy where consumer-facing pressure measurement is dominated by a single non-SI unit, and a US-market product simultaneously sold into Europe will typically carry both psi and bar markings on its label or gauge face.

The megapascal (MPa) is exactly 1,000,000 pascals or exactly 1000 kilopascals, where one pascal equals one newton of force distributed over one square metre of area. The relationship is fixed by SI prefix and SI-derived-unit definition, with the mega- prefix denoting exactly 10⁶. Standard atmospheric pressure at sea level (101,325 Pa) equals approximately 0.101 MPa — too small a figure for atmospheric reporting, where kPa or hPa is the natural unit. The megapascal is the SI-canonical engineering unit for higher-pressure-range applications: hydraulic-system working pressures (5-35 MPa typical), materials-strength testing (yield and ultimate strength 200-2000 MPa for engineering metals), high-pressure water-jet cutting (200-400 MPa), high-pressure die-casting and forging (50-300 MPa). The recognised SI symbol is "MPa", with the uppercase "M" SI prefix (mega-, 10⁶) and the uppercase "Pa" honouring Pascal. The case convention distinguishes M (mega, 10⁶) from m (milli, 10⁻³) — substituting one for the other gives a billion-fold error.

The megapascal emerged alongside the kilopascal as the SI multiple-of-pascal pressure unit for the higher-pressure-range engineering applications where the kPa scale would produce unwieldy four-and-five-digit figures. The pascal was named at the 14th CGPM in 1971, and the megapascal as the everyday-engineering multiple for hydraulics, materials-strength testing and high-pressure industrial processes was an immediate consequence: a typical industrial hydraulic cylinder runs at 20-30 MPa (rather than 20,000-30,000 kPa or 200-300 bar), a structural-steel yield strength is reported in 250-450 MPa (rather than 250,000-450,000 kPa), and a high-pressure water-jet cutting system runs at 200-400 MPa. The MPa scale legibly spans the full higher-pressure-engineering envelope from hydraulic systems through materials-strength testing into ultra-high-pressure machining. ISO 80000-4 standardises MPa as the default SI engineering pressure unit for the high-pressure applications, and ISO 6892-1 (tensile-testing of metallic materials) denominates yield strength, ultimate tensile strength and elastic modulus in MPa or GPa for the structural-engineering reference values.

Hydraulic engineering: industrial-hydraulic-cylinder working pressures (20-30 MPa typical), excavator and construction-equipment hydraulics (25-35 MPa), aerospace-hydraulic systems (20-35 MPa), high-pressure laboratory hydraulic test systems (50-100 MPa). The MPa scale spans the full hydraulic-engineering envelope without resorting to higher-prefix multiples. Materials-strength testing: yield strength and ultimate tensile strength of structural metals are universally reported in MPa under ISO 6892-1 — typical structural steel 250-450 MPa yield, automotive HSLA steel 350-700 MPa, aerospace titanium alloys 800-1100 MPa, ultra-high-strength steels 1500-2000 MPa, advanced fibre composites 1000-3000 MPa. Elastic modulus runs in GPa (200 GPa for steel, 70 GPa for aluminium, 110 GPa for titanium). High-pressure machining: water-jet cutting systems run at 200-400 MPa working pressure, with abrasive water-jet at the upper end of that range. Cold-isostatic pressing for ceramics and metal-injection-moulded parts runs at 100-400 MPa. Concrete strength: concrete-cube compression-strength testing under EN 12390 reports cube strength in MPa, with typical structural-concrete grades C25/30 (25 MPa cylinder strength, 30 MPa cube strength) through C50/60 and high-strength concretes above 80 MPa. Polymer mechanical properties: tensile strength and flexural modulus of engineering plastics are reported in MPa under ISO 527 — typical engineering polymers 30-100 MPa tensile strength.