Kilopascals to Pounds per square inch (kPa to psi)

The kilopascal (kPa) is exactly 1000 pascals, where one pascal equals one newton of force distributed over one square metre of area (1 Pa = 1 N/m²). The relationship is fixed by SI prefix and SI-derived-unit definition. Standard atmospheric pressure at sea level is exactly 101,325 Pa or 101.325 kPa. The kilopascal is the SI-canonical engineering pressure unit for the typical practical pressure ranges: tyre pressures (200-280 kPa typical car, 300-700 kPa typical truck), industrial pneumatic pressures (400-800 kPa typical shop air), HVAC and refrigeration pressures (100-2000 kPa across cycle), hydraulic pressures (typically reported in MPa rather than kPa for the higher ranges). The recognised SI symbol is "kPa", with the lowercase "k" SI prefix and the uppercase "Pa" honoring the unit's namesake. The closely-related unit hectopascal (hPa) at exactly 100 Pa is the meteorological standard for atmospheric pressure reporting, with sea-level pressure typically 1013.25 hPa or 101.325 kPa.

The kilopascal is the SI multiple-of-pascal pressure unit that became the standard everyday SI pressure scale through the 1960s and 1970s as countries metricated and engineering practice migrated from millimetres-of-mercury, technical-atmosphere and bar to a clean SI-prefix-based unit. The pascal itself was named after Blaise Pascal at the 14th CGPM in 1971, formalising the SI derived unit equal to one newton per square metre (1 Pa = 1 N/m²); the kilopascal as the everyday-engineering multiple was a natural consequence of that adoption because typical practical pressures fall in awkward Pa ranges (atmospheric pressure is 101,325 Pa, which is much more readable as 101.325 kPa). The unit gained particular traction in continental European and Australian/NZ tyre-pressure conventions, where modern car-tyre pressure placards print "230 kPa" rather than the bar (2.3 bar) or psi (33 psi) figure. ISO 80000-4 standardises kPa as the default SI everyday-engineering pressure unit, and aviation-meteorology under ICAO Annex 3 uses kPa for some altimeter-setting cross-references alongside hPa and inHg.

European and Australian-NZ automotive tyre-pressure placards: modern car door-jamb tyre placards print kPa as the primary pressure unit, with bar and psi sometimes alongside. A typical small-car cold tyre pressure is 220-240 kPa; SUV and large-car pressures range 230-290 kPa. The kPa convention is mandatory under EU Regulation 661/2009 type-approval requirements, with placards required in at least kPa with optional secondary units. EU industrial pneumatic and hydraulic equipment specs: shop-air system design pressures, pneumatic-tool ratings and small-hydraulic-cylinder specs in EU-manufactured equipment use kPa for the typical 400-800 kPa working-pressure range, with MPa reserved for higher hydraulic pressures (5-30 MPa) above the kPa-readable scale. EU and Australian-NZ HVAC and refrigeration: refrigerant operating-cycle pressures across compression and condensation phases range typically 100-2000 kPa, with the kPa scale legibly spanning the full operating envelope without resorting to MPa. ASHRAE-based US documentation typically uses psia or psig instead, with the kPa-to-psi conversion running at every cross-jurisdictional refrigeration-system spec translation. Aviation meteorology: ICAO altimeter-setting conventions use hPa primarily but kPa appears in cross-disciplinary engineering documentation and high-altitude pressure references. Building-services engineering: water-supply pressure, drainage-system head pressure and HVAC duct static pressure all run at the kPa scale in EU-jurisdiction building-services design.

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.