Pressure Converter

How do you convert pressure units?

To convert pressure units, multiply by the starting unit's value in pascals, then divide by the target unit's value in pascals. Pressure is force divided by area, and every unit on this page has a fixed value in pascals (Pa), so the same two steps work for tires, weather and hydraulics.

For example, to convert 32 psi to bar: 32 × 6,894.757 = 220,632.23 Pa, then 220,632.23 ÷ 100,000 = 2.2063 bar. In practice you can skip the pascal step with a direct factor: multiply psi by 0.0689476 to get bar, or multiply bar by 14.5038 to get psi. The psi to bar converter and bar to psi converter apply these factors instantly, and the psi to kPa converter handles kilopascal placards.

Gauge pressure versus absolute pressure

Most gauges read gauge pressure, the pressure above the surrounding air, sometimes written psig. Absolute pressure (psia) adds the atmosphere on top. A tire reading 32 psi on its gauge is at about 32 + 14.7 = 46.7 psia at sea level. Tire, compressor and plumbing figures are gauge values; weather, altitude, vacuum and most gas-law calculations use absolute pressure. A unit converter changes the unit but not the reference point, so convert gauge to gauge and absolute to absolute.

What is the SI unit of pressure, and how do the units compare?

The pascal (Pa) is the SI unit of pressure, equal to one newton of force spread over one square meter. It is a small unit: a sheet of 80 g/m² office paper lying flat presses on a desk with only about 0.78 Pa, which is why everyday pressures are quoted in kilopascals, bar or psi. The BIPM SI Brochure lists the pascal as a derived unit, and NIST SP 811 gives the exact factors for the customary units.

Several of these are exact by definition. One bar is exactly 100,000 Pa, and the standard atmosphere is exactly 101,325 Pa. One psi is one pound-force on one square inch, which works out to 6,894.757293 Pa from the exact pound, inch and standard gravity. The Torr is exactly 1/760 of an atmosphere, while the millimeter and inch of mercury are conventional values based on a mercury column at 0 °C. Because pressure is force per area, the force converter and the area converter let you rebuild any of these from first principles.

How does air pressure change with altitude?

Air pressure falls as you climb: in the International Standard Atmosphere it drops from 101.325 kPa at sea level to about 89.9 kPa at 1,000 m, and to roughly half at 5,500 m. The standard atmosphere, published by ICAO and as ISO 2533, assumes 15 °C at sea level and a steady cooling with height, so real weather moves the figures by a few percent.

This is where gauge and absolute pressure part ways. A tire gauge measures the difference between the air inside and the air outside, so a tire filled at sea level reads about 2.6 psi higher at one mile up at the same temperature, even though no air has been added. Temperature changes the reading too, because warm air expands, which is why placard pressures are for cold tires; the temperature converter helps when a manual gives °C and your thermometer reads °F. Aircraft altimeters work the other way round, turning a pressure reading in inches of mercury or hectopascals into an altitude.

Where is each pressure unit used?

Psi dominates in the United States, bar and kilopascals in most other countries, and pascals and megapascals in science and engineering, while medicine and weather keep their own mercury-based units. The same tire can carry a placard in all three of the everyday units.

• PSI: tire pressure, air compressors, pressure washers and plumbing in the United States.
• Bar: tire pressure in Europe, espresso machines, scuba cylinders and industrial equipment. One bar is close to one atmosphere, which makes it intuitive.
• Kilopascals: tire placards in Canada, Australia and Japan. Weather maps use hectopascals, where 1 hPa = 0.1 kPa = 1 millibar.
• Megapascals: hydraulics, material strength and concrete ratings; the psi to MPa converter is common in engineering work.
• Atmospheres: chemistry, diving and gas laws, where 1 atm is standard sea-level pressure.
• mmHg and Torr: blood pressure is measured in mmHg (a reading of 120/80 mmHg is about 16.0/10.7 kPa), while vacuum systems use Torr. The two differ by less than one part in seven million.
• Inches of mercury: altimeter settings in US aviation and barometric pressure in US weather reports, where standard pressure is 29.92 inHg.

Pressure is force spread over area, so the force converter and area converter are useful when you work from first principles.