Converting pressure: psi, bar and atmospheres without guessing

Pressure shows up in psi on a tire gauge, in bar on a European pump, in hectopascals on a weather forecast, and in atmospheres in a dive table. All four are fixed multiples of one pascal (Pa), so converting between any two is the same move used for any other unit: route the number through pascals, then convert out.

The method

pressure in target unit = (pressure in source unit × pascals per source unit) ÷ pascals per target unit. The constants worth knowing: 1 bar = 100,000 Pa, 1 psi = 6,894.757 Pa, 1 atmosphere = 101,325 Pa (standard sea-level pressure), 1 mmHg = 133.322 Pa.

A real example: setting a tire to 2.2 bar, in psi

A tire's door sticker reads 2.2 bar; the gauge at hand only shows psi. Convert bar to pascals first: 2.2 × 100,000 = 220,000 Pa. Then divide by pascals-per-psi: 220,000 ÷ 6,894.757 = 31.908, so 2.2 bar is about 31.91 psi.

A real example: a weather report's 1,013 hPa, in atmospheres and psi

1,013 hectopascals is 1,013 × 100 = 101,300 Pa. In atmospheres: 101,300 ÷ 101,325 = 0.99975, essentially one atmosphere — which is exactly the point of "standard pressure," since average sea-level air pressure is close to 1,013 hPa. In psi: 101,300 ÷ 6,894.757 = 14.692, so that forecast's pressure is about 14.69 psi.

A real example: converting back from psi to bar

A compressor gauge reads 125 psi and a spec sheet wants bar. Convert to pascals: 125 × 6,894.757 = 861,844.625 Pa. Divide by pascals-per-bar: 861,844.625 ÷ 100,000 = 8.618, so 125 psi is about 8.62 bar. Multiplying to get into pascals and dividing to get out of them are inverse steps — swapping which one happens first is the usual mistake.

Gauge pressure versus absolute pressure

A tire gauge reads gauge pressure — pressure above the surrounding atmosphere, which is why a flat tire still reads zero rather than a negative number. Absolute pressure (what dive tables and most physics problems use) adds the local atmosphere back in: roughly gauge + 1 atmosphere at sea level. Converting units doesn't fix a gauge-versus-absolute mismatch — check which one a source means before trusting a converted figure.

Run your own numbers

The free Pressure Converter converts between pascals, kilopascals, bar, psi, atmospheres, mmHg and torr instantly, entirely on your device.