Pascals, psi and bar: pressure units explained
How the pascal, psi, kilopascal and bar relate, why the same pressure looks so different across units, and when to use each one.
The pascal is a small unit
The pascal is the SI unit of pressure, defined as one newton acting on one square metre. Because a newton is roughly the weight of a small apple and a square metre is a large surface, a single pascal is a tiny pressure. That is why everyday values run into the thousands or hundreds of thousands of pascals, and why the kilopascal (1000 Pa) shows up so often in weather reports and engineering. Standard atmospheric pressure is about 101,325 Pa, or 101.325 kPa.
Where psi comes from
Psi means pounds-force per square inch and is the everyday pressure unit across the United States. One psi is the pressure of a one pound-force load resting on one square inch, which works out to 6894.757 pascals. Car tyres are usually inflated to around 32 psi, or roughly 220 kPa. Because psi combines an imperial force with an imperial area, converting to metric always involves that fixed 6894.757 factor, which this calculator applies automatically when you switch units.
The bar and the atmosphere
The bar is a metric convenience unit equal to exactly 100,000 pascals, chosen because it sits very close to average sea-level air pressure. Meteorologists often work in millibars or the identical hectopascal. The atmosphere (atm) is a separate reference equal to 101,325 pascals. The tool does not expose bar or atm directly, but you can reach them easily: enter your pressure in pascals or kilopascals and divide by 100,000 for bar or by 101,325 for atmospheres.
Choosing the right unit for the job
Pick the unit your field already speaks. Scientists and most of the world use pascals and kilopascals; American mechanics and plumbers use psi; divers and some industrial gauges use bar. Keeping the whole problem in one system reduces mistakes, but when you must cross systems, let the converter carry the exact factors rather than rounding by hand. That way a tyre spec in psi and a load in newtons still produce a trustworthy answer.