PSI, Bar and Pascal: Making Sense of Pressure Units
Pressure is force spread over an area, and it has more units in daily use than almost anything else. The Pressure Converter handles pascals, bar, PSI and atmospheres.
Why nobody uses the official unit
A pascal is one newton per square metre, which is the correct SI unit and far too small to be convenient - atmospheric pressure is about 101,325 of them. So real use is in kilopascals, and weather forecasts in hectopascals, where 1,013 hPa is a normal day.
A bar is 100,000 pascals, chosen precisely because it is almost exactly atmospheric pressure. It is not an SI unit and it is used constantly in Europe for tyres, diving and industry, because "about one bar" means "about normal".
PSI is pounds per square inch, used in the US and the UK for tyres and plumbing. One bar is about 14.5 PSI, so the quick conversion is to multiply bar by 14.5 or divide PSI by 14.5.
An atmosphere is 101,325 pascals by definition - very slightly more than a bar, by about 1.3 per cent. Close enough that the two are used interchangeably outside precise work.
Gauge or absolute, and why it matters
This catches people more than any arithmetic. A tyre gauge reads zero when the tyre is flat, even though a flat tyre contains air at atmospheric pressure. It is measuring the difference from the surrounding air, which is gauge pressure.
Absolute pressure counts from a vacuum, so a flat tyre is one bar absolute and zero bar gauge. The gap between the two numbers is always one atmosphere, and mixing them up means being out by 14.5 PSI - which is most of a tyre.
Most practical gauges read gauge pressure. Weather pressure, and anything in physics or thermodynamics, is absolute.
Where you meet it
Car tyres run around 30 to 35 PSI, which is just over two bar. A bicycle tyre is far higher - a road tyre at 100 PSI is nearly seven bar. Mains water is typically two to four bar. A car tyre underinflated by a few PSI wears faster and uses more fuel, which is the most useful reason to own a gauge.
Weather: anything above 1,020 hPa is settled, below 1,000 is unsettled, and a rapid fall matters more than the number itself.
Common questions
How many PSI in a bar?
14.504. Multiplying by 14.5 is accurate enough for any tyre.
Why does a tyre read higher when warm?
Pressure rises with temperature. Check tyres cold, because a drive adds several PSI and inflating to spec when warm leaves them under-inflated.
Is a bar the same as an atmosphere?
Within about 1.3 per cent, so yes for practical purposes and no for engineering.
What pressure is a vacuum?
Zero absolute. A perfect vacuum is unreachable; a practical one is a very small fraction of a pascal.
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