Pressure Units Made Simple: PSI, Bar, Pascal & Atmospheres
Updated: July 2026
Pressure units appear everywhere: tire gauges, weather forecasts, espresso machines and scuba tanks. Each field has its favorite unit, and knowing how they relate saves real mistakes — especially with tires.
The four units you actually meet
- PSI (pounds per square inch) — tires and pumps in the US/UK
- Bar — tires in Europe, espresso machines, diving
- Pascal (Pa) — the SI unit; weather uses hectopascals (hPa)
- Atmosphere (atm) — roughly the air pressure at sea level
The key relationships
1 bar = 14.5 PSI = 100,000 Pa ≈ 0.987 atm. Handy anchor: 1 bar is almost exactly one atmosphere, so "2 bar" in your tires means about twice sea-level air pressure. Weather maps show ~1013 hPa as standard pressure; below 1000 hPa usually means unsettled weather.
Tire pressure: the everyday case
A typical car tire runs 32 PSI ≈ 2.2 bar ≈ 220 kPa. Mixing these up is dangerous: inflating to "32" on a bar-calibrated pump would be more than ten times too much. Always check the unit on the gauge and the sticker inside your door frame, which usually lists both.
Espresso, diving and weather
Espresso machines brew at ~9 bar. Recreational scuba tanks hold 200-230 bar. Every 10 m of sea water adds about 1 bar of pressure, which is why dive tables and gauges are bar-based in most of the world. Meteorology sticks to hectopascals because 1 hPa = 1 millibar, preserving decades of weather records.
Frequently asked questions
How do I convert PSI to bar?
Divide PSI by 14.5. A 32 PSI tire is about 2.2 bar. Going the other way, multiply bar by 14.5.
What is normal atmospheric pressure?
About 1013 hPa (millibars) at sea level, equal to 1.013 bar or 14.7 PSI. Weather systems move this a few percent either way.
Why do espresso machines advertise 9 bar?
Nine bar — nine times atmospheric pressure — is the traditional extraction pressure for espresso. Machines advertising 15-20 bar refer to pump maximum, not brewing pressure.