Pressure Converter

Pressure is force per unit area. The SI unit is the pascal (1 N/m²), but engineering and everyday measurements use bar, atmosphere, PSI, mmHg, and others depending on the field. Tire pressure runs in PSI in North America and bar in Europe; weather reports use millibar or hPa; medicine uses mmHg.

Enter a value in any unit and get equivalents across the others. Conversion factors: 1 bar = 100,000 Pa = 14.504 PSI = 0.987 atm = 750.06 mmHg.

The defining unit, the pascal, is small in everyday terms — atmospheric pressure at sea level is about 101,325 Pa. Most working units are scaled to keep numbers readable. The bar is exactly 100 kPa, chosen because it sits within 1 percent of standard atmosphere. PSI (pound-force per square inch) is the imperial workhorse, defined via the pound-force (4.4482 N) and the inch (0.0254 m). The atmosphere is now defined as exactly 101,325 Pa — originally an empirical sea-level average, now a fixed convention. Millimeters of mercury and inches of mercury come from barometers reading the height of a mercury column the gas pressure can support; they're effectively obsolete outside medicine and aviation altimetry but persist for backward compatibility.

A worked example. A car tire spec reads 32 PSI cold. Converting: 32 × 6,894.76 Pa/PSI = 220,632 Pa = 2.21 bar = 2.18 atm = 1,654 mmHg. A European tire placard listing 2.2 bar is recommending essentially the same pressure. Note the units describe gauge pressure (above atmospheric), so absolute pressure inside the tire is roughly 3.2 bar.

Limitations and pitfalls. The biggest one is gauge versus absolute. PSIG measures pressure above local atmosphere; PSIA measures total pressure including atmosphere. A bicycle pump reads 0 PSI in open air because it's reporting gauge; a vacuum spec at 'absolute zero' means PSIA = 0. Specs that don't say which usually mean gauge in industrial contexts and absolute in scientific ones. Altitude affects atmospheric pressure by about 12 mbar per 100 m near sea level. A tire properly inflated to 32 PSI gauge in Denver holds slightly less absolute pressure than the same gauge reading in Miami, but the gauge auto-corrects so the user doesn't notice. Most pressure measurements that 'seem off' come from confusing the two reference frames or mixing gauge readings across altitudes.