Convert Pressure

Gauge pressure measures pressure relative to atmospheric pressure (0 psi gauge = 14.7 psi absolute at sea level). Absolute pressure measures pressure relative to a perfect vacuum (0 psi absolute = perfect vacuum). Most pressure measurements in everyday use are gauge pressure. For example, a car tire at 30 psi gauge contains 44.7 psi absolute pressure.

Different pressure units evolved for different applications and regions. Engineers use pascals (SI standard), meteorologists use millibars/hectopascals, Americans use psi, Europeans often use bar or kgf/cm², and medical professionals use mmHg. Each unit provides convenient numbers for typical pressures in that field - tire pressure in bar gives numbers around 2, while in pascals it would be 200,000.

1 bar = 14.504 psi, so divide psi by 14.5 to get bar, or multiply bar by 14.5 to get psi. For example, 30 psi ÷ 14.5 = 2.07 bar. Many European cars recommend pressures around 2.2-2.5 bar, which equals 32-36 psi. Some regions also use kgf/cm²: 1 kgf/cm² ≈ 14.2 psi ≈ 0.98 bar.

mmHg (millimeters of mercury) measures how high mercury would rise in a column under that pressure. Normal blood pressure around 120/80 mmHg means the pressure could support mercury columns 120mm and 80mm high. This unit persists from early pressure measurement devices that used mercury columns. 1 mmHg = 133.3 pascals = 0.0193 psi.

Standard atmospheric pressure at sea level is defined as 101,325 pascals = 1013.25 mbar = 1013.25 hPa = 1.01325 bar = 14.696 psi = 29.92 inHg = 760 mmHg = 760 torr = 1.033 kgf/cm². This represents average pressure at sea level and is used as a reference for many measurements.

Atmospheric pressure decreases with altitude. At 5,000 feet (1,524m), pressure is about 83% of sea level. At 10,000 feet (3,048m), it's about 69%. At commercial aircraft cruising altitude (35,000 feet), it's only about 24% of sea level pressure. This is why aircraft cabins are pressurized to simulate lower altitudes for passenger comfort.

kPa (kilopascals) is metric, PSI (pounds per square inch) is imperial. For reference: tire pressure is typically 200-250 kPa (30-36 PSI), home water pressure is around 275-415 kPa (40-60 PSI), and espresso machines operate around 900 kPa (130 PSI). To convert: 1 PSI = 6.895 kPa, 1 kPa = 0.145 PSI.