Pressure Converter

Pressure is measured in more incompatible units than almost any other quantity. Weather reports use millibars or inches of mercury. Car tires use PSI or bar. Scuba divers use bar or atm. Medical blood pressure uses mmHg. Industrial pipes use kPa. A tire pump that reads both PSI and bar is fine; a ventilator that mixes cmH₂O and mmHg is a safety incident waiting to happen.

The pascal (Pa) is the SI unit — one newton per square meter — but it's inconveniently small for most everyday uses. One atmosphere is 101,325 Pa, which is why the kilopascal (kPa) and hectopascal (hPa, identical to millibar) are more common. PSI comes from 'pounds-force per square inch' and is the US convention for tires and industrial systems. mmHg comes from old mercury manometers — a column of mercury 1 mm tall exerts about 133.3 Pa at the base, which is why blood pressure 'over 120' is 120 mmHg.

The 'torr' is a synonym for mmHg, named after Evangelista Torricelli (who invented the barometer in 1643). Scientists use torr for vacuum work; the medical world uses mmHg. They're the same unit with different labels.

PSI to bar: divide by 14.504. Bar to PSI: multiply by 14.504. 1 atm = 1.01325 bar = 14.696 PSI = 760 mmHg = 101.325 kPa. For tires: 2 bar = 29 PSI, 2.5 bar = 36 PSI, 3 bar = 44 PSI. These are the common spec ranges you see on tire placards.

For weather: standard atmospheric pressure at sea level is 1013.25 hPa (or mbar, or 29.92 inHg). A strong low-pressure system is below 980 hPa; hurricane eyewalls can drop below 900 hPa. High pressure (fair weather) sits around 1020-1030 hPa.

• Pascals
• Kilopascals
• Bar
• Atmospheres
• PSI
• mmHg (Torr)
• inHg

• Gauge vs absolute pressure. A tire pressure of 32 PSI (gauge) means 32 PSI above atmospheric, so 46.7 PSI absolute. Scientific and engineering contexts often work in absolute; tire pressure is gauge. Check which your spec means.
• PSI vs PSIG vs PSIA. PSIG = gauge, PSIA = absolute. Just 'PSI' usually means gauge, but not always.
• Bar and atm aren't identical. 1 bar = 100 kPa exactly; 1 atm = 101.325 kPa. They're close (0.3% off) but not the same. Scientific papers use atm; industrial specs usually use bar.
• Under-inflating tires by bar-to-PSI confusion. European cars often list tire pressure in bar on the door placard. An American tire pump set to '2.5' means 2.5 PSI — dangerously low. The correct conversion is 2.5 bar = 36 PSI.
• Scuba depth vs pressure. At 10 m depth in seawater, you're at 2 atm absolute (1 atm air + 1 atm water). This doubles the gas consumption rate — a full tank lasts half as long at 10 m as at the surface.

• Sea-level atmospheric pressure: 101,325 Pa = 1 atm = 14.696 PSI = 1013.25 hPa = 760 mmHg.
• Typical car tire (passenger): 32-36 PSI (2.2-2.5 bar).
• Bicycle road tire: 80-130 PSI (5.5-9 bar).
• Healthy blood pressure: 120/80 mmHg systolic/diastolic.
• Espresso machine extraction: 9 bar (130 PSI).
• Kitchen pressure cooker: 1 bar gauge above atmospheric (2 bar absolute).
• Mariana Trench bottom: ~1,100 bar (16,000 PSI) — more than 1,000 atmospheres.
• Hurricane Katrina (2005) peak low pressure: 902 hPa — 11% below sea-level normal.

• Check tire pressure cold. Driving warms the tire and raises pressure by 2-4 PSI. The door placard pressure is the cold pressure.
• Match the unit on your pressure gauge to the spec you're checking. Most US gauges are PSI; European gauges are bar. Don't mix.
• For altitude cooking, every 1,000 m of elevation drops atmospheric pressure by about 12%. Water boils at ~95°C in Denver, 90°C in Mexico City, 72°C at Everest base camp. Boiling times nearly double at 3,000 m.

Related: Force Converter · Flow Rate Converter · Density Converter.