What is a Bar?

The bar equals exactly 100,000 pascals (100 kPa) and is the everyday pressure unit in European tire pressures, hydraulic systems, and meteorology. The unit was chosen to be approximately equal to atmospheric pressure at sea level (1 bar = 0.987 atm). European cars have tire pressures of about 2.0–2.5 bar (29–36 psi), industrial hydraulic systems operate at 100–700 bar, and wine bottles are pressurized to about 6 bar internally for sparkling wines. Although technically not an SI unit, the bar is widely accepted for use with the SI system. Weather forecasts in some countries use 'millibar' (1 mbar = 1 hPa = 100 Pa) for atmospheric pressure. The bar relates to the pascal (100,000 Pa = 1 bar), the kilopascal (100 kPa = 1 bar), the psi (1 bar ≈ 14.504 psi), the atmosphere (1 atm ≈ 1.013 bar), and the mbar (1,000 mbar = 1 bar). It strikes a practical balance between scientific precision and everyday usefulness.

Visual reference for how the bar relates to other pressure units. Each row links to the full converter for that pair.

• European tyre pressure gauges
• Scuba-diving depth pressure
• Hydraulic and pneumatic systems

• ✓1 bar = 100 kPa = 0.987 atm = 14.5 psi.
• ✓Almost 1 atm — convenient approximation.
• ✓Tyre pressures: 2.0–2.5 bar typical car, 6–10 bar truck.

• ✗Assuming 1 bar = 1 atm exactly — 1.3% off.
• ✗Mixing bar with psi — need factor of 14.5.
• ✗Using bar for high-pressure work — switch to MPa.