The formula
bar = atm × 1.01325
Where the factor comes from — each unit by its definition, then its size in SI:
1 atm = (standard atmosphere in Pa, CODATA 2022) × Pa = 101,325 Pa1 bar = 100 × kPa = 100,000 Pa
Dividing the two SI sizes gives the factor: 1.01325, exact — every step above is a definition, not a measurement. The reverse factor is 0.986923266716.
Pressure is force per area; the pascal is one newton per square metre. Tyres are quoted in psi or bar, weather in hPa, blood pressure in mmHg and gas laws in pascals or atmospheres.
Worked examples
- A car tyre (US gauge) (32 psi): 2.17747 atm = 2.20632 bar — 2.17747 × 1.01325 = 2.20632
- Sea-level air: 1 atm = 1.01325 bar — 1 × 1.01325 = 1.01325
- A car tyre (European gauge): 2.17123 atm = 2.2 bar — 2.17123 × 1.01325 = 2.2
- A systolic blood-pressure reading (120 mmHg): 0.157895 atm = 0.159987 bar — 0.157895 × 1.01325 = 0.159987
Frequently asked questions
How many bar is 1 atm?
1 atm = 1.01325 bar, exactly, from the unit definitions.
How do I convert bar back to standard atmospheres?
Divide by 1.01325 — or multiply by 0.986923266716: 1 bar = 0.986923266716 atm.