Convert Millibars to Atmospheres

Formula

atm = mbar × 0.000986923266716

or atm = mbar ÷ 1013.25

Derivation from SI units:

• 1 mbar = 10⁻³ bar = 100 Pa = 1 hPa
• 1 atm = 101 325 Pa (exact, by definition)
• 1 mbar = 100 Pa ÷ 101325 Pa per atm = 0.000986923266716 atm

Both units are defined exactly, so the factor is exact. Millibar definition: millibar; Atmosphere definition: atmosphere.

Worked examples

1. A reading of 20 mbar converts to 0.01973846533 atm.
   20 mbar × 0.0009869232667 = 0.01973846533 atm
2. A value of 900 mbar is 0.88823094 atm.
   900 mbar × 0.0009869232667 = 0.88823094 atm

Where it's used

Millibars are used for weather maps and barometers (sea level ≈ 1013 mbar), while atmospheres appear in chemistry and gas laws (STP and NTP conditions). Both measure pressure, stress and differential pressure (force per unit area), so the conversion is a fixed factor.

Millibar to other pressure units

Frequently asked questions

Sources

• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology , Appendix B.8, "millibar".
• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology , Appendix B.8, "atmosphere, standard (atm)".

Last reviewed: 2026-10-15. Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.