Convert Atmospheres to Millibars

Formula

mbar = atm × 1013.25

Derivation from SI units:

• 1 atm = 101 325 Pa (exact, by definition)
• 1 mbar = 10⁻³ bar = 100 Pa = 1 hPa
• 1 atm = 101325 Pa ÷ 100 Pa per mbar = 1013.25 mbar

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

Worked examples

1. A reading of 1.5 atm converts to 1519.875 mbar.
   1.5 atm × 1013.25 = 1519.875 mbar
2. A value of 10 atm is 10132.5 mbar.
   10 atm × 1013.25 = 10132.5 mbar

Where it's used

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

Atmosphere 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, "atmosphere, standard (atm)".
• 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".

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