Convert Atmosphere to Millibar (atm → mbar)

Converting atmospheres to millibars is straightforward: multiply by the conversion factor. Follow these three steps to do it by hand or in your head.

1. 1
   Take your value in atmospheres

   Start with the number of atmospheres (atm) you want to convert.
2. 2
   Multiply by 1,013.25

   The conversion factor from atm to mbar is 1,013.25. Multiply your value by this number.
3. 3
   Read the result in millibars

   The result is your value in millibars (mbar).

Multiply the value in atmospheres by 1,013.25. For the reverse direction, multiply by 0.000987.

• ✓1 atm = 101,325 Pa = 1.01325 bar = 14.696 psi.
• ✓Note: atm ≠ bar. Close but not identical.
• ✓Modern SI prefers pascals; atm is a legacy reference.

• ✗Using 1 atm = 1 bar exactly — off by 1.3%.
• ✗Confusing standard atm with technical atmosphere (98,066.5 Pa) or atmosphere-absolute in diving.
• ✗Applying 1 atm outside sea level without correction.

What is the Atmosphere?

The atmosphere equals exactly 101,325 pascals (the average atmospheric pressure at sea level, latitude 45°) and is a reference unit in chemistry, diving, and engineering. Defined for scientific convenience to represent 'standard atmospheric pressure,' it is widely used in chemistry (gas laws, reaction conditions), aviation (cabin pressure relative to ambient), and scuba diving (depth pressure: every 10 m of seawater adds about 1 atm). Standard reference conditions in chemistry often specify 1 atm and 25°C. The atmosphere relates to the pascal (101,325 Pa = 1 atm), the kilopascal (101.325 kPa = 1 atm), the bar (1.01325 bar = 1 atm), the psi (14.696 psi = 1 atm), the torr (760 torr = 1 atm), and the meter of seawater (10.33 mH₂O = 1 atm). The 'technical atmosphere' (1 at = 98.066 kPa = 1 kgf/cm²) is a slightly different historical unit no longer in use.

• Chemistry standard conditions
• Aviation cockpit pressurisation references
• Pressure tank and vessel ratings

What is the Millibar?

The millibar equals exactly 100 pascals and is the legacy meteorology unit, identical to the modern hectopascal (hPa). Most weather services worldwide transitioned from millibars to hectopascals in the late 20th century, but the unit persists in aviation weather reports, marine forecasts, and older barometric instruments. Standard sea-level atmospheric pressure is 1,013.25 mbar (or hPa). Hurricane and typhoon central pressures are reported in millibars: a major Category 5 hurricane like Hurricane Andrew (1992) had a central pressure of about 922 mbar. The millibar relates to the hectopascal (1 mbar = 1 hPa exactly), the bar (1,000 mbar = 1 bar), the pascal (100 Pa = 1 mbar), and the atmosphere (1,013.25 mbar = 1 atm). Despite the official preference for hPa, the millibar is retained in some traditional contexts and older texts.

• Weather reports and atmospheric pressure
• Altimetry and aviation
• Historical weather records