Convert Millibar to Atmosphere (mbar → atm)

Converting millibars to atmospheres 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 millibars

   Start with the number of millibars (mbar) you want to convert.
2. 2
   Multiply by 0.000987

   The conversion factor from mbar to atm is 0.000987. Multiply your value by this number.
3. 3
   Read the result in atmospheres

   The result is your value in atmospheres (atm).

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

• ✓1 mbar = 1 hPa = 100 Pa.
• ✓Weather reports: "1013 hPa" and "1013 mbar" mean the same thing.
• ✓Hurricane strength correlates with central pressure drop.

• ✗Confusing mbar and bar — 1000× difference.
• ✗Using mbar for industrial pressures — too small.
• ✗Mixing hPa and mbar in casual writing without noting they are equal.

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

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