Millibar to Atmospheres Converter

Millibar (mbar). A millibar is 100 pascals, identical to the hectopascal used in weather reports. Average sea-level pressure is about 1,013 millibars.

Atmospheres (atm). One atmosphere is exactly 101,325 pascals, the average air pressure at sea level. It is used for gas calculations and diving tables.

Bar, atmosphere and psi are exact definitions. Gauges and weather stations round heavily, so treat the last digit as indicative rather than precise.

Pressure units are all defined against the pascal, so the conversion is a multiplication by a fixed number. What makes this dimension awkward is that the numbers sit within tens of per cent of each other, so a small slip looks plausible.

Three of the units are close together on purpose: one bar is 100,000 pascals, one atmosphere is 101,325, and weather reports use millibars and hectopascals, which are the same size. A reading that looks like a unit error is often a genuine difference between gauge pressure and absolute pressure instead.

Checking a pressure conversion means converting through pascals and comparing with a known reference, such as the fact that a bar and an atmosphere differ by about 1.3%. The error to watch for is quoting a pressure without saying whether it is measured above atmospheric pressure or from a vacuum.