Bar to millibar:
multiply by 1,000

How many millibar are in a bar?

One bar is exactly 1,000 millibar. The prefix milli means one thousandth, so a millibar is a thousandth of a bar and the conversion needs no rounding.

The millibar is the traditional unit on weather charts and barometers. In the metric system the same pressure is often written as hectopascals, because 1 millibar equals 1 hectopascal, or 100 pascals.

Bar to mbar formula and worked example

For the reverse, a barometer reading of 985 mbar is 985 ÷ 1,000 = 0.985 bar.

Chart: bar to millibar

Where bar to mbar is used

Weather forecasters quote air pressure in millibar. Standard sea-level pressure is 1,013.25 mbar, which is 1.01325 bar. A reading below about 1,000 mbar often means a low pressure system, and 985 mbar is 0.985 bar.

Gas engineers work with much smaller pressures. A domestic gas supply is commonly set to a few tens of millibar, which would be a tiny fraction of a bar. Writing 21 mbar instead of 0.021 bar avoids a long run of decimals.

More bar to mbar situations and a second worked example

Pilots meet millibar-sized numbers every flight. The altimeter pressure setting, known as QNH, is given in hectopascals in many countries, and because 1 hPa equals 1 mbar the number on the radio is a millibar figure. A setting of 1,013 is about 1.013 bar, close to standard sea-level pressure.

A second worked example: a pressure of 0.75 bar is 0.75 × 1,000 = 750 mbar, and 1,013.25 mbar is 1,013.25 ÷ 1,000 = 1.01325 bar. Both come from shifting the decimal point three places, to the right for bar to mbar and to the left for mbar to bar.

The typical slip is reading a millibar value as bar. A weather map showing 1,013 is not 1,013 bar, which would crush a submarine. Sanity check: everyday air pressure is just over 1 bar, so any valid millibar reading near sea level should be about 1,000, not 1 and not 1,000,000.

Weather maps, gauges and quick mental conversions

Weather maps give air pressure in millibar, the same size as the hectopascal. Typical sea-level pressure is about 1,013 mbar, which is 1.013 bar. A deep low might fall to 980 mbar (0.98 bar) and a strong high might reach 1,040 mbar (1.04 bar).

Because 1 bar is 1,000 mbar, the conversion needs no calculator: move the decimal point three places. 0.75 bar becomes 750 mbar and 2.5 bar becomes 2,500 mbar. Going the other way, 850 mbar is 0.85 bar.

One document can use both units. Boiler gauges are often in bar, while low gas-appliance supply pressures are commonly given in mbar, such as 20 mbar. Always read the unit next to each figure before comparing: 20 mbar is only 0.02 bar, a tiny fraction of a bar.

Common mistakes when converting bar to mbar

• Using 100 instead of 1,000. The factor of 100 belongs to bar to kilopascal, not to millibar.
• Forgetting that mbar and hPa are equal, so a weather map in hPa needs no conversion from mbar.
• Mixing up mbar with mb as an abbreviation for megabyte or millimetre of mercury.
• Dropping a zero. 0.5 bar is 500 mbar, not 50 mbar.