Atmospheres to millibars:
multiply by 1,013.25

How many millibars are in an atmosphere?

One standard atmosphere is 1,013.25 millibars. The standard atmosphere is defined as exactly 101,325 pascals, and a millibar is 100 pascals, so the factor is exact.

The millibar is the traditional unit on weather charts. It is numerically the same as the hectopascal, so 1,013.25 mbar equals 1,013.25 hPa.

The atmosphere was originally an attempt to describe the average air pressure at sea level, and it was later fixed as exactly 101,325 pascals so that it could be used as a reference in science. The millibar belongs to meteorology, where pressure maps have been drawn in millibars for a long time, and many forecasters now call the same unit the hectopascal. Moving between atm and mbar is common in physics lessons, scuba and aviation, where one source quotes a standard value and another reads a gauge in millibars. Keep in mind that the conversion is exact, so a published pressure such as 1,013.25 mbar needs no rounding when you compare it with a figure given in atmospheres.

The atm to mbar formula

A reading from a weather station gives a second example. A low pressure area at 980 mbar is 980 ÷ 1,013.25 = 0.9672 atm, so it sits about 3.3 percent below standard.

1. 1Write the pressure in atmospheres.
2. 2Multiply by 1,013.25.
3. 3Round: weather work uses whole millibars, lab work may keep the decimals.
4. 4Check that 1 atm is close to 1,000 mbar, and scale from there.

Chart: atmospheres to millibars

Where atm to mbar is used

• Weather forecasting, where average sea level pressure is about 1,013 mbar and storms drop lower.
• Aviation, since altimeters are commonly set to a standard pressure of 1,013.25 hPa.
• Vacuum and pressure testing, where a rating may be given in atmospheres but the gauge reads in mbar.
• Laboratory chemistry, where gas laws often take atm but pumps show millibars.

In vacuum work a 2.5 atm test pressure is 2.5 × 1,013.25 = 2,533.125 mbar.

Quick sanity checks for atmospheres to millibars

1 atm is 101.325 kPa and 1.01325 bar, so it is only 13.25 mbar above 1,000 mbar. Any reading near 1,000 is therefore close to one atmosphere.

Try a forecast figure: 1,050 mbar is 1,050 ÷ 1,013.25 = 1.0363 atm, about 3.6 percent above standard.

Every 100 mbar is about 0.0987 atm, a useful rule when you are scanning a pressure chart.

Common mistakes when converting atm to mbar

• Mixing up bar and atmosphere. 1 bar is 1,000 mbar, while 1 atm is 1,013.25 mbar, so they differ by 1.3 percent.
• Using 1,000 as the factor for exact work, which loses that 1.3 percent.
• Forgetting the units. mbar and hPa are the same size, but a pascal is 100 times smaller.
• Dividing when you should multiply. An answer in mbar should be around a thousand times the atm figure.