About the Inches of Mercury to Millibars
This inHg to mb converter translates barometric pressure between inches of mercury, used by US weather reports and North American altimeter settings, and millibars or hectopascals, used by meteorologists and pilots almost everywhere else. One millibar is exactly one hectopascal, so the two are interchangeable.
Use it to read a home weather station set to the “wrong” unit, to compare a US forecast with a European synoptic chart, to translate a QNH of 1013 hPa into an altimeter setting of 29.92 inHg, or to follow storm reports that quote central pressure in millibars. Kilopascals, millimetres of mercury, psi and atmospheres are shown too.
The converter uses the conventional inch of mercury (3,386.389 Pa, mercury at 0 °C under standard gravity), which is the value used for weather and aviation. It converts the number you enter; it does not correct station pressure to sea level.
How to use the inches of mercury to millibars
- 1Enter the barometer or altimeter reading.
- 2Choose its unit (inHg for US reports, mb or hPa elsewhere).
- 3Choose the unit you want to convert to.
- 4Read the result and use the chart to scan nearby values.
Formula and method
An inch of mercury is the pressure exerted by a column of mercury one inch tall. Using the conventional definition (mercury density 13,595.1 kg/m³ at 0 °C and standard gravity 9.80665 m/s²), 1 inHg = 3,386.389 Pa. A millibar is 100 Pa — exactly the same as a hectopascal — so 1 inHg = 33.86389 mb and 1 mb = 0.02953 inHg.
For every other unit the converter goes through pascals: it multiplies your value by the pascals in the source unit and divides by the pascals in the target unit (1 mmHg = 133.322 Pa, 1 psi = 6,894.757 Pa, 1 atm = 101,325 Pa). The chart lists the typical barometer range from 28 to 31 inHg.
- inHg
- Inches of mercury (conventional, 0 °C)
- mb
- Millibar, identical to the hectopascal (hPa)
- Pa
- Pascal, one newton per square metre
Worked examples
Standard altimeter setting 29.92 inHg
29.92 × 33.86389 = 1013.21 mb (hPa). This is the standard pressure pilots set above the transition altitude in North America.
1013.25 hPa QNH in inches
1013.25 ÷ 33.86389 = 29.921 inHg — the ICAO standard atmosphere at sea level.
Storm low of 980 mb
980 ÷ 33.86389 = 28.94 inHg, a pressure typical of a strong mid-latitude storm or a hurricane on a weather map.
High pressure of 30.50 inHg in kPa
30.50 × 3,386.389 Pa = 103,285 Pa, which is 103.28 kPa or 1032.8 mb — a strong high-pressure system.
Frequently asked questions
How do I convert inHg to millibars?+
Multiply inches of mercury by 33.8639. For example, 30.00 inHg × 33.8639 = 1015.9 mb. To go from millibars to inHg, divide by 33.8639 (or multiply by 0.02953).
Are millibars and hectopascals the same?+
Yes. One millibar equals 100 pascals, and one hectopascal is also 100 pascals, so 1013 mb = 1013 hPa. Weather services switched to the name hectopascal but the numbers did not change.
What is normal barometric pressure?+
Standard sea-level pressure is 1013.25 mb or 29.92 inHg. Real readings commonly range from about 980 to 1040 mb (28.9 to 30.7 inHg) depending on the weather.
Why does my barometer read lower than the forecast?+
Forecasts report pressure corrected to sea level. A barometer at altitude measures station pressure, which falls by roughly 1 inHg per 1,000 ft, unless it has been calibrated to sea level.
What is a high or low pressure reading in inHg?+
As a rough guide, readings above about 30.20 inHg (1023 mb) indicate high pressure and fair weather, while readings below about 29.80 inHg (1009 mb) indicate low pressure and unsettled weather.