Convert a barometer reading
The example is a barometer at 29.80 inHg that read 29.95 inHg three hours earlier. 1 millibar (mb) equals 1 hectopascal (hPa).
A barometer reading can be in inches of mercury (inHg), in millibars (mb) or in hectopascals (hPa), which are the same as millibars. One inch of mercury is 33.8639 hPa, so the standard sea-level pressure of 1013.25 hPa reads 29.92 inches on a barometer. The converter above turns a reading in any of four units into the other three, compares it with that standard and, with the reading of three hours earlier, names the trend in the words of marine forecasts.
hPa (mb) = inHg × 33.8639 · inHg = hPa ÷ 33.8639 · mmHg = inHg × 25.4 · kPa = hPa ÷ 10. One millibar equals one hectopascal, and the standard atmosphere is 1013.25 hPa = 29.92 inHg = 760 mmHg = 101.325 kPa.
How to Convert inHg to Millibars and Back
NIST’s table of conversion factors gives the conventional inch of mercury as 3,386.389 pascals and the conventional millimeter of mercury as 133.3224 pascals; a millibar is 100 pascals, the same as a hectopascal. To convert inches of mercury to millibars, multiply by 33.8639; to convert millibars to inches, divide by it. A reading of 29.80 inHg is 29.80 × 33.8639 = 1009.1 hPa, 4.1 hPa below the standard atmosphere. Because an inch is 25.4 millimeters, inches and millimeters of mercury differ by that factor: 29.80 inHg is 756.9 mmHg. A kilopascal is 10 hPa, so 1009.1 hPa is 100.91 kPa.
| hPa (mb) | inHg | mmHg | kPa |
|---|---|---|---|
| 950 | 28.05 | 712.6 | 95.00 |
| 970 | 28.64 | 727.6 | 97.00 |
| 980 | 28.94 | 735.1 | 98.00 |
| 990 | 29.23 | 742.6 | 99.00 |
| 1000 | 29.53 | 750.1 | 100.00 |
| 1010 | 29.83 | 757.6 | 101.00 |
| 1013.25 (standard) | 29.92 | 760.0 | 101.33 |
| 1020 | 30.12 | 765.1 | 102.00 |
| 1030 | 30.42 | 772.6 | 103.00 |
| 1040 | 30.71 | 780.1 | 104.00 |
| 1050 | 31.01 | 787.6 | 105.00 |
Normal Barometric Pressure at Sea Level
The American Practical Navigator (Bowditch) gives the standard atmosphere a sea-level pressure of 1013.25 hPa, or 29.92 inches of mercury, and uses the same figure, the average sea-level pressure across the globe, as the yardstick for weather systems. Its examples: a high of 1035 hPa (30.56 inHg) is 22 hPa above average, a high of moderate strength; a low of 971 hPa (28.67 inHg) is 42 hPa below average, a strong low. On a weather map, lines of equal pressure called isobars mark the highs and lows, and the closer together the isobars, the stronger the wind.
What a Falling or Rising Barometer Means
Bowditch says that a falling barometer indicates the approach of a low, probably with stormy weather, and that the change between observations, called the pressure tendency, helps predict the approach of a frontal system. The National Weather Service defines the pressure tendency as the character and amount of the change, usually over the 3 hours before an observation. In its marine forecasts the Met Office, the UK’s weather service, describes that 3-hour change in fixed terms:
| Met Office term | Change in 3 hours | In inches of mercury |
|---|---|---|
| Rising or falling slowly | 0.1 to 1.5 hPa | up to about 0.04 inHg |
| Rising or falling | 1.6 to 3.5 hPa | about 0.05 to 0.10 inHg |
| Rising or falling quickly | 3.6 to 6.0 hPa | about 0.11 to 0.18 inHg |
| Rising or falling very rapidly | more than 6.0 hPa | more than about 0.18 inHg |
The Met Office also says “now rising” or “now falling” when the pressure had been falling or rising, or steady, through the three hours but was definitely rising or falling at the time of the observation, and “more slowly” when it changed at a progressively slower rate. To use the terms, enter the reading of three hours ago in the converter: in the example, 29.95 to 29.80 inHg is −5.1 hPa, “falling quickly”.
Wind and Pressure
Wind is air moved by differences in pressure, and the closer the isobars, the stronger it blows (Bowditch). For wind speeds in knots, the Knots to km/h Converter and the Knots to MPH Converter turn them into other units, and on a moving boat the True Wind Calculator works out the true wind from the wind you feel.
Frequently Asked Questions
How do you convert inches of mercury to millibars?
Multiply by 33.8639, the number of hectopascals in one inch of mercury (NIST: 1 inHg = 3,386.389 Pa). For example, 29.80 inHg × 33.8639 = 1009.1 hPa. To go from millibars to inches, divide by 33.8639.
What is normal barometric pressure in inHg and mb?
The standard atmosphere at sea level is 1013.25 hPa, which is 29.92 inches of mercury or 760 millimeters of mercury. Bowditch uses the same value, the average sea-level pressure across the globe, as the reference for highs and lows.
Is a millibar the same as a hectopascal?
Yes. A millibar is 100 pascals, and so is a hectopascal, so 1 mb = 1 hPa. The Met Office, for example, gives pressures in hectopascals and notes that 1 hPa = 1 mb for readers who know the millibar.
What does a falling barometer mean?
A falling barometer indicates the approach of a low pressure system, probably with stormy weather (Bowditch). How fast it falls matters: the Met Office calls a fall of 3.6 to 6.0 hPa in three hours falling quickly, and more than 6.0 hPa falling very rapidly.
How do I convert inHg to mmHg?
Multiply by 25.4. In NIST’s table the conventional inch and millimeter of mercury differ by the 25.4 millimeters in an inch: 29.92 inHg is 760 mmHg.
What is high and low barometric pressure?
Pressure above the average 1013.25 hPa is high and below it low. Bowditch calls a 1035 hPa high, 22 hPa above average, a high of moderate strength, and a 971 hPa low, 42 hPa below average, a strong low. In inches, those are 30.56 inHg and 28.67 inHg.
Sources
- NIST, Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically: inch of mercury, conventional = 3.386 389 kPa; millimeter of mercury, conventional = 133.3224 Pa; millibar = 100 Pa; standard atmosphere = 101.325 kPa (read Oct 21, 2026).
- NGA, The American Practical Navigator (Bowditch), Vol. I (2019): art. 3800 (wind from differences in pressure; standard atmosphere, 1013.25 hPa or 29.92 inches), art. 3811 (average sea-level pressure as the reference for highs and lows, the 1035 hPa and 971 hPa examples, isobars and wind), art. 3812 (falling barometer, pressure tendency), art. 3826 (a falling barometer and an approaching low), Appendix B (1013.250 hPa = 760 mmHg = 29.92126 inHg) (read Oct 21, 2026).
- Met Office, Marine forecasts glossary: pressure tendency in station reports, from slowly to very rapidly, now rising or falling, more slowly; 1 hPa = 1 mb (read Oct 21, 2026).
- National Weather Service, Glossary: Pressure Tendency (read Oct 21, 2026).