Air pressure is written several different ways depending on where you are. The one thing worth knowing first: a millibar and a hectopascal are exactly the same size. A barometer reading 1013 mbar and a forecast saying 1013 hPa agree completely.
Standard pressure in every unit
Standard pressure at sea level is 1013.25 hPa, and this is that same figure written every way it is commonly written:
| Unit | Standard sea-level pressure | Used by |
|---|---|---|
| hPa | 1013.25 | Weather services in most of the world |
| mbar | 1013.25 | Home weather stations, shipping forecasts |
| mmHg | 760 | Russia and neighbouring countries |
| inHg | 29.92 | The United States, and aviation in the US, Canada and Japan |
| kPa | 101.325 | Canada |
Every page on this site has a switch between hectopascals, millimetres, inches and kilopascals, so you rarely need to convert anything by hand. Millibars need no switch, because they are the same number as hectopascals.
Converting
- hPa to mmHg: multiply by 0.75. So 1013 hPa is 760 mmHg.
- hPa to inHg: divide by 33.864. So 1013 hPa is 29.91 inHg.
- hPa to kPa: divide by 10. So 1013 hPa is 101.3 kPa.
- hPa to mbar: nothing to do; same unit, two names.
| hPa | mmHg | inHg | kPa |
|---|---|---|---|
| 980 | 735 | 28.94 | 98.0 |
| 1000 | 750 | 29.53 | 100.0 |
| 1013 | 760 | 29.91 | 101.3 |
| 1030 | 773 | 30.42 | 103.0 |
One detail that trips people up: the famous 29.92 inHg belongs to the exact standard figure of 1013.25 hPa. A plain 1013 hPa works out at 29.91. The difference never matters to a weather reading, and it matters a great deal if you are checking someone's arithmetic.
Where the units come from
For most of three centuries, measuring pressure meant reading the height of a column of mercury in a glass tube. Heavier air pushed the column higher, so the reading was literally a length. That is where inches and millimetres of mercury come from, and both are still in daily use.
The scientific unit is the pascal, which is very small, so forecasters count in hundreds of them. A hundred pascals is a hectopascal. The millibar is the older name, and it was already exactly the same size, so during the 1980s most weather services changed the label without changing any numbers. Old books and many home weather stations still say millibar, and the unit itself is not out of date.
Two things that catch people out
A Russian barometer reading 747 is not low. Russian forecasts publish the reading at the station rather than corrected to sea level. Moscow stands about 150 m above the sea, which is enough to take some 13 mmHg off the reading, so 747 mmHg there is an ordinary day rather than a storm.
An airport figure is not quite the same quantity. Aviation quotes an altimeter setting, which is worked out through a standard column of air rather than the real one. Near the coast the two agree within about a hectopascal. High up they drift apart, by several hectopascals at Denver and by ten or more at Mexico City, and which of the two is higher depends on how warm the day is. Neither is wrong; they answer different questions.
Which unit should you use?
Whichever your local forecast uses, since that is the one your neighbours and your barometer's manual will both be using. The site starts in the unit for your part of the world, and the switch at the top of any page changes every figure at once.
There is a second switch worth knowing about. This site leads with the sea-level figure everywhere, and can also show the pressure at the city's own height, which is what a barometer in the room reads. Air pressure and altitude explains the difference.
If two sources disagree, check three things before assuming one is wrong: the same unit, the same quantity (sea level or local reading), and the same hour.
