If you’ve ever looked closely at a barometer and felt slightly bewildered by the units printed around the dial, you’re in good company. Atmospheric pressure gets measured in several different scales depending on where you are in the world and what tradition the instrument comes from, and while they all describe the same thing, the numbers look wildly different. Understanding them makes every pressure reading you ever see far more useful.
The unit you’ll meet most often in modern meteorology is the hectopascal, abbreviated hPa. It’s the standard across most of the world’s weather services. Conveniently, one hectopascal is exactly equal to one millibar, the older unit it replaced, so if you see "mb" on an older instrument or forecast, you can read it as identical to hPa. There is no conversion to do — 1013 mb and 1013 hPa are the same pressure. The millibar lingered for decades and still shows up in marine forecasting and aviation in some regions, which is why both labels survive.
Then there’s inches of mercury, written inHg, which is the unit you’ll see almost universally in the United States and on a lot of household barometers sold there. This one traces straight back to the original mercury barometers, where pressure was literally read off as the height of a column of mercury in a glass tube. When pressure is higher, it pushes the mercury further up the tube. A typical reading sits around 29.92 inHg, which is the same pressure as the 1013.25 hPa you’d see elsewhere. To convert roughly in your head, one inch of mercury is about 33.86 hPa.
So what counts as high or low? Standard atmospheric pressure at sea level is defined as 1013.25 hPa, or 29.92 inHg. Readings above that are considered high pressure, and readings below are low pressure. In practice, most barometers spend their lives wandering between roughly 980 hPa and 1040 hPa. Anything down around 980 hPa or below usually means a vigorous storm system is overhead, while a reading pushing toward 1030 hPa or higher signals a strong, settled high — typically clear, calm conditions.
Many older dial barometers add a layer of confusion by printing words instead of, or alongside, numbers: "Stormy," "Rain," "Change," "Fair," "Very Dry." These verbal labels are a charming holdover but should be taken with a heavy pinch of salt. They were calibrated for a generic European climate generations ago and assume your barometer reading maps neatly onto weather. It doesn’t, reliably — the absolute number matters far less than which direction it’s heading. A barometer pointing at "Fair" but falling rapidly is far more ominous than one sitting on "Rain" but rising steadily.
If you’re configuring a digital weather station, you’ll usually be able to pick your preferred unit in the settings, and it’s worth choosing the one your local forecasts use so you can compare like with like. Someone in London or Sydney will want hPa; someone in Ohio will find inHg easier to sanity-check against the local broadcast. The instrument is measuring exactly the same air either way — the unit is just the language it speaks.