A brand-new barometer almost never reads correctly out of the box, and that catches a lot of people off guard. The needle sits wherever the factory left it, and unless you tell the instrument where it lives, it has no way of knowing whether it’s sitting at sea level in Florida or a mile up in Denver. Setting it properly takes about five minutes, and once it’s done you rarely have to touch it again.
The thing to understand first is that there are two different pressure numbers in play, and confusing them is the single most common mistake. The actual air pressure pressing down where you are standing is called station pressure, and it drops as you climb higher. The number you hear in weather reports is sea-level pressure, which has been mathematically adjusted as if your location were at sea level. Weather services do this so that a map of pressure across a country actually means something — otherwise every mountain town would look like a permanent storm. When you "set" a barometer, what you are almost always doing is adjusting it to display sea-level pressure for your altitude.
Start by finding a trustworthy reference reading for your exact area. The best source is your nearest airport’s METAR report, which lists an altimeter setting updated every hour. A local National Weather Service or Met Office station works too. Avoid grabbing a number from a phone app that’s pulling from a city forty miles away — small differences matter here, and a distant reading defeats the purpose. Write down the current sea-level pressure for your location and note the time.
For an aneroid barometer — the classic round dial with a needle — look on the back. You’ll find a small recessed screw, usually brass, that you turn with a flathead screwdriver. Gently rotate that screw until the needle points to your reference pressure. Don’t force it; these movements are delicate. Tap the glass lightly with a fingertip afterward, because the needle can stick slightly, and a tap settles it onto the true reading. That’s genuinely all there is to it. The second needle you may see is not powered — it’s a manual marker you set by hand to "remember" the last reading, so you can see at a glance whether pressure has risen or fallen since you last looked.
A digital barometer is set through its menu rather than a screw, but the logic is identical. Find the calibration or "reference pressure" setting, and enter the same sea-level value you looked up. Some units instead ask for your elevation in metres or feet; if yours does, enter your altitude accurately and it will handle the conversion itself. You can find your elevation from a topographic map or a GPS reading.
Once it’s set, resist the urge to recalibrate it constantly. A barometer that you reset every day is useless, because you’ve erased exactly the changes you’re trying to watch. The whole value of the instrument is in the trend — whether pressure is climbing, holding steady, or falling, and how fast. Set it once against a good reference, then leave it alone and read the movement. Check it against your reference station maybe once a season to make sure it hasn’t drifted.
If your barometer simply refuses to hold a setting, or the needle moves erratically, the aneroid capsule inside may be damaged, and no amount of adjustment will fix that. But for the vast majority of instruments, a single careful calibration is all they’ll ever need.