Air pressure is measured using an instrument called a barometer, and the method you choose depends on whether you want a wall-mounted gauge, a portable digital device, or simply an app on your phone. Standard air pressure at sea level is 1013.25 hectopascals (also called millibars), which equals 29.92 inches of mercury. Any measurement you take is compared against that baseline to tell you whether pressure is rising, falling, or holding steady.
Units of Air Pressure
Before you measure anything, it helps to know how the result will be expressed. Air pressure shows up in several different units depending on where you live and what you’re doing with the data. In the United States, weather reports typically use inches of mercury (inHg). Most of the rest of the world, along with international aviation, uses hectopascals (hPa) or millibars (mb), which are identical in value. Scientists sometimes refer to standard atmospheres (atm), and tire gauges use pounds per square inch (psi). At sea level, these all describe the same thing: 29.92 inHg, 1013.25 hPa, 1 atm, or 14.7 psi.
Converting between units is straightforward. To go from inches of mercury to millibars, multiply by 33.86. To go from millibars to inches of mercury, multiply by 0.0295. Most digital instruments let you toggle between units with a button press, so you rarely need to do the math yourself.
Mercury Barometers
The original method, invented in 1643, uses a glass tube filled with mercury and inverted into an open dish of mercury. Atmospheric pressure pushes down on the mercury in the dish, which forces the column of mercury up inside the tube. You read the height of that column, and that height is your pressure measurement, literally “inches of mercury.”
Mercury barometers are extremely accurate and were the standard in weather stations for centuries. They’re also bulky, fragile, and contain a toxic metal. Readings need to be corrected for the temperature of the mercury itself, and on a moving ship, the column sloshes around and becomes unreliable. For these reasons, mercury barometers have been almost entirely replaced in professional settings and are essentially unavailable for home use.
Aneroid Barometers
An aneroid barometer is the round, dial-faced instrument you might see mounted on a wall in a cabin or boat. Inside the case sits a small metal capsule with corrugated sides that has been partially evacuated of air. When atmospheric pressure increases, it compresses the capsule. When pressure drops, the capsule expands. A system of tiny levers and springs amplifies that movement and transfers it to a needle on the dial.
These devices are portable, need no liquid, and give a continuous visual readout. The tradeoff is drift: over months and years, the metal of the vacuum chamber loses some of its elasticity, and the readings slowly shift. If you own one, it needs periodic calibration. The simplest way to calibrate is to compare it to a known accurate reading, such as the current pressure reported by your nearest airport (available on any aviation weather site), and adjust the set screw on the back of the instrument until it matches.
Many aneroid barometers have a second “reference” needle you can manually set to the current reading. Come back a few hours later and compare the measurement needle to the reference needle. If the measurement needle has moved clockwise (higher), pressure is rising. Counterclockwise (lower) means pressure is falling.
Digital Barometers and Weather Stations
Digital barometers use electronic pressure sensors instead of a mechanical capsule. Most consumer models rely on a tiny chip that detects changes in electrical capacitance or resistance as a thin membrane flexes under pressure. These sensors are small enough to fit on a circuit board and accurate enough for home weather monitoring, typically within plus or minus 1 hPa of the true value. Professional-grade instruments used in meteorology and aviation tighten that to 0.1 to 0.5 hPa.
A standalone digital weather station usually displays current pressure, a pressure trend graph (often the last 12 to 24 hours), temperature, and humidity. The trend graph is the most useful feature for casual weather watchers because it makes rising or falling pressure obvious at a glance. Many models connect to a computer or home network so you can log data over time.
Using Your Smartphone
Most smartphones manufactured in the last decade contain a built-in barometric pressure sensor. The phone uses it primarily for altitude estimation to improve GPS accuracy, but you can access the raw pressure data through a barometer app. Search your app store for “barometer” and you’ll find dozens of free options for both iPhone and Android. After installing one, grant it permission to use your location. The app will display the current atmospheric pressure immediately.
Smartphone barometers are convenient but have some quirks. The sensor sits inside a sealed phone case, and changes in internal temperature from the processor can affect readings slightly. You’ll get the most reliable results if the phone has been sitting at ambient temperature for a few minutes rather than running intensive apps. Still, for tracking pressure trends throughout the day or noticing a significant drop before a storm, a phone barometer works surprisingly well.
Why Altitude Matters
Air pressure decreases as you go higher because there’s less atmosphere stacked above you. A barometer at 5,000 feet elevation will naturally read much lower than one at sea level, even on a perfectly calm day. This is why weather reports always adjust pressure readings to their sea-level equivalent: it lets you compare conditions across locations regardless of elevation.
The relationship between pressure and altitude is not linear, but at low to moderate elevations, pressure drops roughly 1 hPa for every 8 meters (about 26 feet) of altitude gained. The National Weather Service uses a more precise formula that accounts for the way air compresses under its own weight. For practical purposes, if you’re setting up a home barometer, you need to enter your elevation so the device can calculate the sea-level equivalent pressure. Without this correction, a barometer in Denver would always read about 840 hPa instead of something near 1013, making it useless for weather comparison.
Most digital barometers and weather stations let you enter your elevation during setup. If you don’t know your exact elevation, look it up on a topographic map website or simply check your property’s listing on a site like Google Earth. You can also work backward: find the current sea-level pressure from your nearest airport’s weather report, then adjust your barometer’s altitude setting until it displays that same value.
Reading Pressure Trends
A single pressure reading tells you very little. What matters is the direction and speed of change over several hours. Rising pressure generally signals improving weather: clearing skies, lighter winds, and drier air moving in. Falling pressure indicates a low-pressure system approaching, which brings clouds, wind, and precipitation.
The speed of the change gives you a sense of urgency. A slow, steady decline of a few millibars over 6 to 12 hours suggests a gradual weather shift. A rapid drop of 5 to 10 hPa in just a few hours can signal a strong storm. On the high end, a sharp, fast rise after a pressure drop usually means the storm is passing and conditions will clear quickly. For context, typical fair-weather pressure sits between about 1015 and 1030 hPa, while the center of a strong mid-latitude storm might pull pressure down to 980 or lower.
If your barometer has a trend indicator or graph, check it a few times a day. Over time, you’ll develop an intuitive sense of how your local weather responds to pressure changes, and you’ll start predicting rain before the forecast tells you about it.
Choosing the Right Method
For most people, a digital weather station or a smartphone app covers everything you need. A weather station is better if you want continuous logging and a dedicated display you can glance at from across the room. A smartphone app is ideal if you’re hiking, traveling, or simply curious and don’t want to buy additional hardware.
An aneroid barometer works well if you prefer an analog display and don’t mind occasional calibration. It requires no batteries and no software updates, and a quality one will last decades with minimal maintenance. Whichever method you choose, the key to getting useful information is always the same: set it up correctly for your altitude, and pay attention to how the readings change over time rather than fixating on any single number.