Your phone reads 24.75 inHg on a calm March morning in Denver. Your weather app describes the same sky over the same rooftop and says 30.05 inHg — and both numbers are correct.
The gap of more than five inches of mercury exists because each number answers a different question. One describes the air actually pressing on your phone, and the other describes the air that would press on an imaginary sea-level floor about a mile beneath your feet.
Station pressure is the actual weight of the air at your elevation. Sea-level pressure is that reading adjusted to what it would be at sea level, so a Denver station and a Miami station can share one weather map.
We covered what rising and falling pressure means for tomorrow's sky in our barometric pressure explainer. This piece is about the number itself: the units, the sea-level adjustment, the altimeter setting pilots use, and whether a barometer can tell you how high you are.
What Your Phone Barometer Is Actually Measuring
Most modern phones carry a small MEMS pressure sensor, which is a sealed chamber with a flexible membrane that bends as the outside air pushes on it. The sensor reports absolute pressure, meaning the full weight of the column of air sitting above the phone at that moment.
Meteorologists call that value station pressure. It is the rawest pressure number there is, and it depends mostly on one thing: how much atmosphere is stacked above you.
Station pressure changes quickly with height. Climb a single flight of stairs and a good phone sensor will register it, and if you drive from Boulder to Estes Park the reading falls by more than most storm systems will move it in a week.
A few everyday factors can also nudge the raw reading, including but not limited to:
- Building pressurization. Large HVAC systems keep some office towers and hotels slightly above outdoor pressure. That can shift an indoor reading by a fraction of a hectopascal.
- Moving air. Wind gusting across the phone, or a car with the fan on high, creates small dynamic pressure swings. On a graph they look like noise.
- Sensor temperature. The chips are temperature-compensated. Even so, a phone left on a sunny dashboard can read a touch off until it settles.
Of course, none of these explains a five-inch gap. That gap comes entirely from elevation.
Why Weather Apps Report Sea-Level Pressure
Imagine a weather map built from raw station pressure. Denver would show a permanent deep low near 835 hPa and Miami a permanent high near 1013 hPa, and every real storm would be buried under the shape of the terrain.
Sea-level reduction fixes this. Each station takes its measured pressure and adds the weight of a fictional column of air reaching from the station down to sea level, which puts every location on the same footing.
Weather apps show sea-level pressure because raw station pressure mostly reflects elevation. Adjusting every station to sea level removes the terrain, so highs, lows, and fronts can be compared across a map.
Once elevation is removed, the real signal appears. The highs and lows in our guide to reading surface pressure maps are drawn from these sea-level values, and that is the only reason a front crossing the Rockies can be traced at all.
What's more, sea-level values make the pressure gradient readable, and the gradient is what drives the wind. Our guide to reading a wind forecast builds directly on that idea.
The hard part of the method is the fictional column. Its weight depends on its temperature, because cold air is denser than warm air, and there is no thermometer inside a mile of imaginary rock.
In the United States, the National Weather Service estimates that column by averaging the current station temperature with the temperature from twelve hours earlier. This smooths out the daily swing, although at high, cold stations the result can still drift away from what nearby lowland stations suggest.
Converting inHg To hPa
Two families of units dominate pressure readings. Inches of mercury (inHg) come from the old mercury barometer, which measured how high a column of mercury the air could hold up, while hectopascals (hPa) are metric and are the international standard for weather reporting.
Keep in mind that the millibar (mb), which still appears on older maps and in hurricane bulletins, is exactly the same size as the hectopascal. A reading of 1004 mb and a reading of 1004 hPa describe identical air.
One inch of mercury equals 33.864 hectopascals, and one hectopascal equals about 0.0295 inHg. Standard sea-level pressure is 29.92 inHg, or 1013.25 hPa.
Here is how common sea-level readings translate between the two systems:
| inHg | hPa | What it usually signals at sea level |
|---|---|---|
| 28.50 | 965.1 | An intense storm, such as a hurricane or a powerful winter low |
| 29.00 | 982.1 | A well-developed low-pressure system |
| 29.50 | 999.0 | Moderately low and often unsettled |
| 29.92 | 1013.2 | The standard atmosphere, the textbook average |
| 30.00 | 1015.9 | Near average, slightly high |
| 30.50 | 1032.9 | A strong high, often clear and cold in winter |
| 31.00 | 1049.8 | An exceptionally strong high, typically an Arctic air mass |
For quick mental math, about 0.03 inHg equals one hectopascal, and 0.10 inHg equals about 3.4 hPa. That means a forecast drop from 30.10 to 29.80 inHg, three-tenths of an inch, is a fall of about 10 hPa.
The conversion is linear, so you don't need a lookup table in the field. Multiply inHg by 33.864 to get hPa, or multiply hPa by 0.02953 to get inHg.
What A Specific Reading Means At Your Elevation
A pressure number means little until you know which kind it is. A reading of 25.00 inHg (847 hPa) at sea level would be far below the deepest typhoon on record, Typhoon Tip's 870 hPa in 1979, yet it is an ordinary afternoon at a station roughly 4,900 feet up.
The table below shows typical station pressure on an average day at several elevations, based on the International Standard Atmosphere. Actual readings move above and below these values with the weather, usually by less than an inch of mercury.
| Location | Elevation | Typical station pressure | What the weather app shows |
|---|---|---|---|
| Miami | Sea level | 1013 hPa / 29.92 inHg | About 29.92 inHg |
| Atlanta | About 1,000 ft | 977 hPa / 28.86 inHg | About 29.92 inHg |
| Denver | 5,280 ft | 834 hPa / 24.64 inHg | About 29.92 inHg |
| High trailhead | 10,000 ft | 697 hPa / 20.58 inHg | About 29.92 inHg |
The last column is the point of the table. On an average day, all four phones read differently and all four weather apps read the same.
Near sea level, pressure falls about 1 hPa for every 8 meters (27 feet) of climb, or roughly 1 inHg per 1,000 feet. That is why Denver's station pressure sits near 24.6 inHg on an ordinary day.
This is also why the pressure in our Denver weather and style guide will never match the barometer widget on a phone in Larimer Square. The app is describing the weather, while the phone is mostly describing the mile of altitude.
That said, station pressure tells you something of its own. At 10,000 feet each breath holds about 31 percent less oxygen than at sea level, whatever the sea-level number says about highs and lows.
The Altimeter Setting: A Third Number
Aviation adds one more pressure value. The altimeter setting, which pilots call QNH, is the number dialed into an aircraft altimeter so it reads the airport's true elevation while the plane sits on the runway.
The altimeter setting is the pressure pilots dial into an altimeter so it reads field elevation on the ground. It uses a standard atmosphere, while sea-level pressure uses actual temperature, so the two can differ.
Because the altimeter setting assumes a standard temperature profile instead of the real one, it answers a slightly different question than sea-level pressure. Near sea level the two usually agree closely, but at high-elevation airports they can differ by several hectopascals, especially in hard cold snaps or summer heat.
You can see both in a raw airport observation, called a METAR. Here is how to read the pressure parts of a typical report:
- A3002. The altimeter setting in inches of mercury with the decimal removed. Here, that is 30.02 inHg.
- SLP165. Sea-level pressure in the remarks section, in hectopascals with the leading 9 or 10 and the decimal removed. Here, that is 1016.5 hPa.
- Q1017. Outside North America, the altimeter setting is usually reported in whole hectopascals. It carries a Q prefix instead of an A.
Pilots also use QFE, the pressure at field elevation, which is simply station pressure. With QFE set, an altimeter reads zero on the runway, a convention some glider pilots and military operators still prefer.
Outside a cockpit, a simple check works. A number near 29 or 30 inHg (about 1000 to 1020 hPa) has been adjusted to sea level one way or another, and a number that looks strangely low is probably station pressure.
Can Pressure Do Your Altitude Math?
It can, and your phone's fitness app already uses it to count flights of stairs and log elevation gain on a hike. The real question is how long the math stays accurate.
Near sea level, a change of 1 hPa corresponds to about 8.3 meters (27 feet) of height. Higher up the air thins, so each hectopascal covers more height, roughly 10 meters (32 feet) near Denver's elevation.
Over a few minutes the atmosphere barely changes, so a pressure difference is almost entirely a height difference. A good sensor can resolve a meter or two, and that is why a barometer is better than GPS for counting the climb on a staircase or a switchback.
Over a few hours, however, the weather starts showing up in the altitude reading. A passing front can drop sea-level pressure by 10 hPa in a day, and a phone sitting motionless on a table would conclude it had climbed about 80 meters, roughly 270 feet.
Aviation has a mnemonic for this hazard: "high to low, look out below." If you fly from high pressure into low pressure without resetting, the altimeter reads higher than the aircraft actually is, so controllers pass along fresh altimeter settings throughout a flight.
Temperature matters too. In air colder than the standard atmosphere, pressure falls faster with height, so a barometric altimeter will overstate how high you have climbed.
The same habits apply on the ground for hikers, skiers, and photographers hunting a specific ridgeline. Here's how to keep a barometric altitude reading accurate:
- Calibrate at a known point. Set your watch or app to the elevation printed on a trailhead sign or topo map before you start. Don't rely on a calibration from the night before.
- Recalibrate on long days. Each time you pass a summit marker, lake, or junction with a published elevation, reset to it.
- Watch for drift at rest. If your altitude climbs while you sit at camp, the pressure is falling. That tells you about the weather, not the terrain.
- Use the altimeter setting when you have it. The current A-value from the nearest airport METAR is a better starting reference than a sea-level pressure from a weather app.
The last tip is easy to miss. Phone and watch altimeters assume the same standard atmosphere as the altimeter setting, so it is the matching input, while sea-level pressure built from actual temperatures can add a small extra error at high elevation.
A phone barometer tracks elevation changes over minutes, often to within a meter or two. Over hours, the weather moves pressure enough to throw it off by tens of meters unless you recalibrate to a known height.
How To Read Any Pressure Number You Meet
This comes down to a short decision rule. Before you interpret a reading, work out which of the three numbers you are looking at, then use it for the job it was built to do.
The three-number rule. Station pressure tells you about altitude and the air your lungs are working with. Sea-level pressure tells you about the weather, and its trend tells you about tomorrow.
The altimeter setting tells an altimeter where the ground is. Match the number to the question and most of the confusion goes away.
Where your phone sits compared with the official station matters as well. Our piece on weather station siting explains why two sensors a few blocks apart can disagree, and the same logic applies to the barometer in your pocket.
When you want the pressure number turned into a decision, such as whether the wind is about to shift or whether the evening light will hold, our guide to reading the daily brief shows how it fits into the rest of the day's forecast. Pressure is one input among several, and it matters most when it moves.
Common Questions
How do I convert a pressure reading from inHg to hPa?
Multiply the inHg value by 33.864. A reading of 30.12 inHg becomes about 1020 hPa, and 29.50 inHg becomes about 999 hPa. To go the other way, multiply hPa by 0.02953. Millibars convert exactly the same way as hectopascals, because one millibar and one hectopascal are the same unit.
Can my phone's barometer tell me my altitude?
Yes, for changes over minutes. Near sea level, each hectopascal equals about 8 meters of height, and phone sensors can detect changes of a meter or two. Over hours, weather systems shift pressure enough to throw the reading off by tens of meters, so recalibrate at a trailhead sign or summit marker with a published elevation.
Why is the altimeter setting different from sea-level pressure?
The altimeter setting assumes a standard atmosphere. Sea-level pressure uses the station's actual recent temperatures to estimate the weight of the imaginary air below it. At low elevations the two usually agree within a hectopascal or so, but at high, very cold, or very hot stations they can be several hectopascals apart.
What is a normal station pressure where I live?
It depends almost entirely on your elevation. On an average day, expect about 29.92 inHg (1013 hPa) at sea level, 28.86 inHg (977 hPa) at 1,000 feet, 24.64 inHg (834 hPa) at Denver's 5,280 feet, and 20.58 inHg (697 hPa) at 10,000 feet. Weather moves those values up or down, usually by less than an inch.
Should I set my home weather station to sea-level pressure?
If you want it to match forecasts and weather maps, yes. Most home weather stations have a sea-level or relative mode. Enter your elevation, or adjust the offset until the display matches the pressure reported by your nearest airport. Keep it on absolute or station pressure only if you want to track altitude rather than weather.