By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed July 16, 2026
Barometric pressure is the weight of the atmosphere pressing down on a surface. At sea level on a standard day it averages 1013.25 hPa, which is 14.70 psi, 29.92 inHg, or 101.325 kPa. It is the same quantity engineers also call atmospheric pressure, ambient pressure, or air pressure.
The number is never quite fixed. It falls as you climb, because less air sits above you, and it drifts a few percent as weather systems move through. Those two movements are why a barometric reading has to say where and when it was taken, and why the instrument that reads it has to be built the right way.
This guide covers what barometric pressure is, the units it comes in, the three kinds of barometer that measure it, how a modern barometric pressure sensor works, and where an atmospheric reference matters in process instrumentation.
Contents
- What barometric pressure is
- Units and the standard atmosphere
- How barometric pressure is measured
- How a barometric pressure sensor works
- Barometric pressure and altitude
- Where a barometric reference matters in process work
- FAQ
What barometric pressure is
The atmosphere is a column of air roughly 100 km deep, and it has mass. Gravity pulls that mass down, and the force it exerts on each unit of area at the bottom is barometric pressure. Stand at sea level and about 10 tonnes of air press on every square meter above you, which you never feel because the same pressure pushes in from every side and from inside your body.
Two things change the reading. Altitude sets the baseline: the higher you stand, the less air is stacked above you, so the pressure is lower. There are roughly half as many air molecules above 5,500 m as above sea level. Weather moves it around that baseline: warm, moist, rising air weighs less and gives low pressure, associated with clouds and storms; cool, dense, sinking air gives high pressure and clear skies. A passing front shifts a fixed site by up to about 3 kPa (30 hPa).
Units and the standard atmosphere
Barometric pressure carries more unit conventions than almost any other measurement, because meteorology, aviation, and process engineering each kept their own. They all describe the same standard atmosphere at sea level:
| Unit | Standard atmosphere at sea level |
|---|---|
| Hectopascal (hPa) – meteorology, SI | 1013.25 |
| Millibar (mbar) – older weather charts | 1013.25 (1 mbar = 1 hPa exactly) |
| Inches of mercury (inHg) – US aviation and weather | 29.92 |
| Millimeters of mercury (mmHg, Torr) – vacuum, medical | 760 |
| Pounds per square inch (psi) – US process | 14.70 |
| Kilopascal (kPa) – SI process | 101.325 |
The mercury units come from the original barometer: a standard atmosphere holds a column of mercury 760 mm, or 29.92 inches, high.
Weather reports mostly use hPa and inHg; a US pilot setting an altimeter dials in inHg, while a European chart shows hPa. In process plants the atmosphere shows up as the 14.7 psi or 101.325 kPa you add or subtract when converting between gauge and absolute readings. The full set of pressure unit conversions is in our pressure units guide, and the gauge-versus-absolute suffixes are covered in the PSI vs PSIA vs PSIG guide.
How barometric pressure is measured
Every barometer balances the weight of the atmosphere against something known. Three designs have carried the job across four centuries.
Mercury barometer. The original, from Torricelli in 1643: a glass tube closed at the top, filled with mercury, standing in an open reservoir. The atmosphere pushes on the reservoir and holds the column up; its height, 760 mm at standard sea-level pressure, is the reading. Accurate and self-referencing, but fragile, temperature-sensitive, and full of mercury, which is why almost every country has phased it out of routine service.
Aneroid barometer. A sealed metal capsule with most of the air evacuated. Rising pressure squeezes it, falling pressure lets it expand, and a lever train moves a needle on a dial. No liquid, compact, and the mechanism behind most dial weather barometers and mechanical altimeters.
Digital barometric pressure sensor. The modern standard. A pressure-sensitive diaphragm flexes with the atmosphere and the deflection becomes an electrical signal, in hPa or mbar. Weather stations, aircraft, and phones use tiny MEMS versions; laboratories, industrial references, and vacuum systems use a precision absolute pressure transmitter. That electrical output is the whole advantage: it can be logged, transmitted, and fed into a control system, which a column of mercury cannot.
How a barometric pressure sensor works
A barometric pressure sensor has to read the full weight of the atmosphere, which means it needs a reference that does not move with the atmosphere. That reference is a vacuum. This is what makes a barometric sensor an absolute pressure sensor, and the rest of this section builds on it.
Behind the sensing diaphragm sits a small chamber evacuated and sealed at the factory to near-zero pressure. The atmosphere presses on the front of the diaphragm; the sealed vacuum sits behind it and never changes. The diaphragm therefore deflects by the true, total pressure of the air, and the electronics turn that deflection into a reading. Nothing in the signal path depends on today’s weather, because the reference is empty space.
Contrast that with an ordinary gauge sensor, which vents the back of its diaphragm to the outside air. A gauge sensor reads process pressure minus atmosphere, so pointed at nothing but the atmosphere it reads zero. It cannot measure the barometer at all. The distinction between the two references, and when each is correct, is the whole subject of our absolute vs gauge pressure guide.
The sensing element behind the diaphragm is usually piezoresistive silicon or a capacitive cell. A consumer weather station or a phone uses a MEMS chip a few millimeters across; an industrial reference uses a stable, temperature-compensated absolute pressure transmitter with a 4-20 mA or digital output. The physics is identical; the accuracy, stability, and packaging are not.
Barometric pressure and altitude
Because barometric pressure falls predictably with height, the same physics that measures it also measures altitude, which is how a mechanical altimeter works. The standard atmosphere (ISO 2533) sets the reference curve:
| Altitude | Barometric pressure | In inHg |
|---|---|---|
| Sea level | 1013 hPa (101.3 kPa) | 29.92 |
| 500 m (1,640 ft) | 955 hPa (95.5 kPa) | 28.2 |
| 1,000 m (3,280 ft) | 899 hPa (89.9 kPa) | 26.5 |
| 1,600 m (Denver) | 835 hPa (83.5 kPa) | 24.7 |
| 2,000 m (6,560 ft) | 795 hPa (79.5 kPa) | 23.5 |
| 3,000 m (9,840 ft) | 701 hPa (70.1 kPa) | 20.7 |
ISO 2533 standard atmosphere. Real weather adds roughly ±30 hPa on top of any row.
This is why a weather report has to say whether a pressure is station pressure (the raw absolute reading at that elevation) or sea-level pressure (QNH, corrected as if the station were at sea level). Without the correction, a mountain town would always report a “low” and the map would be useless. Take a station at 500 m that measures 955 hPa: reduced to sea level it becomes about 1014 hPa, which now compares directly with a coastal station. Aviation uses the same idea when a pilot sets 29.92 inHg or the local QNH so the altimeter reads true height.
Where a barometric reference matters in process work
Outside the weather station, barometric pressure shows up as a correction that decides whether a measurement is right.
The first case is the gauge-versus-absolute choice. Any process that ignores the atmosphere, a vacuum process, a leak test, a gas-density or mass-flow calculation, must be read against a vacuum, not against the local air, so it needs an absolute reference exactly like a barometer. If a low-range reading near atmosphere is taken on an absolute transmitter, the daily 30 hPa of weather rides straight into the data unless it is compensated; on a vented gauge transmitter the same weather cancels out. Picking the wrong reference is the most common way a low-pressure loop drifts for no apparent reason.
The second case is altitude. A transmitter calibrated at sea level and installed on a plateau sees a different atmosphere, and a barometric reference lets a control system correct for it. The error is only large when the span is small: 30 hPa against a 100 kPa gauge span is 3 percent of the reading, but against a 40 MPa span it is 0.0075 percent and invisible.
Both cases point to the same instrument. An absolute pressure transmitter is the industrial barometric reference, and it installs like any other 4-20 mA transmitter in the pressure transmitter line. For a two-way conversion with the altitude correction built in, our absolute and gauge pressure calculator does the arithmetic, and the wider pressure instruments range covers gauge, absolute, and differential references. Barometric pressure is one form of static pressure; how it sits alongside dynamic and total pressure is in our static vs dynamic vs total pressure guide.
Application note
Absolute reference near atmosphere. A recurring request is a transmitter that has to read a process sitting just above or below the local air, a blanketed tank or a light vacuum, on a span of a few tens of kPa. On a gauge instrument the weather would wander through the reading all day. The fix is an absolute-reference transmitter with a span chosen around the true pressure, so the barometer is measured, not smuggled into the signal. It is the same reason a weather barometer is built absolute, applied to a process line.
FAQ
What is barometric pressure?
Barometric pressure is the pressure exerted by the weight of the atmosphere on a surface, also called atmospheric or air pressure. At sea level on a standard day it averages 1013.25 hPa, equal to 14.70 psi, 29.92 inHg, or 101.325 kPa. It decreases with altitude and shifts a few percent with the weather.
How is barometric pressure measured?
With a barometer. A mercury barometer balances the atmosphere against a column of mercury; an aneroid barometer uses a sealed metal capsule that flexes with pressure; a digital barometric pressure sensor uses a diaphragm and a sealed vacuum reference to produce an electrical reading in hPa or mbar. Digital sensors are the modern standard because the output can be logged and transmitted.
How does a barometric pressure sensor work?
It measures absolute pressure against a sealed vacuum. The atmosphere presses on a sensing diaphragm while an evacuated, permanently sealed chamber sits behind it as a fixed reference. The diaphragm deflects by the full pressure of the air, and that deflection becomes an electrical signal. A vented gauge sensor cannot do this, because it subtracts the atmosphere and would read zero.
What is a normal barometric pressure reading?
The standard sea-level value is 1013.25 hPa (29.92 inHg). Day to day at sea level, readings typically range from about 980 hPa in a deep low to 1040 hPa in a strong high. At elevation the normal reading is lower: around 835 hPa at 1,600 m, which is why weather reports correct station pressure to sea level before comparing sites.
Specify an absolute or barometric-reference transmitter
Send us the pressure range, the units you work in, and whether the reading must hold steady against weather and altitude. We will confirm whether an absolute, gauge, or compound reference fits and quote the matching build. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.