A bar is a unit of pressure equal to 100,000 pascals, which is very close to the average atmospheric pressure at sea level. If you have ever checked a tire-pressure gauge, read a weather map from a European forecast service, or looked at the specs on a scuba regulator, you have probably seen pressure expressed in bars or millibars. The unit sits at a convenient crossroads: one bar is almost exactly one atmosphere, which makes it intuitive for everyday use, yet it slots neatly into the metric system, which makes it practical for science and engineering. How that pressure is actually measured, though, depends enormously on the situation, from mercury columns that date back nearly four centuries to tiny silicon chips that fit inside a smartphone.
Defining the Bar in Plain Terms
Pressure is force spread over an area. A bar represents a force of 100,000 newtons acting on each square meter of surface, which in the standard metric system equals exactly 100,000 pascals (Pa).1Japanese Journal of Applied Physics. Acoustics: Sound Fields and Transducers Standard atmospheric pressure, the benchmark for “normal” air pressure at sea level, is 101,325 Pa, so one bar undershoots it by about 1.3 percent. For most practical purposes the two are treated as nearly the same thing, which is why the bar caught on as a convenient shorthand.
You will often see the millibar (mbar), which is one-thousandth of a bar, or equivalently 100 Pa. Meteorologists worldwide report surface pressure in millibars or hectopascals (hPa), and those two are identical: 1 mbar equals 1 hPa. Standard sea-level pressure is roughly 1,013 mbar. The microbar, which equals 0.1 Pa, shows up in acoustics, where sound-pressure levels can be vanishingly small.1Japanese Journal of Applied Physics. Acoustics: Sound Fields and Transducers So a single root unit, the bar, scales from the faintest whisper to pressures deep inside planets, just by adding the right prefix.
How the Bar Fits Among Other Pressure Units
Pressure has accumulated a confusing zoo of units over the centuries, and they still coexist depending on the industry. In the United States, tire shops and HVAC technicians work in pounds per square inch (psi); one bar is about 14.5 psi. Physicians measure blood pressure in millimeters of mercury (mmHg), a unit rooted in the earliest barometers; one bar equals roughly 750 mmHg. Divers often talk in atmospheres (atm), where one bar is about 0.987 atm. And the formal SI unit is the pascal, where one bar is exactly 100,000 Pa.
The bar itself is not an official SI unit, but it is accepted for use alongside SI and appears throughout European engineering standards, weather services, and industrial pressure ratings. Its main advantage over the pascal is scale: telling someone that their car tire is inflated to 220,000 Pa is technically correct but unwieldy, whereas 2.2 bar communicates the same thing instantly. Millibars serve the same role in meteorology, keeping numbers in a human-friendly range rather than in the hundreds of thousands.
The First Pressure Measurement and the Road to the Bar
Before anyone could define a unit like the bar, someone had to prove that air exerts pressure at all. In 1644, Evangelista Torricelli filled a glass tube with mercury, inverted it into a dish, and watched the mercury column settle at a height that varied with weather conditions. He described the phenomenon in a letter containing the now-famous phrase: “We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight.”2PubMed Central. Torricelli and the ocean of air: the first measurement of barometric pressure Torricelli’s mercury column stood about 760 mm high under normal conditions, and that height became the baseline reference for atmospheric pressure for centuries.
The mercury barometer worked because the weight of the atmosphere pushing down on the open mercury dish balanced the weight of mercury in the tube. A taller column meant more atmospheric pressure; a shorter column meant less. This instrument remained the gold standard for pressure measurement well into the twentieth century, and it is the reason blood pressure is still quoted in “millimeters of mercury.” The bar, introduced in 1909 by the British meteorologist Napier Shaw, was designed to replace these length-based units with a proper force-per-area measure that fit the metric system. One standard atmosphere, 760 mm Hg, mapped to approximately 1.013 bar, and the unit spread through European meteorology and then into broader industry.
How Pressure in Bars Is Measured Today
Measuring pressure boils down to detecting how much force a fluid (gas or liquid) exerts on a known area. The technologies range from the elegantly simple to the microscopically complex, depending on how much precision and speed you need.
Mechanical Instruments
The aneroid barometer, invented in the mid-1800s, replaced mercury with a small, sealed metal capsule that flexes as outside pressure changes. A system of levers amplifies that flex and moves a needle across a dial. These instruments are still common in home weather stations and aircraft altimeters. They are rugged, need no power source, and are accurate enough for general weather observation. Their main limitation is drift: the metal capsule slowly deforms over time, so aneroid barometers need regular calibration against a reference standard.
Bourdon-tube gauges are the workhorses of industrial pressure measurement. A curved metal tube tends to straighten when pressure inside it rises, and that motion drives a pointer. You see Bourdon gauges on compressed-gas cylinders, hydraulic lines, and steam boilers. They read directly in bar (or psi, depending on the face plate) and handle pressures from well below one bar up to several hundred bar without electronics of any kind.
Electronic Sensors
Most modern pressure readings come from electronic sensors that convert mechanical deformation into an electrical signal. One widespread approach uses piezoresistive elements: a thin membrane or cantilever made of silicon changes its electrical resistance when it bends under pressure. Microelectromechanical systems (MEMS) have shrunk these sensors to chip scale. Researchers have developed MEMS piezoresistive-cantilever-based barometric sensors that achieve high sensitivity by coupling the cantilever with a sealed air chamber, allowing detection of very small pressure changes at rapid speeds.3Japanese Journal of Applied Physics. Time response characteristics of a highly sensitive barometric pressure change sensor based on MEMS piezoresistive cantilevers Tuning the dimensions of the cantilever, its length and thickness, lets engineers adjust the sensor’s frequency response to suit different applications.4Japanese Journal of Applied Physics. Influence of cantilever dimensions on time response characteristics of a MEMS piezoresistive-cantilever-based barometric pressure change sensor
Capacitive sensors take a different approach: two conductive plates sit close together, and when pressure pushes one plate closer to the other, the electrical capacitance changes. This change is measured and converted into a pressure reading. Capacitive sensors tend to be very stable over time and handle a wide range of pressures. Many of the barometric-pressure chips inside smartphones and GPS watches use either piezoresistive or capacitive designs, which is why your phone can estimate your elevation by reading atmospheric pressure accurate to a fraction of a millibar.
Thermal Gauges for Very Low Pressures
When pressure drops far below one bar, into vacuum territory, conventional mechanical and MEMS sensors lose sensitivity. Thermal gauges fill this gap. A heated filament loses heat faster when more gas molecules are around to carry the energy away. By measuring how quickly the filament cools, the gauge infers the surrounding pressure. One design pulses the filament temperature and tracks heating and cooling rates, covering an impressive range from 0.0001 mbar all the way up to 5 bar.5Vacuum. New thermal pulse vacuum gauge That kind of range makes thermal gauges useful in semiconductor fabrication, vacuum furnaces, and laboratory systems where pressure might swing across many orders of magnitude.
Keeping Measurements Honest Through Calibration
A pressure gauge is only as good as the standard it is checked against. At the top of the calibration chain sit dead-weight testers, devices that balance the pressure of a fluid column against precisely known masses sitting on a piston of precisely known area. Since pressure equals force divided by area, if you know the mass (and therefore the gravitational force) and the piston’s cross-sectional area to extreme accuracy, you know the pressure. The U.S. National Institute of Standards and Technology (NIST) maintains a primary dead-weight tester built around a piston and cylinder assembly whose diameter is known to within about 15 nanometers, and whose roundness is within roughly 30 nanometers. This instrument covers the range from 0.05 to 1.0 megapascals (0.5 to 10 bar) with a relative uncertainty of only about two parts per million.6PubMed Central. A Primary Dead-Weight Tester for Pressures (0.05-1.0) MPa
National metrology labs around the world maintain similar primary standards and periodically compare them to ensure consistency. When an engineer buys a factory-floor pressure gauge rated at “±0.1 bar,” that accuracy claim traces back, through a chain of comparisons, to a dead-weight tester like the NIST unit. Without that chain, the number on any gauge is just a guess.
Why Pressure Changes with Altitude
If you have ever popped your ears driving up a mountain pass, you have felt the bar dropping. Atmospheric pressure decreases with altitude because there is simply less air stacked above you. The relationship is not linear: pressure falls roughly exponentially, dropping faster in the first few kilometers and more slowly higher up. The barometric formula describes this altitude dependence mathematically, accounting for the fact that gravity pulls gas molecules downward while their thermal energy pushes them upward.7ChemTexts. Barometric formulas: various derivations and comparisons to environmentally relevant observations
At sea level, you start at about 1.013 bar. By 1,500 meters (roughly 5,000 feet), pressure has dropped to around 0.85 bar. At the summit of Everest, roughly 8,849 meters, it is about 0.33 bar, one-third of sea level. This is why your phone’s barometric sensor can estimate altitude: it reads the local atmospheric pressure, compares it to a reference, and backs out your height above sea level. The accuracy depends on local weather conditions, since a passing storm system can shift sea-level pressure by 30 to 40 mbar, throwing off the altitude estimate by a few hundred meters if the reference is not updated.
Bars in Extreme Environments
The bar is convenient for everyday pressures, but nature and engineering push far beyond the single-digit range. The bottom of the Mariana Trench sits under roughly 1,100 bar of water pressure. Industrial hydraulic systems routinely work between 200 and 700 bar. Diesel fuel injectors fire at pressures exceeding 2,000 bar. And inside the Earth’s mantle or at the cores of giant planets, pressures reach millions of bar.
Measuring Megabar Pressures
At pressures above roughly 10,000 bar, conventional gauges fail. Scientists studying deep-Earth minerals or creating exotic states of matter use diamond anvil cells, devices that squeeze a tiny sample between two gem-quality diamond tips. Since you cannot stick a Bourdon gauge into a space smaller than a grain of sand, researchers read pressure optically. A tiny ruby chip is placed alongside the sample. When illuminated by a laser, ruby emits fluorescent light at characteristic wavelengths, and those wavelengths shift predictably as pressure rises. This ruby fluorescence method has become the most widely used pressure gauge in high-pressure science.8PubMed. High-pressure ruby and diamond fluorescence: observations at 0.21 to 0.55 terapascal
Using this technique, researchers have reached static pressures of 0.55 terapascals, equivalent to 5.5 million bar, inside laboratory diamond cells at room temperature.8PubMed. High-pressure ruby and diamond fluorescence: observations at 0.21 to 0.55 terapascal At those pressures, the diamond anvils themselves begin to fluoresce and their signal overlaps with the ruby’s, forcing spectroscopists to use increasingly creative techniques to separate the two signals. These extreme measurements help scientists understand how materials behave deep inside planets and how new phases of matter, like metallic hydrogen, might form.
Pressures on Other Planets
Planetary science puts the bar into cosmic perspective. Mars has a surface pressure of roughly 0.006 bar, so thin that liquid water cannot exist on the surface. Earth sits at 1.013 bar. And Venus, our nearest neighbor in size, has a crushing surface pressure of about 97 atmospheres, roughly 98 bar, paired with a surface temperature near 770 kelvin.9PDS: The Planetary Atmospheres Node. Summary of the Venus Venera Mission Those numbers come from the Soviet Venera landers of the late 1960s and 1970s, which survived on the Venusian surface for only minutes before succumbing to the heat and pressure. Gas-giant planets like Jupiter and Saturn have no solid surface at all; pressure simply increases continuously with depth, reaching millions of bar in their interiors.
Everyday Encounters with the Bar
Even if you never set foot in a metrology lab or a planetary-science department, bars show up in your life more than you might realize. European tire markings commonly list recommended inflation in bar (a standard passenger-car tire runs around 2.2 to 2.5 bar). Espresso machines are rated by brewing pressure, typically 9 bar. Pressure cookers operate at roughly 1 bar above atmospheric, bringing the total inside to about 2 bar, which raises the boiling point of water and speeds cooking. Home water-supply systems in much of Europe are rated in bar, with typical mains pressure running 2 to 5 bar.
In diving, every 10 meters of seawater adds approximately 1 bar of pressure. A recreational dive to 20 meters means your body and equipment are subjected to about 3 bar of absolute pressure (1 bar from the atmosphere plus 2 from the water). Dive computers display this in bar, and tank pressure gauges read in bar as well, often up to 200 or 300 bar for a fully charged cylinder. Hyperbaric oxygen therapy chambers, used to treat decompression sickness and certain wounds, typically operate at modest overpressures. One pilot study evaluated safety at a partial oxygen pressure of 1.4 bar and found it was well tolerated, though it produced moderate cardiovascular changes and a rise in body temperature.10PubMed Central. Combined Hyperbaric Oxygen Partial Pressure at 1.4 Bar with Infrared Radiation: A Useful Tool To Improve Tissue Hypoxemia?
Common Misconceptions About Bars and Pressure
One persistent confusion is between “bar” and “bar gauge” (often written “barg”). When a tire-pressure gauge reads 2.2 bar, it is showing you 2.2 bar above atmospheric pressure, not 2.2 bar in absolute terms. The absolute pressure inside the tire is actually about 3.2 bar (2.2 + 1.013). In most consumer contexts, gauge pressure is what you see, but in scientific and process-engineering contexts, absolute pressure (bara) matters. Mixing the two up can cause real problems: setting a chemical reactor to the wrong reference can mean operating at dangerously higher or lower pressures than intended.
Another misconception is that pressure and force are the same thing. A person standing on one foot exerts far more pressure on the floor than when standing on both feet, even though the force (their weight) has not changed. The smaller area concentrates the force, raising the pressure. This is why a thumbtack pushed with modest force can pierce a surface: the tiny point area creates enormous pressure. Understanding this distinction is what makes the bar meaningful. It is not describing a lump of force; it is describing how that force is distributed across a surface.
A subtler misunderstanding involves weather maps. When a forecaster says a low-pressure system is at 990 mbar, that number has been adjusted to mean sea-level equivalent pressure, not the actual pressure at the weather station, which might be sitting at 500 meters of elevation. Without this adjustment, every mountain station would always look like a deep low-pressure zone, and pressure patterns related to actual weather would be invisible. So the number on the map is a corrected value, and the correction itself depends on temperature, humidity, and the station’s elevation, all folded into standard reduction formulas. It is one of those quiet pieces of math that makes weather forecasting possible but that most people never think about.
The Bar in Acoustic and Dynamic Pressure
Sound is a pressure wave: alternating regions of slightly higher and slightly lower pressure rippling through air. The pressures involved are tiny compared with atmospheric pressure. The threshold of human hearing corresponds to a pressure fluctuation of about 20 micropascals, or 0.0000000002 bar. Even a painfully loud sound, near 120 decibels, involves pressure swings of only about 20 pascals, or 0.0002 bar. Acousticians historically used the microbar (0.1 Pa) as a convenient reference unit for sound-pressure measurements.1Japanese Journal of Applied Physics. Acoustics: Sound Fields and Transducers Although most modern acoustic work has shifted to pascals, the microbar still appears in older literature and in some American standards, so recognizing it prevents confusion when reading legacy measurements.
Dynamic pressure in flowing fluids is another context where the bar shows up. When air rushes past an aircraft wing, the kinetic energy of the air converts into pressure. At cruising altitude, the dynamic pressure on a commercial jet is typically in the range of 0.1 to 0.3 bar, a figure that matters for structural loading and control-surface design. Wind tunnels, jet-engine test cells, and automotive aerodynamics labs all measure and report pressures in bar or millibar as a matter of course, linking everyday engineering back to the same unit Napier Shaw introduced over a century ago for weather forecasting.