Why Does Pressure Have So Many Different Units?

Check the side of a tire and you may see pressure in psi. Look at a European car manual and it may use bar instead.

A weather forecast might report 1,013 hectopascals. A doctor writes blood pressure in millimeters of mercury. A physics textbook uses pascals. Vacuum equipment may be labelled in torr.

All of them are measuring pressure.

This is not because pressure has several different meanings. It is mostly the result of history. Different trades learned to measure pressure with different instruments, in different countries, for different purposes. Their units became familiar long before the world had one international system of measurement.

The pascal eventually became the official SI unit. The older units did not disappear.

Pressure is force spread over an area.

Push with the same force over a smaller area and the pressure rises. Spread that force over a larger area and the pressure falls.

That is why a narrow high heel can put enormous pressure on a floor even though the person wearing it weighs far less than a car. The force is concentrated into a tiny area.

The SI unit follows directly from this definition. One pascal is one newton of force acting over one square metre.

Written out, 1 Pa = 1 N/m².

The definition is tidy. Everyday pressures usually are not.

One pascal is a very small amount of pressure.

Normal atmospheric pressure at sea level is about 101,325 Pa. Writing numbers of that size all day is inconvenient, so practical measurements usually add an SI prefix.

A kilopascal, or kPa, is 1,000 Pa. A megapascal, or MPa, is 1,000,000 Pa.

The underlying unit has not changed. The prefix simply makes the number easier to handle.

That is why a scientist might write pressure in pascals, a weather service in hectopascals, and an engineer in megapascals while all three remain within the same basic SI system.

Psi is short for pounds per square inch. The name almost explains the unit by itself.

One psi is one pound-force acting over one square inch.

It belongs to the older family of inch-and-pound measurements that became deeply established in British and American engineering. In countries where those units remained common, pressure gauges, compressors, pumps and vehicle equipment were built and labelled around them.

That history still shows up on tire gauges today.

In the United States, a driver is far more likely to think of a tire as needing something like 32 psi than 221 kPa, even though those numbers describe roughly the same pressure.

In countries that use metric units more heavily, the same tire may instead be labelled in kPa or bar.

The bar is not the official SI unit, but it fits comfortably beside SI units.

One bar is defined as exactly 100,000 Pa, or 100 kPa.

That happens to be close to ordinary atmospheric pressure at sea level. One standard atmosphere is 101,325 Pa, or about 1.01325 bar.

That makes bar a convenient size for many real machines. A pressure of 2 bar, 5 bar or 200 bar is easier to say and read than the equivalent value written out in pascals.

The unit became common in engineering and remains widespread in compressors, hydraulics, diving equipment and vehicle specifications in many parts of the world.

It is a good example of why an official standard does not automatically erase a practical unit. A unit can survive simply because an entire industry already speaks it fluently.

The strangest-looking pressure unit may be mmHg: millimeters of mercury.

The name comes from a very literal way of measuring pressure.

A column of liquid mercury rises or falls as pressure changes. Measure the height of that column in millimeters and you have a pressure reading in millimeters of mercury.

Mercury manometers became central to blood-pressure measurement. A reading such as 120/80 mmHg described pressures in terms of the heights they would support in a mercury column.

Modern digital blood-pressure monitors do not need a visible column of mercury. The unit remained anyway.

Medicine had already built its measurements, reference ranges, textbooks, research and everyday language around mmHg. Replacing the unit would change the numbers without changing the underlying pressure.

So a digital machine can use electronic sensors and still display a unit named after a column of metal.

Meteorology has its own version of the same story.

Weather services commonly used the millibar for atmospheric pressure. Standard sea-level pressure is 1,013.25 millibars.

When scientific measurement moved more firmly toward SI units, meteorologists could have started reporting that as 101,325 Pa.

Instead, the hectopascal offered an unusually painless change.

One hectopascal is exactly 100 Pa. One millibar is also exactly 100 Pa.

That means 1,013 millibars and 1,013 hectopascals have the same numerical value.

NOAA notes that meteorology had used the millibar since 1929. When the field moved toward scientific SI units, the hectopascal allowed forecasters to adopt the pascal without forcing everyone to relearn the familiar scale.

The label changed. The weather-map numbers did not.

Another pressure unit begins with a useful reference rather than a particular measuring instrument.

The standard atmosphere, written atm, is defined as exactly 101,325 Pa.

It is close to the typical atmospheric pressure near sea level, which made one atmosphere an intuitive reference for experiments involving gases.

A pressure of 2 atm is therefore easy to picture as roughly twice standard atmospheric pressure. Half an atmosphere is roughly half that reference pressure.

The atmosphere is especially familiar in chemistry and gas calculations, even though the pascal is the SI unit.

Vacuum science often introduces another unit: the torr.

The name honours Evangelista Torricelli, whose work with mercury helped establish the principles behind the barometer.

A torr is now defined as exactly one 760th of a standard atmosphere.

That makes 1 Torr extremely close to 1 mmHg, and the two are often treated as interchangeable in ordinary work.

Strictly speaking, however, their modern definitions are not exactly the same. Torr is tied to the defined standard atmosphere, while the conventional millimeter of mercury has its own standardized relationship to the pascal.

The difference is tiny, but it is another reminder that units which grew from the same physical idea can take slightly different paths as measurement becomes more precise.

The variety starts to look less random once the units are placed beside the jobs that use them.

Two pressure readings can use the same unit and still mean different things if they use different zero points.

A tire gauge usually measures gauge pressure. It treats the surrounding air pressure as zero and tells you how much higher the pressure inside the tire is than the air outside it.

Absolute pressure uses a perfect vacuum as zero.

That distinction matters because the atmosphere is already pressing on everything around us. A tire gauge showing zero does not mean there is literally no pressure inside the tire. It means the pressure has fallen to roughly the same level as the surrounding atmosphere.

So pressure measurement carries two questions: which unit is being used, and what is the measurement being compared with?

In principle, they could.

Every pressure in this article can be converted into pascals. NIST publishes conversion factors for units including psi, bar, atmospheres, torr and millimeters of mercury.

But measurement systems do not live only in equations.

They live on gauges, machine drawings, tire stickers, weather maps, medical records, regulations, textbooks and in the habits of people who use them every day.

Once an industry has spent decades building around a unit, replacing it has a cost. Instruments have to change. Documentation has to change. People have to learn new reference numbers. Old records still have to be understood.

And in many cases, there is little practical reward for doing so.

A doctor reading mmHg, a mechanic reading psi and a meteorologist reading hPa are not disagreeing about what pressure is. They are using the vocabulary their fields inherited.

There is one official SI unit of pressure: the pascal.

The reason the world still has psi, bar, atmospheres, millimeters of mercury, torr and millibars is not that physics needs all of them.

People do.

Some units came from the way pressure was physically measured. Some came from older systems of force and length. Some provided numbers that were convenient for a particular job. Once those units became standard inside a field, familiarity helped keep them there.

That is why the pressure in a tire, an artery and the atmosphere can all be the same kind of physical quantity and still arrive with completely different labels.

The units are measuring pressure.

They are also carrying a little history with them.