What are the units of pressure?

Short Answer:

Pressure is the normal force acting per unit area, and its unit depends on the system of measurement used. The SI unit of pressure is Pascal (Pa), which is equal to one newton per square meter (1 N/m²). Other commonly used units include bar, atmosphere (atm), millimeters of mercury (mmHg), and pounds per square inch (psi).

In simple words, pressure can be measured in different units depending on the type of application. For example, atmospheric pressure is measured in bar or mmHg, while tire pressure or hydraulic pressure is often measured in psi or Pascal.

Detailed Explanation :

Units of Pressure

Pressure is defined as the force exerted per unit area on a surface, and it plays a major role in the study of fluids, gases, and solid mechanics. Since pressure is derived from force and area, its units are determined from the relationship:

where,

  • P = pressure
  • F = force
  • A = area

The standard unit of pressure depends on the system of units being used—SI (International System), CGS (Centimeter-Gram-Second System), FPS (Foot-Pound-Second System), and other engineering or practical systems like bar and atmosphere.

SI Unit of Pressure

The SI unit of pressure is Pascal (Pa).
1 Pascal is defined as the pressure exerted when a force of one newton acts uniformly over an area of one square meter.

Hence,

Although Pascal is the official SI unit, in engineering applications, pressure values are often very large, so other convenient multiples are used:

  • 1 kPa (kilopascal) = 1,000 Pa
  • 1 MPa (megapascal) = 1,000,000 Pa

For example, tire pressure or hydraulic system pressure is usually expressed in MPa instead of Pa.

CGS Unit of Pressure

In the CGS system (centimeter-gram-second), the unit of pressure is dyne per square centimeter (dyne/cm²).
Since,

and

we get:

Hence,

This unit is mostly used in theoretical studies and older scientific references, while Pascal is preferred in modern engineering systems.

FPS Unit of Pressure

In the FPS (foot-pound-second) system, pressure is expressed in pounds per square inch (psi) or pounds per square foot (psf).

and,

This unit is commonly used in countries like the United States for industrial, automotive, and hydraulic applications. For instance, car tire pressures are often around 30–35 psi.

Bar and Its Relation to Pascal

The bar is a commonly used practical unit of pressure, especially in meteorology and engineering. It is defined as:

This is approximately equal to 1 atmosphere (the average air pressure at sea level).
Smaller submultiples of the bar are often used:

  • 1 millibar (mbar) = 0.001 bar = 100 Pa

Example: Meteorologists often use millibars to express atmospheric pressure. Standard atmospheric pressure is 1013 mbar or 1.013 bar.

Atmosphere as a Unit of Pressure

The standard atmosphere (atm) is another important unit of pressure, defined as the pressure exerted by the Earth’s atmosphere at sea level.

This unit is often used in physics, chemistry, and thermodynamics to express gas pressures and boiling points.

Millimeter of Mercury (mmHg)

This unit is based on the height of a mercury column in a barometer that supports the pressure of the atmosphere. It is widely used in medical science and meteorology.

  • 1 mmHg = Pressure exerted by a mercury column 1 mm high.
  • 1 mmHg = 133.3 Pa

The standard atmospheric pressure is 760 mmHg, which is approximately equal to 1 atm.

Torr

The Torr is another unit of pressure, named after Evangelista Torricelli, who invented the barometer.

It is identical in magnitude to 1 mmHg and is mainly used in vacuum measurements and laboratory work.

Conversions Between Common Units

Some important conversion relationships between common pressure units are:

 

These conversions are very useful in engineering and scientific calculations, where pressure might be given in different units depending on the context.

Examples in Practical Use

  • Meteorology: Atmospheric pressure is often expressed in bar or mmHg.
  • Automotive Engineering: Tire pressures are measured in psi or bar.
  • Hydraulic Systems: Hydraulic pressures are given in MPa.
  • Medical Field: Blood pressure is measured in mmHg (for example, 120/80 mmHg).
  • Industrial Applications: Steam and gas pressures are commonly measured in bar or atm.

Importance of Using Correct Units

Using correct pressure units is extremely important in engineering because:

  • It ensures accuracy in design and analysis.
  • It avoids errors in conversions, especially when working with international standards.
  • It helps in standardizing measurements across different systems (hydraulics, pneumatics, thermodynamics).

A small mistake in pressure unit conversion can lead to serious mechanical or operational failures in systems such as pipelines, boilers, or pressure vessels.

Conclusion

In conclusion, pressure can be measured in different units depending on the system of measurement and application. The SI unit of pressure is Pascal (Pa), which is equivalent to one newton per square meter. Other commonly used units include bar, atmosphere (atm), millimeters of mercury (mmHg), and pounds per square inch (psi). Each unit is suited for a particular field—engineering, meteorology, medicine, or laboratory science. Correct usage and conversion between these units are essential for accurate design and analysis in mechanical engineering.