Key Takeaway
The SI unit for pressure gauges is the Pascal (Pa), which is defined as one Newton per square meter (N/m²). However, because the Pascal is a small unit, larger multiples are often used in practical applications:
kPa (kilopascal) = 1,000 Pa
MPa (megapascal) = 1,000,000 Pa
In many industries, pressure is commonly measured in bar (1 bar = 100,000 Pa) or psi (1 psi ≈ 6.895 kPa), depending on the region. Although the SI unit is Pascal, you will often see pressure gauges marked with psi or bar, especially in automotive, HVAC, or industrial machinery, due to convention and ease of reading.
Introduction to Pascal (Pa) and Its Definition
The Pascal (Pa) is the SI (International System) unit of pressure. It’s defined as 1 newton per square meter (N/m²). That means if a force of one newton is applied to an area of one square meter, the pressure is one Pascal.
While this unit is scientifically correct, it’s often too small for practical use. So we use:
kPa (kilopascal) = 1,000 Pa
MPa (megapascal) = 1,000,000 Pa
In HVAC, microelectronics, and medical applications, Pascals are useful because of their precision. But in heavy industries, we prefer bar or PSI for readability.
Still, understanding Pascals is important when working across international standards, scientific calculations, or pressure simulation models.
Derivation of 1 Pascal from Basic SI Units
The Pascal (Pa) is the SI unit of pressure, defined as:
1 Pa = 1 Newton / 1 square meter (N/m²)
To break it down:
Force is measured in Newtons (N)
Area is in square meters (m²)
So, if you apply a force of 1 Newton evenly over 1 m², the pressure is 1 Pascal.
Since Pascal is a small unit, we often use:
kPa (1,000 Pa)
MPa (1,000,000 Pa)
Understanding this derivation helps engineers connect pressure with mechanical stress, force, and surface area. It’s a fundamental building block in fluid mechanics, structural analysis, and instrumentation design.
Why Pascal Is Used in Science and Engineering
The Pascal (Pa) is the SI unit of pressure, defined as 1 Newton per square meter. It’s the standard in scientific and engineering contexts due to its:
Simplicity
Direct relation to force and area
Compatibility with other SI units
Though impractical for industrial use due to its small size (1 Pa = ~0.000145 PSI), it scales well:
1 kPa = 1000 Pa
1 MPa = 1,000,000 Pa
In simulations, stress calculations, and fluid mechanics, using Pascals ensures consistency across equations and systems — which is why it’s the go-to unit in scientific models.
High-Pressure Variants: kPa, MPa
For high-pressure systems like CNG fueling, hydraulic presses, or aerospace testing, pressure is typically expressed in:
kPa (kilopascal): 1 kPa = 1,000 Pascal
MPa (megapascal): 1 MPa = 1,000,000 Pascal = 10 bar = ~145 PSI
These units offer better clarity for large values (e.g., 21 MPa instead of 21,000,000 Pa). They’re also part of the SI system — making them ideal for scientific, regulatory, and technical use.
Gauges and sensors rated in MPa must be built with reinforced materials, safety blowouts, and overrange protection due to the enormous forces involved.
Where Non-SI Units Are Still Used
Despite SI (International System of Units) being the global standard, many industries still rely on non-SI pressure units:
psi (pounds per square inch): Common in the US and industries like automotive, HVAC, and hydraulics.
inHg (inches of mercury): Used in vacuum and weather applications.
bar: Widely used in Europe and global industries.
atm (atmospheres): Used in diving, vacuum science.
Torr/mmHg: Predominantly used in vacuum applications, medical and scientific fields.
Why do they persist?
Legacy equipment and processes
Industry norms and documentation
Simpler communication in field work (e.g., “35 psi tire pressure”)
Transitioning to SI units (like Pascal or kPa) is ongoing — but for practical purposes, many non-SI units will remain in active use for years. Always check unit settings in digital gauges and ensure consistency across data sheets and logs.
Conclusion
The International System of Units (SI) was developed to unify measurements worldwide, and pressure is no exception. The Pascal (Pa), the SI unit for pressure, simplifies global operations and reduces unit confusion in engineering calculations, reporting, and product design. Using SI units eliminates costly conversion errors and ensures compatibility between equipment sourced from different regions. Whether working in aerospace, pharmaceuticals, or manufacturing, standardizing on SI units also helps in meeting ISO, ASTM, or IEC requirements. It’s especially critical in automation and sensor-based systems where unit mismatches can cause calibration drift or inaccurate feedback loops. For engineers aiming at precision and global compliance, SI units are not just ideal—they’re essential.