1. Pascal (Pa) – International System (SI)

The Pascal is the official pressure unit of the International System of Units (SI).

1 Pa = 1 N/m²

It is used in physics, scientific research, and precise calculations.

2. Bar and mbar (bar / millibar)

Widely used in industry and technical applications.

1 bar = 100,000 Pa
1 mbar = 0.001 bar

Millibar is commonly used in meteorology.

3. Atmosphere (atm)

Represents the average atmospheric pressure at sea level.

1 atm = 101,325 Pa ≈ 1.013 bar

4. psi, psia, and psig (pounds per square inch)

Traditional U.S. pressure scale, widely used in hydraulic, pneumatic systems, and compressors.

• psi: pounds per square inch
• psia: absolute pressure
• psig: gauge pressure (relative pressure), excluding atmospheric pressure

Example:
0 psig means atmospheric pressure.

5. Torr and mmHg (millimeters of mercury)

Mainly used in vacuum systems, chemistry, and medicine.

1 Torr ≈ 1 mmHg
760 mmHg = 1 atm

6. inHg (inches of mercury)

Used in aviation and meteorology.

Example:
Atmospheric pressure ≈ 29.92 inHg

7. kgf/cm² (kilogram-force per square centimeter)

Very common in industrial applications, hydraulics, pumps, and boilers.

1 kgf/cm² ≈ 14.22 psi ≈ 0.98 bar

8. inH₂O and mmH₂O (water column)

Used to measure small pressures, mainly in:

• Ventilation systems
• HVAC systems
• Low-pressure systems
• U-tube manometers

Pressure Classification by Reference Type

In addition to units, pressure can be measured according to three reference types:

A) Absolute Pressure (abs or “a”)

Reference: absolute vacuum

Example:
1 atm = 1.013 bar abs

B) Gauge Pressure (g or “gauge”)

Reference: atmospheric pressure

Example:
0 bar(g) = atmospheric pressure

C) Differential Pressure (ΔP)

Difference between two pressure points.

Used in filters, ducts, pumps, and similar applications.

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