Whether you’re reading a client spec in MPa, a P&ID in bar, or a field gauge in psi, mixing pressure units can slow you down. This one-page guide gives the exact conversions you need—and a pocket-size table you can save to your phone.

Why Units Get Confusing

• Colloquial use of “kilogram” for both force and pressure.

• Coexistence of SI, Imperial and metric engineering units.

• Simultaneous appearance of MPa, bar and psi in the same data set.

First, we need to understand that in the context of valves and pipe flanges, PN stands for Nominal Pressure. 

It is a dimensionless designation (e.g., PN10, PN16, PN25, PN40) used to classify the maximum allowable working pressure of a flange, valve, or piping component at a specified reference temperature—usually ambient.

The PN value is not a pressure unit itself. 

For instance, PN16 indicates that the flange is rated for a maximum allowable working pressure of 16 bar at ambient temperature, but PN16 does not numerically equal 16 bar in the same way that 1 bar equals 100 kPa.

In summary: do not treat PN as a pressure unit like Pa, bar, or MPa. It is an engineering pressure-class index whose nominal reference is approximately equal to the bar value (PN number ≈ bar number).

And, in China, it is still common to use “kilogram” (kilogram-force or kg f/cm²) as a pressure unit, where 1 kg f/cm² represents one kilogram of force acting on one square centimeter.

Conversion among MPa, bar, kg f/cm², and other units:

• Pascal (Pa) is a very small unit; a peanut on a desk exerts more than 10 Pa. For engineering convenience, multiples are used:

1 MPa = 1 000 kPa = 1 000 000 Pa

• PSI (pounds per square inch) is widely used in Europe and North America:

1 psi ≈ 6.895 kPa ≈ 6 894.757 Pa

• Bar is accepted within the SI system (Système International d’Unités) and common in industry:

1 bar = 0.1 MPa = 100 kPa = 100 000 Pa

• Standard atmosphere (atm):

1 atm = 760 mmHg = 101.325 kPa = 101 325 Pa

• Millimeter of mercury (mmHg):

1 mmHg = 1 Torr = 0.133 322 4 kPa ≈ 133.322 Pa

• Inch of mercury (inHg):

1 inHg = 3.386 388 2 kPa ≈ 3 386.388 Pa

• Millimeter of water (mmH₂O):

1 mmH₂O ≈ 9.807 Pa

• Kilogram-force per square centimeter (kgf/cm²):

1 kgf/cm² ≈ 0.098 MPa ≈ 98.066 5 kPa ≈ 98 066.5 Pa

The 10-Second Conversion Table

1 MPa = 10 bar ≈ 145 psi ≈ 10.197 kgf/cm²

1 bar = 0.1 MPa = 100 kPa ≈ 14.5 psi ≈ 1.02 kgf/cm²

1 psi ≈ 6.895 kPa ≈ 0.06895 bar ≈ 0.0703 kgf/cm²

1 kgf/cm² ≈ 0.098 MPa ≈ 0.981 bar ≈ 14.22 psi

Worked Examples

1. Design Input

    Spec = 4 bar → Design pressure = 0.4 MPa

    Hydrotest = 1.5 × 0.4 MPa = 0.6 MPa

2. Site Calibration

    Gauge reads 0–10 bar → Record 1 bar = 0.1 MPa

3. International Procurement

    US spec 150 psi → European PN10 (10 bar) covers requirement.