Pressure is everywhere. Civil and geotechnical engineers see it in soil bearing capacity and lateral earth pressures. Mechanical engineers see it in pipe ratings and compressed air systems. HVAC designers deal with static pressure and fan curves. Process engineers live in a world of pressure drops. The quantity is common, but the conventions are not.

Most conversion mistakes with pressure are not from mixing kPa and psi. They come from converting the wrong kind of pressure. Gauge pressure, absolute pressure, and differential pressure can all be expressed in kPa, but they do not mean the same thing.

The unit conversion is the easy part

A few reference values are useful:

• 1 psi is about 6.895 kPa
• 1 bar is 100 kPa
• Standard atmospheric pressure is about 101.325 kPa

The issue is that a pressure value without a reference is incomplete. If a system is specified as 300 kPa, you need to know whether that is: 300 kPa gauge, 300 kPa absolute, or 300 kPa differential across a component. All three are plausible depending on the application.

Gauge vs absolute: the cause of many "it should work" moments

Gauge pressure is measured relative to local atmospheric pressure. Absolute pressure is measured relative to vacuum. When you convert between units, you keep the same reference. When you change reference, you must add or subtract atmospheric pressure.

Example: A tank is listed at 200 kPa gauge. What is the absolute pressure? Assuming local atmosphere near sea level, absolute pressure is about:

200 kPa gauge + 101.325 kPa = 301.325 kPa absolute

Converting 200 kPa directly to psi is fine as a gauge value, but it is not the same as converting the absolute pressure. If you are using the pressure for gas density calculations, absolute is usually required.

Differential pressure: the "quiet" unit mismatch

Differential pressure is often used in filters, orifice plates, and HVAC ductwork. A specification might say "filter rated for 250 Pa pressure drop at design flow." That is not a system pressure, it is a delta.

In HVAC, you will also see inches of water column. It is a perfectly valid unit, but it is different from psi in a way that invites mistakes. Inches of water is typically used for small pressure differences, not absolute system pressure.

A realistic engineering example: pipe class and test pressure

Many piping specs list a nominal pressure class in one unit system and a hydrotest pressure in another. For example, a component may be rated for 150 psi, while the project documentation uses kPa. The conversion is simple, but the engineering question is: is the rating a maximum operating pressure, a design pressure, or a test pressure with a factor applied?

A common hydrotest practice is to test at 1.5 times design pressure. If you convert pressure without tracking whether the factor is already included, you can easily end up testing too low or specifying too high. Neither outcome is fun, and only one is expensive in the way you wanted.

Practical checks that catch pressure mistakes

Engineers who are good at unit consistency usually do small sanity checks:

• Compare to atmosphere. If your absolute pressure is below 0 kPa, something is wrong.
• Check typical ranges. Tire pressure is tens of psi. Municipal water is often 40 to 100 psi. Soil bearing values are often tens to hundreds of kPa.
• Confirm reference. Gauge, absolute, or differential.
• Track temperature when needed. For gases, pressure alone is not the full story.

A small note on humor and pressure

Engineers sometimes say they work well under pressure. The unit system does not care. It will happily convert your value correctly while you apply it incorrectly. That is why context matters more than the conversion factor.

Convert the units, then confirm the meaning. That order saves time.

Related tools: Pressure, Force, Area, Temperature.