Pressure Units: A Practitioner's Guide for Engineers

The 4 most common pressure units in engineering

1. psi (lb/in²) — US pneumatic, hydraulic, gas cylinders (200 bar ≈ 2900 psi)
2. bar — European process, pneumatic, hydraulics
3. kPa — building HVAC, low-pressure gas, medical
4. MPa — high-pressure hydraulics, structural

Gauge vs absolute: the silent killer

psig (gauge) reads zero at atmosphere. psia (absolute) reads atmospheric + gauge. A 50 psig tire gauge at sea level is 64.7 psia. At 10,000 ft (10.1 psia atmospheric), the same tire shows 60.1 psia.

When in doubt, ask: "is the zero at vacuum or atmosphere?"

Conversions you should memorize

• 1 bar = 14.5038 psi
• 1 atm = 14.696 psi = 1.01325 bar
• 100 kPa = 1 bar ≈ 14.5 psi
• 1 inch H₂O = 0.0361 psi (low-pressure HVAC)
• 1 inch Hg = 0.4912 psi (vacuum)

Tools to use

• PSI to Bar — the everyday conversion
• PSIG to PSIA — for absolute pressure calculations
• MPa to PSI — for hydraulic systems