Gauge vs absolute pressure — psig, psia, barg, bara
psia − 14.696 = psig at sea level. Miss the offset in a compressor sizing calc and you either overspend on horsepower or starve the tools.
Every pressure gauge on the planet zeros at whatever the atmosphere happens to be that day. Every gas-law equation assumes zero means vacuum. The 14.7 psi offset between them silently corrupts calculations across process, HVAC, refrigeration and pneumatics.
The distinction
psia (absolute) = psig (gauge) + p_atm > At sea level: p_atm ≈ 14.696 psi = 101.325 kPa = 1.01325 bar
When to use each
Absolute: ideal gas law, compressor thermodynamics, altitude-corrected performance, vacuum measurements. Gauge: pipe and vessel ratings, safety relief settings, everyday instrumentation, tyre pressures.
Worked example — receiver sizing
Compressor delivers 100 CFM at 100 psig into a 60 gal receiver from 60 to 100 psig. Wrong (treating psig as psia): pressure ratio 100/60 = 1.67. Right: (100+14.7)/(60+14.7) = 1.53. Cycle time changes 9% — enough to burn out an under-cycled motor.
Regional cues
US: often just 'psi' with 'g' or 'a' inferred from context — dangerous. Europe: 'barg' vs 'bara' explicit on most datasheets. Vacuum work: always absolute (torr, mbar, Pa).
Recap
Suffix matters. Assume gauge on instrumentation, absolute in gas-law formulas, and refuse spec sheets that leave the letter off.
Try the converters mentioned in this article
FAQ
Is atmospheric pressure exactly 14.7 psi?+
14.696 psi at sea level, standard day. Colorado shops work closer to 12.2 psi — enough to shift compressor performance measurably.
Is 'kPa (a)' the same as kPaA?+
Yes — many notations. 'kPaG', 'kPa gauge', 'kPa(g)' all mean gauge; 'kPaA', 'kPa abs' mean absolute.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.