PSIA vs PSIG: Pressure Units Explained

Quick Answer: PSIG (pounds per square inch gauge) measures pressure relative to atmospheric pressure, while PSIA (pounds per square inch absolute) measures pressure relative to a perfect vacuum. The conversion is: PSIA = PSIG + 14.696. Standard atmospheric pressure equals 0 PSIG or 14.696 PSIA.

Technical Definitions

PSIG (gauge pressure) reads zero at atmospheric pressure. A pressure gauge on a pipe or vessel displays PSIG—it shows only the pressure above the surrounding atmosphere. Most piping design pressures, hydrostatic test pressures, and valve ratings are expressed in PSIG.

PSIA (absolute pressure) includes the full atmospheric pressure component. It references a perfect vacuum (0 PSIA). Absolute pressure is always positive and is required for thermodynamic calculations, gas law equations, and vacuum system design.

Conversion Between PSIA and PSIG

Condition	PSIG	PSIA	Notes
Perfect vacuum	-14.696	0	Theoretical zero absolute pressure
Full vacuum (practical)	-14.0 to -14.5	0.2 to 0.7	Achievable with mechanical pumps
Atmospheric pressure (sea level)	0	14.696	Standard reference per API and ASME
Typical instrument air	100	114.696	Common utility pressure
ASME Class 150 flange rating at ambient	285	299.696	Per ASME B16.5 pressure-temperature table
High-pressure steam (600 PSIG)	600	614.696	Boiler and power piping

Conversion formulas:

• PSIA = PSIG + 14.696 (at standard atmospheric conditions)
• PSIG = PSIA - 14.696
• At altitudes above sea level, actual atmospheric pressure is lower (e.g., ~12.2 PSIA at 5,000 ft elevation)

When to Use PSIG vs PSIA

Application	Unit	Reason
Piping design pressure (ASME B31.3)	PSIG	Code calculations reference gauge pressure
Flange pressure-temperature ratings	PSIG	ASME B16.5 tables list gauge pressure
Valve specifications	PSIG	Valve nameplates and datasheets use gauge
Hydrostatic test pressure	PSIG	Test gauges read gauge pressure directly
Gas law calculations (PV = nRT)	PSIA	Thermodynamic equations require absolute pressure
Vacuum system design	PSIA (or inHg abs)	Gauge pressure becomes negative; absolute avoids confusion
Relief valve set pressure	PSIG	Per API 520/521, set pressure is gauge
Relief valve back pressure	PSIA or PSIG	Superimposed back pressure may use either; accumulation uses gauge
Compressor suction/discharge	PSIA	Compression ratio requires absolute values
Vapor pressure of fluids	PSIA	Thermodynamic property; always absolute

Common Mistake

Using PSIG in gas law or compressibility calculations gives incorrect results. The ideal gas law PV = nRT and all equations of state require absolute pressure. A system at 100 PSIG is actually at 114.7 PSIA—failing to add atmospheric pressure introduces a 13% error in volume or flow calculations.

Metric Equivalents

Imperial Unit	Metric Equivalent	Conversion
1 PSIG	0.0689 barg	Gauge pressure in bar
1 PSIA	0.0689 bara	Absolute pressure in bar
14.696 PSIA	1.01325 bara	Standard atmosphere
1 PSIG	6.895 kPag	Kilopascals gauge

Pro Tip

On P&IDs and process datasheets, always specify the pressure suffix (g or a). Writing “150 psi” without clarification creates ambiguity. ASME and API documents default to gauge unless explicitly stated otherwise, but project-specific practice may differ.

Pressure unit selection is a fundamental part of the design basis documented in the pipe class specification, which defines the design pressure (in PSIG or barg) for every piping service.

Read the full guide to pipe class specifications