PSIG vs PSIA: Difference Explained, Conversion Formulas & Comparison
Discover the key differences between PSIG and PSIA pressure, why 0 PSIG equals 14.7 PSIA, and how to change from PSIG to PSIA accurately.
The PSIG vs PSIA difference lies in the reference zero point: PSIG measures gauge pressure relative to ambient atmospheric air, whereas PSIA measures absolute pressure relative to a perfect vacuum. Understanding the difference between PSIG and PSIA is essential across mechanical engineering, HVAC/R technology, pneumatics, and process safety.
For automated calculations, use our free online PSIG to PSIA calculator on the home page.
1. Understanding the PSIG vs PSIA Difference: Gauge vs Absolute Pressure
What is PSIG in the PSIG vs PSIA Difference?
PSIG (Pounds per Square Inch Gauge) measures pressure relative to ambient atmospheric air.
- Datum (Zero Point): Ambient atmospheric pressure.
- Reading at Sea Level: Open air reads 0 PSIG.
- Negative Values: Values between 0 PSIG and -14.7 PSIG represent vacuum levels.
What is PSIA in the PSIG vs PSIA Difference?
PSIA (Pounds per Square Inch Absolute) measures total pressure relative to absolute zero pressure (a total vacuum).
- Datum (Zero Point): Absolute vacuum ($0\text{ PSIA}$).
- Reading at Sea Level: Open air reads 14.7 PSIA ($14.6959\text{ psi}$).
- Non-Negativity: Absolute pressure can never be negative ($PSIA \ge 0$).
2. Converting the PSIG vs PSIA Difference Using Core Formulas
Core Engineering Rule: To change from PSIG to PSIA you must add ambient atmospheric pressure to the gauge pressure reading.
$$\text{PSIA} = \text{PSIG} + P_{\text{atm}}$$
At standard sea level conditions ($P_{\text{atm}} = 14.7\text{ psi}$):
$$\text{PSIA} = \text{PSIG} + 14.7$$
Quick answer: PSIG vs PSIA Difference
Answer: The fundamental PSIG vs PSIA difference is that PSIG uses atmospheric pressure as zero, whereas PSIA uses a perfect vacuum as zero. To change from PSIG to PSIA you must add 14.7 psi ($\text{PSIA} = \text{PSIG} + 14.7$). 0 PSIG equals 14.7 PSIA, and 14.7 PSIG equals 29.4 PSIA.
3. Comparison Matrix: PSIG vs PSIA Difference Across Industrial States
| Physical State / Environment | Gauge Pressure (PSIG) | Absolute Pressure (PSIA) | Notes / Applications |
|---|---|---|---|
| Absolute Perfect Vacuum | -14.7 PSIG | 0.0 PSIA | Space vacuum / Deep chamber |
| HVAC Vacuum Evacuation | -7.35 PSIG | 7.35 PSIA | 50% partial vacuum |
| Open Ambient Air (Sea Level) | 0.0 PSIG | 14.7 PSIA | Standard ambient atmosphere |
| Car Tire (Standard) | 32.0 PSIG | 46.7 PSIA | Tire gauge measurement |
| Shop Air Compressor | 90.0 PSIG | 104.7 PSIA | Pneumatic tool supply line |
| Industrial Steam Line | 150.0 PSIG | 164.7 PSIA | High pressure steam header |
4. How Do You Convert PSIG to PSIA & Vice Versa?
Converting PSIG to PSIA
To perform conversion of PSIG to PSIA: $$\text{PSIA} = \text{PSIG} + 14.7$$
- Example: $0\text{ PSIG} + 14.7 = 14.7\text{ PSIA}$.
- Example: $14.7\text{ PSIG} + 14.7 = 29.4\text{ PSIA}$.
Detailed guide: How to Convert PSIG to PSIA.
Converting PSIA to PSIG
To perform conversion of PSIA to PSIG: $$\text{PSIG} = \text{PSIA} - 14.7$$
- Example: $14.7\text{ PSIA} - 14.7 = 0\text{ PSIG}$.
- Example: $100\text{ PSIA} - 14.7 = 85.3\text{ PSIG}$.
Detailed guide: How to Convert PSIA to PSIG.
5. Practical Engineering Guidance
When designing HVAC systems, pneumatics, or gas storage, always verify whether your sensor outputs PSIG or PSIA.
For elevation adjustments, see our guide on altitude atmospheric pressure PSIG PSIA calculator.
Convert any pressure value instantly on our PSIG to PSIA home calculator.
Related Resources
- PSIG to PSIA Tool: Homepage PSIG to PSIA Calculator – Online unit converter.
- PSIG to PSIA Guide: How to Convert PSIG to PSIA – Full step-by-step conversion.
- PSIA to PSIG Guide: How to Convert PSIA to PSIG – Reverse conversion calculations.
- Altitude Effects: Altitude Atmospheric Pressure Guide – Elevation barometric corrections.
Frequently asked questions
What is the difference between PSIG and PSIA?
PSIG measures pressure relative to local atmospheric air (reading 0 PSIG at sea level), whereas PSIA measures total pressure relative to a absolute zero vacuum (reading 14.7 PSIA at sea level).
To change from PSIG to PSIA you must do what?
To change from PSIG to PSIA you must add the ambient atmospheric pressure (14.7 psi at sea level) to the PSIG value.
Why does 0 PSIG equal 14.7 PSIA?
Because standard Earth atmosphere exerts 14.7 psi of pressure at sea level. A gauge open to air measures 0 differential pressure (0 PSIG), but experiences 14.7 psi of total absolute air pressure (14.7 PSIA).
When should you use PSIA instead of PSIG?
Use PSIA for thermodynamic gas laws (PV=nRT), steam tables, altitude calculations, and deep vacuum processing where absolute pressure values are required.