PSI to PSIG Calculator & Conversion: How to Convert PSI to PSIG
Learn how to convert PSI to PSIG with conversion formulas, online PSI to PSIG calculator, and thermodynamics steam table pressure reference values.
Performing PSI to PSIG conversion allows engineers to translate absolute thermodynamic values into physical gauge pressure readings. Understanding how to convert PSI to PSIG is essential whether interpreting steam tables, boiler gauges, or sensor data.
For rapid conversion, use our free online PSI to PSIG calculator on the home page.
1. Understanding PSI to PSIG Conversion in Thermodynamics
In thermodynamic calculations and steam tables:
- PSI (Absolute) / PSIA: Measures pressure relative to zero molecular momentum (absolute vacuum). Steam saturation tables always publish properties indexed by PSIA.
- PSIG (Gauge Pressure): Measures pressure relative to local atmospheric pressure ($P_{\text{atm}} = 14.7\text{ psi}$ at sea level).
Read our in-depth article on PSIG vs PSIA difference explained to learn why zero reference points matter.
2. Step-by-Step Formula for PSI to PSIG Conversion
To convert PSI absolute to PSIG, subtract 14.7 psi (standard atmospheric pressure at sea level):
$$\text{PSIG} = \text{PSI absolute} - 14.7$$
$$\text{PSIG} = \text{PSIA} - 14.7$$
Quick answer: PSI to PSIG Calculator & Conversion
Answer: To perform PSI to PSIG conversion, subtract 14.7 from absolute PSI ($\text{PSIG} = \text{PSI absolute} - 14.7$). For example, 14.7 PSI absolute to PSIG equals 0 PSIG, and 100 PSI absolute to PSIG equals 85.3 PSIG.
3. Step-by-Step Numerical Examples of PSI to PSIG Conversion
Example 1: How to Convert 1 PSI Absolute to PSIG
$$\text{PSIG} = 1 - 14.7 = -13.7\text{ PSIG}$$
Example 2: How to Convert 8 PSIA to PSIG
$$\text{PSIG} = 8 - 14.7 = -6.7\text{ PSIG}$$
Example 3: How to Convert 14.7 PSI to PSIG (Atmospheric Level)
$$\text{PSIG} = 14.7 - 14.7 = 0\text{ PSIG}$$
Example 4: Convert 20 PSIA to PSIG
$$\text{PSIG} = 20 - 14.7 = 5.3\text{ PSIG}$$
Example 5: Convert 25 PSIA to PSIG
$$\text{PSIG} = 25 - 14.7 = 10.3\text{ PSIG}$$
Example 6: Convert 30 PSIA to PSIG
$$\text{PSIG} = 30 - 14.7 = 15.3\text{ PSIG}$$
Example 7: Convert 50 PSIA to PSIG
$$\text{PSIG} = 50 - 14.7 = 35.3\text{ PSIG}$$
Example 8: Convert 100 PSI to PSIG
$$\text{PSIG} = 100 - 14.7 = 85.3\text{ PSIG}$$
4. Steam Table Pressure Conversion Matrix (PSIA vs PSIG)
| Absolute Pressure (PSIA) | Gauge Pressure (PSIG) | Saturation Temperature (°F) | Enthalpy $h_fg$ (Btu/lb) |
|---|---|---|---|
| 8 PSIA to PSIG | -6.7 PSIG | 182.86 °F | 988.5 |
| 14.7 PSIA to PSIG | 0 PSIG | 212.00 °F | 970.3 |
| 20 PSIA to PSIG | 5.3 PSIG | 227.96 °F | 960.1 |
| 30 PSIA to PSIG | 15.3 PSIG | 250.33 °F | 945.3 |
| 50 PSIA to PSIG | 35.3 PSIG | 281.01 °F | 924.0 |
| 100 PSIA to PSIG | 85.3 PSIG | 327.81 °F | 888.8 |
| 150 PSIA to PSIG | 135.3 PSIG | 358.42 °F | 863.6 |
5. Related Thermodynamics Guides
For reverse conversions, read how to convert PSIA to PSIG and how to convert PSIG to PSIA. For ideal gas equations, consult gas laws & ideal gas PSIA calculations.
Test any value immediately with our online PSIG to PSIA converter tool.
Related Resources
- PSIG to PSIA Tool: Homepage PSIG to PSIA Calculator – Online unit converter engine.
- PSIA to PSIG Guide: How to Convert PSIA to PSIG – Reverse conversion calculations.
- Gas Laws & Thermodynamics: Gas Laws & Ideal Gas PSIA Calculations – Gas law state equations.
- Altitude Adjustments: Altitude Atmospheric Pressure Calculator – Elevation barometric rules.
Frequently asked questions
How do you convert PSI to PSIG?
If starting with total absolute pressure (PSI absolute or PSIA), subtract 14.7 psi to get PSIG: PSIG = PSI - 14.7. If starting with gauge PSI, 1 PSI gauge = 1 PSIG.
What is 14.7 PSI to PSIG?
14.7 PSI absolute (14.7 PSIA) equals 0 PSIG at standard sea level.
What is 100 PSI to PSIG?
If 100 PSI refers to total absolute pressure, 100 PSI - 14.7 = 85.3 PSIG. If 100 PSI refers to gauge pressure, it equals 100 PSIG.
Why do steam tables use PSIA instead of PSIG?
Steam tables specify thermodynamic properties (such as saturation temperature and enthalpy) based on total absolute pressure (PSIA) regardless of atmospheric variations.