PSIG to PSIA Converter
Enter a gauge pressure in PSIG to get the absolute pressure in PSIA. Switch the direction to convert PSIA back to PSIG, adjust the atmospheric pressure for your altitude, and see equivalent values in bar and kPa. Negative gauge pressures (vacuum) are supported. Your result updates as you type.
Formula
Worked example
A compressed-air system reads 100 PSIG at sea level (P_atm = 14.696 PSI). PSIA = 100 + 14.696 = 114.696 PSIA, or 0.7907 MPa absolute. At 5,000 ft altitude the same gauge reading gives 100 + 12.228 = 112.228 PSIA, about 1.7% lower.
PSIG vs. PSIA: what is the difference?
Pressure can be expressed relative to two different reference points. PSIA (pounds per square inch absolute) measures pressure relative to a perfect vacuum, the lowest possible reference. PSIG (pounds per square inch gauge) measures pressure relative to the local atmospheric pressure. When you read a tyre gauge or a boiler gauge, you are reading gauge pressure: the instrument zeroes itself to the surrounding air and shows only the excess above it. Absolute pressure equals gauge pressure plus the atmospheric pressure at that location. At sea level, atmospheric pressure is approximately 14.696 PSI, so a gauge reading of 0 PSIG corresponds to 14.696 PSIA. A reading of -14.696 PSIG would correspond to a perfect vacuum (0 PSIA).
How to convert PSIG to PSIA
The conversion is a simple addition: PSIA = PSIG + atmospheric pressure. At sea level the standard value is 14.696 PSI (some references round to 14.7 PSI, which is accurate enough for most engineering work). For the reverse direction, PSIG = PSIA - atmospheric pressure. Because atmospheric pressure decreases with altitude, the conversion factor changes if you are not at sea level. At 5,000 feet the atmospheric pressure is about 12.228 PSI, so a gauge reading of 50 PSIG corresponds to 50 + 12.228 = 62.228 PSIA rather than the 64.696 PSIA you would get at sea level. This matters in altitude testing, aviation systems, and high-altitude process plants.
Vacuum pressures and negative gauge readings
A negative PSIG value means the system is at a pressure below atmospheric, that is, a partial vacuum. A reading of -5 PSIG at sea level corresponds to 14.696 - 5 = 9.696 PSIA. Perfect vacuum (0 PSIA) would read -14.696 PSIG at sea level. Vacuum levels are sometimes expressed as inches of mercury (in Hg) rather than PSIG or PSIA. 1 PSI equals 2.036 in Hg, so a vacuum of -5 PSIG is equivalent to approximately 10.18 in Hg below atmospheric. This calculator handles negative PSIG inputs correctly, giving you the corresponding PSIA value.
Practical applications
Gas compression: PSIA is required for thermodynamic calculations using the ideal gas law and real-gas equations. Piping and vessel design: codes such as ASME B31.3 and ASME Section VIII specify design pressures in PSIG, while flow calculations use absolute pressure. HVAC and refrigeration: refrigerant saturation tables are indexed by absolute pressure, but service gauges read in PSIG. Aviation: altimeters use absolute pressure referenced to a standard atmosphere. Whenever you use a pressure in a calculation that involves ratios (P1/P2), compressibility, or thermodynamic equations, you must use absolute pressure (PSIA).
PSIG to PSIA quick-reference chart (sea level, 14.696 PSI atm)
| PSIG | PSIA | bar (abs) | kPa (abs) |
|---|---|---|---|
| -14.696 | 0 | 0.0000 | 0.00 |
| -10 | 4.696 | 0.3238 | 32.38 |
| -5 | 9.696 | 0.6685 | 66.85 |
| 0 | 14.696 | 1.0133 | 101.33 |
| 5 | 19.696 | 1.3580 | 135.80 |
| 10 | 24.696 | 1.7027 | 170.27 |
| 15 | 29.696 | 2.0475 | 204.75 |
| 20 | 34.696 | 2.3922 | 239.22 |
| 30 | 44.696 | 3.0817 | 308.17 |
| 50 | 64.696 | 4.4606 | 446.06 |
| 75 | 89.696 | 6.1843 | 618.43 |
| 100 | 114.696 | 7.9080 | 790.80 |
| 150 | 164.696 | 11.3554 | 1135.54 |
| 200 | 214.696 | 14.8028 | 1480.28 |
| 300 | 314.696 | 21.6975 | 2169.75 |
| 500 | 514.696 | 35.4871 | 3548.71 |
| 1000 | 1014.696 | 69.9609 | 6996.09 |
Values are rounded to two decimal places. For other atmospheric pressures, add your local value instead of 14.70.
Frequently asked questions
What is the formula for converting PSIG to PSIA?
PSIA = PSIG + atmospheric pressure. At sea level the standard atmospheric pressure is 14.696 PSI, so PSIA = PSIG + 14.696. Many sources round to 14.7 PSI, which is accurate to within 0.03%. For precise work, use the actual barometric reading at your location and altitude.
What does 0 PSIG mean in PSIA?
A gauge pressure of 0 PSIG means the system is exactly at atmospheric pressure. In PSIA this equals the local atmospheric pressure: 14.696 PSIA at sea level, or less at higher altitudes. It does not mean zero absolute pressure.
Can PSIG be negative?
Yes. A negative PSIG value indicates the pressure is below atmospheric, that is, a partial vacuum. For example, -5 PSIG at sea level equals 14.696 - 5 = 9.696 PSIA. PSIA itself can never be negative, because that would require a pressure below a perfect vacuum, which is physically impossible.
Does altitude affect the PSIG to PSIA conversion?
Yes. The conversion adds the local atmospheric pressure, which decreases with altitude. At sea level you add approximately 14.696 PSI, but at 5,000 feet you add only about 12.228 PSI. This calculator includes altitude presets so you can apply the correct atmospheric pressure for your location.
When should I use PSIA instead of PSIG?
Use PSIA any time your calculation involves pressure ratios or thermodynamic relationships. The ideal gas law (PV = nRT), compressibility factors, and flow equations all require absolute pressure. Gauge pressure is convenient for reading instruments and specifying process conditions relative to the environment, but absolute pressure is needed for physics-based calculations.
How do I convert PSIG or PSIA to bar or kPa?
Multiply PSI by 0.0689476 to get bar, or by 6.89476 to get kilopascals. For example, 100 PSIA x 0.0689476 = 6.8948 bar absolute. This calculator shows bar and kPa equivalents for both your input and your result automatically.
What is the difference between PSI, PSIG, and PSIA?
PSI stands for pounds per square inch, a unit of pressure. Without a suffix it is ambiguous. PSIG specifies gauge pressure (relative to atmospheric). PSIA specifies absolute pressure (relative to perfect vacuum). In practice, instruments and everyday usage default to gauge pressure, while engineering calculations require absolute pressure. Always check which reference point applies before plugging a PSI value into a formula.