PSIG to PSIA Converter
The PSIG to PSIA converter handles a pressure-reference change, not just a unit change. Both PSIG and PSIA use pounds per square inch, but they measure from different zero points. PSIG means pounds per square inch gauge: the gauge reads zero when it is open to the surrounding atmosphere. PSIA means pounds per square inch absolute: zero is a perfect vacuum. The difference is why this calculator adds or subtracts a standard atmospheric offset.
This distinction is essential in compressed air, refrigeration, vacuum systems, process piping, gas cylinders, and thermodynamics. A shop air gauge that reads 100 psig is not saying the gas has 100 psi above vacuum. Near standard sea-level conditions, its absolute pressure is about 114.6959 psia in this calculator. Conversely, a vacuum chamber at 10 psia is below the standard atmosphere and corresponds to a negative gauge pressure.
The form uses 14.6959 psi as its standard-atmosphere offset. That is a rounded standard reference, not a live barometer. Local atmospheric pressure can be lower at altitude and can drift with weather. The calculator is therefore best for standard-reference calculations, documentation, examples, and quick engineering checks. If a procedure specifies local barometric pressure, use that procedure’s value instead.
Formula
In PSIG to PSIA mode:
In PSIA to PSIG mode:
The calculator also enforces the physical lower bound for absolute pressure. In the forward direction, an entered gauge pressure less than -14.6959 psig would produce negative psia, so the result is invalid. In the reverse direction, psia must be zero or greater, but the calculated psig may be negative.
Examples
For a compressor gauge reading of 35 psig, the calculator adds the standard offset:
The primary result is 49.6959 psia. The item rows list 35 psig as the gauge pressure and 14.6959 psi as the atmospheric offset.
For a vacuum-related absolute pressure of 10 psia, switch the direction to PSIA to PSIG. The calculator subtracts:
The displayed result is -4.6959 psig. That negative sign does not mean negative absolute pressure. It means the pressure is 4.6959 psi below the standard atmospheric reference used by the form.
Reference table
| Gauge pressure | Absolute pressure with 14.6959 psi offset | Interpretation |
|---|---|---|
| -10 psig | 4.6959 psia | Partial vacuum relative to standard atmosphere |
| -5 psig | 9.6959 psia | Mild vacuum reading |
| 0 psig | 14.6959 psia | Gauge open to standard atmosphere |
| 15 psig | 29.6959 psia | Common compressed-air scale |
| 35 psig | 49.6959 psia | Worked example |
| 100 psig | 114.6959 psia | Shop or process pressure |
| Absolute pressure | Gauge pressure with 14.6959 psi offset | Interpretation |
|---|---|---|
| 0 psia | -14.6959 psig | Perfect-vacuum limit under this reference |
| 10 psia | -4.6959 psig | Below standard atmosphere |
| 14.6959 psia | 0 psig | Standard atmosphere |
| 50 psia | 35.3041 psig | Above standard atmosphere |
When gauge and absolute pressure matter
Gas-law calculations generally require absolute pressure. If you enter 35 psig into an ideal gas relation as though it were 35 psia, the pressure is understated by about one standard atmosphere. That can distort volume, mass, density, and temperature relationships. Absolute pressure is also common in vacuum specifications because a vacuum chamber’s remaining gas is naturally measured above zero pressure.
Gauge pressure is often the right field value for mechanical service. Tire gauges, shop air regulators, hydraulic gauges, and many process instruments are designed around pressure above ambient atmosphere. A pressure gauge reading of zero usually means the device is equalized with the surrounding air, not empty of pressure. That is why the label matters: psi alone is incomplete when the reference could be gauge or absolute.
Altitude and weather add practical nuance. The calculator’s 14.6959 psi offset approximates a standard atmosphere. At higher elevations, local atmospheric pressure can be much lower, so a gauge that reads zero may correspond to less than 14.6959 psia locally. Calibration work should follow the local or specified reference. For unit-only changes, use the pressure converter, psi to atm conversion, or bar to psi. For gauge units related to bar, see bar to psig conversion.
Pitfalls to avoid
- Using psig directly in formulas that require absolute pressure.
- Assuming the standard offset equals your local atmosphere on every day and at every elevation.
- Treating negative psig as impossible; it is common for vacuum relative to ambient pressure.
- Treating negative psia as possible; absolute pressure cannot go below zero.
- Mixing a unit conversion problem with a reference conversion problem. Pa to psi changes unit size, while psig to psia changes the zero point.
Accuracy and limits
The calculator keeps the defined or cited relationship through the calculation and rounds only the displayed result. A converted number does not become more precise than the source measurement. Keep additional digits for chained calculations, then round to the precision justified by the original value; also preserve any reference basis or notation convention named with the input.
Sources
- NIST, standard atmosphere constant — standard atmosphere pressure reference used to derive atmospheric offsets.
- NOAA Ocean Service, What is a barometer? — explanation of changing atmospheric pressure.
- National Weather Service, pressure altitude calculator — aviation/weather context showing pressure variation with altitude.
- NIST, SI units — pressure-unit context through the SI system.