Article — PSIG to PSIA Converter
PSIG to PSIA: Gauge vs Absolute Pressure
PSIA equals PSIG plus atmospheric pressure: at sea level, that is PSIG + 14.696 psi. PSIG measures pressure relative to the surrounding air (zero on the gauge means matching atmospheric). PSIA measures pressure relative to a perfect vacuum (zero means total absence of pressure). Most gauges read PSIG; most engineering calculations require PSIA.
The two units differ only by the reference point, but they are not interchangeable. Plugging PSIG into the ideal gas law produces nonsense. Treating PSIA as gauge pressure overestimates by 14.7 psi at sea level — enough to overdesign equipment by 10–20%. Knowing which one a specification calls for is half the work.
What is PSIG?
PSIG stands for "pounds per square inch, gauge." The G suffix marks it as a gauge measurement: the pressure above (or below) local atmospheric pressure. A tire gauge, a water pressure gauge, a compressor manifold, and a refrigeration manifold all show PSIG. When the system is open to the atmosphere, the gauge reads zero — not because there is no pressure, but because the reading is relative.
The unit is American imperial. The pound-force is the gravitational force on a one-pound mass at standard gravity; the square inch is exactly 0.00064516 m². The PSI dates to 19th-century steam engineering and persists in US industry because of inertia. Metric countries use bar (1 bar ≈ 1 atm) or kPa (101 kPa ≈ 1 atm) instead.
The American Society of Mechanical Engineers (ASME) requires pressure vessel ratings in PSIG for the design pressure, but boiler steam tables are in PSIA. Reading one and applying it to the other is one of the most common sources of safety calculation errors in pressure vessel work.
What is PSIA?
PSIA stands for "pounds per square inch, absolute." The A suffix marks it as a reading relative to a perfect vacuum. PSIA can never be negative; the minimum is zero, corresponding to a complete absence of pressure. At sea level under standard conditions, atmospheric pressure is 14.696 psia by definition.
Absolute pressure is what enters every fundamental physics and chemistry equation: ideal gas law, Bernoulli's equation, isentropic compression formulas, refrigerant pressure-temperature relationships. A compressor producing 100 psig of compressed air at sea level is actually delivering 114.696 psia — the absolute pressure ratio is 7.8:1, not 100:1.
The PSIG to PSIA formula
The conversion is addition, not multiplication:
PSIA = PSIG + Patm PSIG = PSIA − PatmPatm (sea level) = 14.696 psi Patm (Denver) = 12.09 psi0 psig = 14.696 psia −14.696 psig = 0 psia (vacuum)The atmospheric pressure constant varies with altitude and, to a lesser degree, with weather. Sea-level standard is 14.696 psi. At Denver (1610 m / 5280 ft), it is 12.09 psi. At 10,000 ft, it is 10.11 psi. For everyday work near sea level, rounding to 14.7 is fine; for precision work and high-altitude installations, use the local value or the barometric formula.
When to use PSIG vs PSIA
The rule of thumb: use whichever your data source uses, and convert when crossing boundaries. Practical pressure measurements (tires, water, compressed air, hydraulics) come in PSIG. Engineering calculations involving gases, vapor, or vacuum come in PSIA.
Three concrete cases where conversion matters:
- Compressor sizing. A compressor specified at 100 psig sea-level discharge produces a pressure ratio of 114.696 / 14.696 = 7.80. At 10,000 ft, the same gauge pressure means 110.11 / 10.11 = 10.89 — a much harder ratio that may exceed the machine's adiabatic limit.
- Refrigeration manifolds. R-410A at 32°F (0°C) has an absolute pressure of about 117 psia. A manifold gauge reads 102 psig at sea level — but the same gas in Denver reads 105 psig (102 + 12.09 still gives 117 psia).
- Steam tables. Boiler operators set safety valves at PSIG, but the steam tables give boiling temperature as a function of PSIA. A 100 psig steam boiler at sea level produces saturated steam at 338°F (114.696 psia). The same boiler at 10,000 ft produces steam at slightly lower temperature.
PSIG below zero: vacuum pressure
Gauge pressure can go negative — it just means the system pressure is below local atmospheric. Vacuum pumps, suction lines, and low-pressure chambers routinely operate in this range. The convention can be confusing: a vacuum gauge reading −10 psig means 10 psi below atmosphere, equivalent to 4.696 psia absolute.
Many vacuum gauges read in inches of mercury (inHg) rather than negative PSIG. 1 inHg = 0.4912 psi. A vacuum of 25 inHg corresponds to about −12.3 psig or 2.4 psia. Industrial vacuum systems often combine PSIG gauges (for positive-pressure side) with inHg gauges (for vacuum side) on the same equipment.
The lowest possible gauge reading is −14.696 psig at sea level (corresponding to 0 psia, perfect vacuum). At altitude, the minimum is whatever the local atmospheric pressure happens to be — in Denver, −12.09 psig. Pumps cannot produce gauge readings below the local atmosphere because there is nothing left to remove.
PSIG and PSIA at altitude
Atmospheric pressure drops with altitude approximately exponentially. Half the sea-level pressure is reached at 5500 m (18,000 ft). The change is smooth and predictable, given by the barometric formula:
- Sea level (0 ft) = 14.696 psia
- 1,000 ft = 14.18 psia
- 2,000 ft = 13.66 psia (Atlanta)
- 5,280 ft = 12.09 psia (Denver)
- 10,000 ft = 10.11 psia (mid-range mountain altitude)
- 29,029 ft = 4.78 psia (Mt. Everest)
Industrial equipment sold in the US is usually rated assuming 14.7 psia atmospheric. Installations at high altitude often need derating. A compressor producing 7 bar of gauge pressure delivers less mass flow per stroke at lower atmospheric pressure because the suction air is thinner. HVAC chillers, vacuum pumps, and pneumatic tools all show similar effects.
PSIG in industrial applications
Different fields run at characteristic pressures. Plant air systems target 100–125 psig. Hydraulics run 1500–3000 psig for tools, up to 5000+ psig for heavy equipment. Steam systems range from 15 psig (low-pressure heating) to 600+ psig (power generation). Vacuum work goes deep into negative gauge territory.
Pressure relief and safety valves are always specified in PSIG (or barg in metric countries). Confusing PSIA with PSIG on a relief valve means the valve will not lift until pressures are 14.7 psi higher than intended — a catastrophic safety error in vessel design. Always confirm the unit before sizing.
Refrigeration is a special case: the manifold gauges show PSIG, but the refrigerant pressure-temperature charts use PSIA. Service technicians constantly add or subtract 14.7 psi to read between the two. Modern digital gauges often display both, with a switch to convert on the fly.
Common PSIG-PSIA mistakes
- Mixing units in calculations. The ideal gas law (PV = nRT) requires absolute pressure. Using PSIG instead of PSIA gives volume or temperature off by factors that can exceed 100% at low pressures.
- Ignoring altitude. A pressure spec written in PSIG assumes sea-level atmosphere. Equipment installed in Denver, Bogotá, La Paz, or any mountain location operates at lower absolute pressure for the same gauge reading.
- Using 15 psi instead of 14.696. A 2% rounding error on atmospheric pressure compounds in any calculation that subtracts or adds it. Use the exact value or take the local barometric reading.
- Misreading vacuum. A vacuum of 25 inHg is roughly half-atmosphere, not a near-perfect vacuum. Industrial vacuum specifications give absolute (PSIA, microns, or millibar) for clarity.
- Forgetting weather variability. Atmospheric pressure changes by 1–2% in a storm. For high-precision work (analytical chemistry, gas chromatography), the local barometer reading matters.