How this calculator works
Gauge pressure is measured against the air around the instrument, while absolute pressure is measured against a perfect vacuum. The two differ by the local atmospheric pressure: psia = psig + atmosphere. At sea level the standard atmosphere is 14.6959487755 psi, so a gauge reading of 30 psig is about 44.696 psia. Tyre gauges, compressor gauges and water-pressure gauges read gauge pressure. Gas-law calculations, vacuum systems and many specifications need absolute pressure, which is why both labels exist.
Formula
psia = psig + p atm. psig = psia - p atm. At the standard atmosphere, p atm = 14.6959 psi, so psia = psig + 14.6959 and psig = psia - 14.6959.
Worked example
30 psig at one standard atmosphere is 44.6959 psia. 44.6959 psia at the same atmosphere is 30 psig. 0 psig is 14.6959 psia. A vacuum reading of -5 psig is 9.6959 psia. On a day when the local atmosphere is 14.0 psi, 30 psig is 44.0 psia.
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Frequently asked questions
Why is psia always larger than psig?
Absolute pressure counts from a perfect vacuum, and the atmosphere presses down with about 14.7 psi at sea level. Gauge pressure ignores that atmosphere, so the absolute value is higher by the local atmospheric pressure.
Which one belongs in the ideal gas law?
Use absolute pressure in the ideal gas law and in most thermodynamic equations. A gauge reading must first have the atmospheric pressure added, otherwise a gas at 0 psig would be treated as having no pressure at all.
Does altitude change the conversion?
Yes. Local atmospheric pressure usually decreases with height, so the gap between psig and psia shrinks. Enter actual local station pressure. A sea-level-adjusted weather reading does not describe the ambient pressure at an elevated instrument.
What does a negative psig mean?
It means the pressure is below the surrounding air, which is a partial vacuum. The most negative possible gauge reading is minus the atmospheric pressure, at which point the absolute pressure is zero. The calculator rejects readings below that limit.