PSIG to PSIA Converter
Convert gauge pressure (psig) to absolute (psia) and back, at any atmospheric pressure or altitude.
Atmospheric pressure by altitude
| Altitude | psi | millibar |
|---|---|---|
| Sea level | 14.696 psi | 1,013.2 mbar |
| 500 m | 13.845 psi | 954.6 mbar |
| 1,000 m | 13.035 psi | 898.7 mbar |
| 1,600 m (Denver) | 12.114 psi | 835.2 mbar |
| 2,400 m | 10.969 psi | 756.3 mbar |
| 3,500 m | 9.538 psi | 657.6 mbar |
| 5,000 m | 7.835 psi | 540.2 mbar |
Gauge versus absolute
A pressure gauge reads zero when it is open to the air, so it measures the difference from atmospheric pressure. Absolute pressure counts from a perfect vacuum instead:
psia = psig + atmospheric
psig = psia − atmospheric
A tyre at 32 psig contains air at about 46.7 psia. Both numbers describe the same tyre.
Which one you need
Anything mechanical — tyres, hydraulics, compressors, boilers — is quoted in gauge, because that is what the instrument shows and what the vessel wall actually has to hold back. Gas-law calculations need absolute: the ideal gas law and compression ratios break completely if you feed them gauge pressure, because zero psig is not zero pressure.
Altitude matters
Atmospheric pressure drops roughly 0.5 psi per 1,000 feet of altitude. In Denver at about 1,600 m it is near 12.1 psi rather than 14.7, so a tyre inflated to 32 psig there holds about 44.2 psia — a real difference for anything sensitive to absolute pressure. Weather shifts it by a few percent too.
Negative gauge pressure means a partial vacuum. The most a gauge can read below ambient is the ambient pressure itself, since absolute pressure cannot go below zero.