PSIG to PSIA Conversion Calculator
Convert gauge pressure (psig) to absolute pressure (psia) and back, with an adjustable atmospheric pressure input and vacuum (psiv) support.
The formula
psia = psig + P_atm and psig = psia − P_atmWhich pressure tool do you need? This page converts between pressure reference frames (gauge ↔ absolute ↔ vacuum) via atmospheric offset. To convert pressure magnitudes between units (psi ↔ bar, kPa, atm, inHg, …), use the psi conversion hub.
Standard atmospheric pressure is 14.6959 psi (101,325 Pa ÷ 6,894.757293168 Pa per psi). For vacuum: psiv = P_atm − psia.
Worked examples
Gauge vs absolute vs vacuum
PSIG (gauge) measures pressure relative to the surrounding air — the zero point moves with weather and altitude. PSIA (absolute) measures against a perfect vacuum, so its zero never moves; gas laws and compressor math need psia. PSIV (vacuum) measures how far below the atmosphere you are: 0 psiv is open air, 14.6959 psiv is a perfect vacuum. The three are linked by one number: atmospheric pressure.
Quick reference table (standard atmosphere)
| Gauge (psig) | Absolute (psia) |
|---|---|
| 0 | 14.6959 |
| 15 | 29.6959 |
| 30 | 44.6959 |
| 50 | 64.6959 |
| 100 | 114.6959 |
| 150 | 164.6959 |
| 200 | 214.6959 |
| 300 | 314.6959 |
Frequently asked questions
What is the formula to convert psig to psia?
Add atmospheric pressure to the gauge reading: psia = psig + P_atm. With the standard atmosphere of 14.6959 psi, the shortcut is psia = psig + 14.6959. For example, 100 psig + 14.6959 = 114.6959 psia. This calculator also lets you enter your actual barometric pressure for better accuracy.
How do I convert psia to psig?
Subtract atmospheric pressure: psig = psia − P_atm, or psig = psia − 14.6959 with the standard atmosphere. For example, 114.6959 psia − 14.6959 = 100 psig. This calculator converts in both directions automatically.
What is 100 psig in psia?
114.6959 psia. Add the standard atmosphere (14.6959 psi) to the 100 psig gauge reading: 100 + 14.6959 = 114.6959 psia.
What is 0 psig in psia?
14.6959 psia. A gauge reading of zero means the pressure equals the surrounding atmosphere, which is 14.6959 psi absolute at sea level under standard conditions.
What is the difference between psig and psia?
PSIG (pounds per square inch gauge) measures pressure relative to the surrounding atmosphere, so a tire gauge reads 0 when open to the air. PSIA (pounds per square inch absolute) measures against a perfect vacuum, so the same open gauge reads 14.6959 psia. They differ by atmospheric pressure: psia = psig + 14.6959.
What is psiv and how does it relate to psig and psia?
PSIV is vacuum pressure measured downward from the atmosphere: psiv = P_atm − psia. A 10 psiv vacuum equals 14.6959 − 10 = 4.6959 psia, or −10 psig. Zero psiv is atmospheric pressure; 14.6959 psiv is a perfect vacuum (0 psia).
Why is the standard atmosphere 14.6959 psi?
One standard atmosphere is defined as exactly 101,325 Pa, and 1 psi equals 6,894.757293168 Pa. Dividing 101,325 by 6,894.757293168 gives 14.6959487755 psi, which rounds to 14.6959 psi. Many references round further to 14.7 psi.
Is using 14.7 psi instead of 14.6959 psi accurate enough?
Usually yes. The difference is 0.0041 psi, an error of about 0.03 percent. At 100 psig that shifts the result by 0.0041 psia (114.7 vs 114.6959). For weather-dependent precision, enter your actual barometric reading instead.
Can psig be negative?
Yes. Negative psig means pressure below atmospheric — a vacuum. For example, −10 psig equals 14.6959 − 10 = 4.6959 psia. The lowest physically possible gauge reading is −14.6959 psig, which is 0 psia (a perfect vacuum).
When should I use psia instead of psig?
Use psia whenever the absolute amount of gas matters: Boyle's law, the ideal gas law (PV = nRT), compressor ratios, and altitude corrections all require absolute pressure. Using psig in these formulas gives wrong answers because it ignores the atmosphere pushing on the system.
How does altitude change the psig to psia conversion?
The atmosphere gets thinner with altitude, so P_atm drops. At high elevations a barometer might read 14.2 psi instead of 14.6959. Then 90 psig becomes 90 + 14.2 = 104.2 psia instead of 104.6959 psia. Enter your local barometric pressure in the converter above for correct results.
How do I convert psig to absolute pressure in kPa?
Convert to psia first, then multiply by 6.894757293168. For example, 100 psig = 114.6959 psia, and 114.6959 × 6.894757293168 = about 790.8 kPa absolute.
Does a perfect vacuum equal 0 psia?
Yes. Zero psia is the total absence of pressure — a perfect vacuum. On a gauge that reads −14.6959 psig at sea level, and 14.6959 psiv. Real vacuum pumps approach but never quite reach 0 psia.
Do tire gauges read psig or psia?
PSIG. Consumer pressure gauges are vented to the atmosphere, so they read 0 when unpressurized. A car tire at 32 psig contains 32 + 14.6959 = 46.6959 psia of absolute pressure.
How precise should my barometric pressure input be?
Match it to your gauge's precision. Weather and altitude swing sea-level-corrected barometric pressure by about 0.3 psi or more, which shifts psia by the same amount. If your gauge resolves 0.1 psi, a local barometer reading to 0.01 psi is plenty; the 14.6959 default is fine for ordinary work.
What is 14.6959 psia in psig?
0 psig. Subtracting the standard atmosphere (14.6959 − 14.6959) leaves zero gauge pressure — this is just atmospheric pressure, exactly what an open gauge reads.
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