By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed July 11, 2026
PSIA and PSIG measure the same pressure from two different zero points. PSIG (gauge) counts up from the local atmosphere, so it reads 0 at ambient air. PSIA (absolute) counts up from a perfect vacuum, so ambient air already reads about 14.7. Plain PSI names the unit without saying which zero it uses, and PSID is the difference between two pressures. Get the suffix wrong on a datasheet and a reading can be off by a full atmosphere, roughly 14.7 psi. This guide gives you the at-a-glance table, the conversion with real numbers, and which reference to specify on a transmitter.
Contents
- PSI, PSIA, PSIG, and PSID at a glance
- PSIG: gauge pressure
- PSIA: absolute pressure
- PSID: differential pressure
- Converting between PSIG and PSIA
- Which reference to specify on a transmitter
- FAQ
PSI, PSIA, PSIG, and PSID at a glance
All four use pounds per square inch. The suffix only tells you the reference point the number is counted from.
| Suffix | Name | Zero reference | Sea-level air reads | Typical use |
|---|---|---|---|---|
| PSI | Pounds per square inch | Not stated; set by context | n/a | Generic; on a gauge, read as gauge |
| PSIG | Gauge | Local atmosphere | 0 PSIG | Tires, pumps, most process pressure |
| PSIA | Absolute | Perfect vacuum | 14.7 PSIA | Vacuum, barometric, altitude-sensitive work |
| PSID | Differential | The second port | Depends on both sides | Filter drop, flow, level in closed tanks |
PSIG: gauge pressure
Gauge pressure uses the local atmosphere as its zero. Open a valve to the air and the gauge reads 0 PSIG, no matter what the barometer says that day. A car tire at 32 PSIG holds 32 psi above the surrounding air. Pull below atmosphere and PSIG goes negative: a partial vacuum reads about −9.7 PSIG, and a perfect vacuum reads about −14.7 PSIG at sea level. Most industrial pressure is gauge, because operators care about the pressure a system holds above the air around it.
This is also why a bare “psi” almost always means gauge. The US standard for pressure gauges, ASME B40.100, treats an unmarked dial as gauge pressure unless it is labeled absolute or differential. A gauge or transmitter that reads gauge is usually vented to atmosphere through a small port or a breathing cable, so the reference tracks the real ambient air.
PSIA: absolute pressure
Absolute pressure uses a perfect vacuum as its zero, so it already includes the weight of the atmosphere. Sea-level air is about 14.7 PSIA. A sealed chamber pumped down toward a hard vacuum approaches 0 PSIA but never quite reaches it; a semiconductor process might target 0.1 PSIA. Because the reference is a fixed vacuum, an absolute reading moves when the weather or the elevation changes: sea level is 14.7 PSIA, 1,000 ft is about 14.2 PSIA, and Denver at 5,280 ft averages about 12.2 PSIA. That is exactly why barometric and weather sensors are absolute. A gauge sensor would read 0 at every one of those elevations and tell you nothing about the real air pressure.
PSID: differential pressure
Differential pressure is the difference between two points, and it does not care about the absolute level at either one. If the pressure before a filter is 100 PSIG and after it is 95 PSIG, the differential across the filter is 5 PSID. The same 5 PSID would read the same whether the line sat at 100 psi or 1,000 psi. Differential is the basis of most flow elements, filter monitoring, and level in a closed or pressurized tank, where you measure the head between the bottom and the vapor space.
Converting between PSIG and PSIA
The two are one atmosphere apart. Add the local atmospheric pressure to go from gauge to absolute, subtract it to go the other way:
PSIA = PSIG + atmospheric pressure
PSIG = PSIA − atmospheric pressure
Use 14.7 psi for the atmosphere at sea level, or the real barometric value at altitude. The table works a few everyday readings at sea level.
| Reading | PSIG | PSIA | Math |
|---|---|---|---|
| Flat tire | 0 PSIG | 14.7 PSIA | 0 + 14.7 |
| Car tire | 32 PSIG | 46.7 PSIA | 32 + 14.7 |
| Compressed-air line | 100 PSIG | 114.7 PSIA | 100 + 14.7 |
| Partial vacuum | −9.7 PSIG | 5.0 PSIA | 5.0 − 14.7 |
| Hard vacuum | −14.7 PSIG | 0 PSIA | 0 − 14.7 |
At altitude the offset shrinks. A gauge reading 30 PSIG in Denver is only about 42.2 PSIA (30 + 12.2), not the 44.7 PSIA the same gauge implies at sea level. Our absolute and gauge pressure calculator does the conversion with an altitude estimate built in. For the wider family of pressure units, bar, kPa, and MPa, see the pressure units guide.
Which reference to specify on a transmitter
The suffix is a purchasing decision, not just a label. Pick it from what the reading has to stay true to.
Specify gauge (PSIG) for most process pressure, where you care about pressure above the surrounding air: tanks, pumps, compressed-air headers, and line pressure. That is the default for a gauge pressure transmitter. Specify absolute (PSIA) for vacuum and low-pressure work, barometric measurement, and any application where weather or elevation must not shift the reading; that calls for an absolute pressure transmitter. Specify differential (PSID) when you measure the difference across two points, a filter, a flow element, or level in a closed tank, which is the job of a differential pressure transmitter. When you are not sure, browse the full range on the pressure instruments hub and tell us the process.
Application example
Vacuum process equipment, semiconductor and lab. A US vacuum-equipment builder needed pressure transmitters for a process that runs below atmosphere. A gauge unit was the wrong reference: near a vacuum it would read a large negative PSIG that drifts every time the barometer moves, and it cannot resolve how close the chamber is to a true vacuum. We supplied absolute transmitters referenced to vacuum, so the reading is 0 PSIA at a perfect vacuum and climbs to about 14.7 PSIA at atmosphere, independent of the weather. The reference, not the range, was the decision that made the measurement usable.
For the concept behind the suffixes, including an altitude table and a vented, sealed, or absolute selection table, read our absolute vs gauge pressure guide.
FAQ
When should I use PSIA instead of PSIG?
Use PSIA (absolute) when the reading must not move with the weather or elevation: vacuum systems, low-pressure and barometric measurement, altitude compensation, and scientific work. Use PSIG (gauge) for everyday process pressure, tires, pumps, and lines, where you only care about pressure above the surrounding air.
Is 14.7 PSIA equal to 0 PSIG?
At sea level, yes, closely. Standard atmosphere is about 14.7 PSIA, which is the zero point for gauge pressure, so 14.7 PSIA equals roughly 0 PSIG. It is only exact when the local barometric pressure is actually 14.7 psi; at altitude the atmosphere is lower, so 0 PSIG corresponds to a smaller PSIA value.
What is 100 PSIG in PSIA?
About 114.7 PSIA at sea level. Add the atmospheric pressure to the gauge reading: 100 PSIG + 14.7 psi = 114.7 PSIA. At a higher elevation, add the local atmospheric value instead of 14.7.
How do I convert PSIA to PSIG?
Subtract the local atmospheric pressure: PSIG = PSIA − atmospheric pressure. At sea level that is PSIA − 14.7. For example, 30 PSIA is about 15.3 PSIG (30 − 14.7). A result below zero means the pressure is below atmosphere, that is, a vacuum.
Match the pressure reference to your process
Tell us the process, the range, and whether the reading has to hold true against weather and altitude, and we will confirm whether you need a gauge, absolute, or differential transmitter and quote the right one. Tell us the application and we configure one unit, not a shelf part.
Written and technically reviewed by Wu Peng and the Instranova engineering team. Based on standard atmospheric references, ASME B40.100, and field experience specifying gauge, absolute, and differential transmitters. Questions? Reach our application engineers.