PSI, or pounds per square inch, is a pressure unit, not a pressure reference. A value written only as PSI does not show whether its zero point is a perfect vacuum or the local atmospheric pressure.
PSIG removes this uncertainty by stating that the pressure is measured relative to local atmospheric pressure. PSIA identifies absolute pressure measured from a perfect vacuum. This difference must be clear when reading gauges and datasheets or using pressure values in gas and steam valve calculations.
Source: Kimray
PSI vs PSIG: the short answer
PSI tells you the unit, while PSIG and PSIA also tell you the pressure reference. In an engineering record, a value such as 100 PSI is incomplete unless the document makes clear whether it means 100 PSIG or 100 PSIA.
| Term | What it tells you | Zero reference | Example |
|---|---|---|---|
| PSI | Pressure unit only | Not specified | 100 PSI needs a reference check |
| PSIG | Gauge pressure in PSI | Local atmospheric pressure | A vented gauge reads 0 PSIG when open to air |
| PSIA | Absolute pressure in PSI | Perfect vacuum | Used in many gas and steam equations |
At standard atmospheric pressure, a properly vented gauge exposed to the surrounding air reads 0 PSIG but about 14.7 PSIA. The exact difference is not fixed at every site because local atmospheric pressure changes with altitude and weather.
What PSI, PSIG and PSIA actually mean
These terms differ because each reading is tied to a specific pressure reference. The reference determines what 0 means and whether atmospheric pressure is included in the reading.
PSI: a unit, not a pressure reference
PSI means pounds-force per square inch. It tells you how pressure is expressed, but it does not state where the measurement starts. A document may use PSI for gauge, absolute, sealed-gauge or differential pressure, so the instrument type, suffix or surrounding specification must supply the reference.
PSIG: pressure relative to local atmosphere
PSIG means pounds per square inch gauge. A vented pressure gauge compares the process pressure with the surrounding atmospheric pressure. When both sides are at the same pressure, a correctly zeroed gauge reads 0 PSIG.
A PSIG value can be negative when the process pressure is below local atmospheric pressure, provided the instrument has a suitable vacuum or compound range. This does not mean that the absolute pressure is negative.
PSIA: pressure relative to perfect vacuum
PSIA means pounds per square inch absolute. Its zero point is a perfect vacuum, so one standard atmosphere is about 14.7 PSIA rather than 0 PSIA. Absolute pressure cannot fall below 0 PSIA.
Many gas and steam equations use absolute pressure because density and other fluid properties depend on the total pressure above a perfect vacuum. A PSIG reading must therefore be converted before it is entered into an equation that specifies PSIA.
How to convert PSIG and PSIA
PSIG and PSIA differ by the atmospheric pressure at the measurement location. Both values must use the same pressure unit before the calculation is made.
PSIA = PSIG + atmospheric pressure
PSIG = PSIA – atmospheric pressure
One standard atmosphere is approximately 14.6959 psi. This value is suitable for standard-condition examples, but it is not the exact atmospheric pressure at every site. For calibration or site-specific work, use the actual local barometric pressure at the instrument.
Can you convert PSI directly to PSIG?
A value written only as PSI cannot be converted to PSIG until its pressure reference is known. If the value is already gauge pressure, the number stays the same and should be labelled PSIG. If it is absolute pressure, subtract the local atmospheric pressure to obtain PSIG.
PSIG and PSIA calculator
Enter the pressure value, select the conversion direction and adjust the atmospheric pressure when local data are available.
Worked examples at standard atmospheric pressure
| Gauge pressure | Absolute pressure | Calculation |
|---|---|---|
| 0 PSIG | 14.6959 PSIA | 0 + 14.6959 |
| 40 PSIG | 54.6959 PSIA | 40 + 14.6959 |
| 50 PSIG | 64.6959 PSIA | 50 + 14.6959 |
| 100 PSIG | 114.6959 PSIA | 100 + 14.6959 |
These examples use standard atmospheric pressure. At a different altitude or under different weather conditions, replace 14.6959 psi with the actual local atmospheric pressure.
When “PSI” usually means PSIG and when it does not
In everyday maintenance work, people often shorten PSIG to PSI because the gauge type already makes the reference clear. That shortcut may be understood in conversation, but it can create an error when the value is copied into a datasheet, test record or calculation.
When the context usually points to PSIG
Tyre-pressure gauges, workshop air-compressor gauges and many vented process gauges measure pressure above the surrounding atmosphere. A correctly zeroed vented gauge reads about 0 when it is open to the same air pressure on both sides, so a spoken reading of “80 PSI” usually means 80 PSIG in these settings.
The word “usually” matters. A dial face may show only PSI even though the instrument is gauge, absolute or sealed-gauge type. The unit printed on the scale does not prove which reference the instrument uses.
When PSI must not be assumed to mean PSIG
An absolute-pressure sensor compares the process pressure with a sealed vacuum reference. A differential-pressure instrument compares two pressure ports. A sealed-gauge sensor uses a sealed reference close to one standard atmosphere instead of following changes in the local atmospheric pressure. These instruments may all display values in PSI, but their readings are not interchangeable.
Vacuum and compound gauges also need clear labels. A compound gauge can show both pressure above the atmosphere and vacuum below it, while an absolute sensor remains above 0 PSIA. Treating either reading as ordinary positive PSIG can change the meaning of the measurement.
How to check a gauge or datasheet
Check the pressure type as well as the unit. The instrument face, model code, datasheet or calibration certificate should identify the reference as gauge, absolute, sealed gauge, compound or differential. A range of 0 to 100 PSIA is not the same as 0 to 100 PSIG, even though both ranges use PSI.
If the reference is missing, confirm it from the instrument documentation before using the number in an engineering calculation or permanent record. Writing the complete unit, such as PSIG or PSIA, prevents the next reader from having to infer what the original gauge measured.
Where the pressure reference changes a valve calculation
The pressure suffix matters whenever a valve calculation uses total absolute pressure or compares pressure with an absolute physical limit. Entering a PSIG number where the method requires PSIA removes atmospheric pressure from the input and can change the result.
Gas and steam valve sizing
Gas and steam sizing methods commonly require upstream or downstream pressure on an absolute basis. If the stated pressures are 100 PSIG upstream and 60 PSIG downstream, they are about 114.7 PSIA and 74.7 PSIA at standard atmospheric pressure when the sizing method calls for PSIA.
The pressure drop remains 40 psi because the same atmospheric pressure is added to both values and cancels when they are subtracted. Add atmospheric pressure to a gauge reading only when converting that reading to absolute pressure. Do not add it to the pressure drop.
The MacoTango control valve Cv calculator separates liquid, gas and steam calculations. Follow the pressure basis shown for the selected method rather than entering the number printed on a gauge without checking its reference.
Liquid pressure drop and vapour-pressure checks
For a liquid pressure-drop calculation, two gauge pressures can be subtracted directly when both use the same atmospheric reference. A change from 100 PSIG to 60 PSIG gives a pressure drop of 40 psi, just as 114.7 PSIA minus 74.7 PSIA gives 40 psi.
Cavitation, choking and flashing checks are different because they compare the liquid pressure with its vapour pressure at the operating temperature. Vapour pressure is an absolute value, so the process pressure used in that comparison must also be absolute. Comparing PSIG directly with an absolute vapour pressure mixes two reference points and can give the wrong conclusion.
Pressure class and test pressure are separate concepts
A valve marked Class 150 is not limited to 150 PSI. It is a rating designation. Where ASME B16.34 applies, the permitted pressure depends on the valve material group, operating temperature and applicable pressure-temperature table. Other valve types may be covered by a different product standard or project specification.
Shell and closure test pressures are also separate from the normal operating pressure. Their values and acceptance rules come from the applicable valve and test procedure. They cannot be found by treating the class number as PSI or by adding atmospheric pressure. The valve markings guide gives more detail on reading class and pressure information from a valve body or nameplate.
Conclusion
A pressure value is ready for engineering use only when its reference is clear. Use PSIG for pressure relative to the local atmosphere and PSIA for pressure relative to a perfect vacuum. If a value is labelled only PSI, confirm the reference before using it in a conversion, gas or steam sizing equation, or liquid vapour-pressure check.