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What Do PSI, PSIA, and PSIG Mean? A Beginner’s Guide

TIME: 2025.06.24 AUTHOR: Admin NUMBER OF VIEWS 1034
PSI is a common pressure measurement unit in the United States. It is very important to understand its meaning and application in pressure measurement. In this article, we will introduce the concepts of PSI, PSIA and PSIG, their uses and differences.

What is PSI?

PSI is a unit of pressure measurement, which is the abbreviation of Pounds Per Square Inch. It is a relatively common unit of pressure measurement, mainly used in the United States, Canada and the United Kingdom. Generally speaking, people usually use PSI to refer to PSIG, which is the pressure relative to a standard atmospheric pressure. In specific industries, PSIG and PSIA are still distinguished. PSI is mainly used in tire pressure monitoring, hydraulic and pneumatic fields, and industrial manufacturing fields.

PSIA Vs PSIG

PSIA measures the pressure relative to an absolute vacuum, that is, the relative pressure. The PSIA = PSIG + current 1atm (in PSI). A completely sealed container with a vacuum inside has a pressure of 0 PSIA. At sea level, a standard atmospheric pressure is about 14.7 PSIA. It is mainly used in aerospace, HVAC, meteorology and scientific research, vacuum technology and other fields.

PSIG

PSIG refers to the pressure value relative to a standard atmospheric pressure, that is, gauge pressure. Gauge pressure is divided into positive pressure and negative pressure, so PSIG has positive and negative values. The conversion formula is PSIG = PSIA - current 1atm (in PSI). For example, at sea level, the pressure value of a tire is 32 PSIG, and the standard atmospheric pressure at that time is 14.7PSIA, then the actual absolute pressure value is 32+14.7=46.7PSIA.

PSIA PSIG Conversion

As mentioned above, the conversion formula between PSIA and PSIG is:
PSIA = PSIG + current 1atm (in PSI）
 PSIG = PSIA - current 1atm (in PSI)
It can be seen that the value of PSIG is always lower than that of PSIA, so the values between the two are easily interchangeable, provided that you know the local atmospheric pressure value, or you can use a standard atmospheric pressure value (14.7) for estimation.
PSIA = PSIG + 14.7 (in PSI）
PSIG = PSIA - 14.7 (in PSI)

Conclusion

PSI is a universal pressure measurement unit, PSIA and PSIG are two different unit variants. In specific applications, the choice of PSIG or PSIA needs to be determined based on the actual working conditions. Although they are not very different, they are very helpful for us to choose products.

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