In HVAC, we are mostly dealing with PSIG pressure (gauge). That’s why we often need to convert PSIA to PSIG. Both are pressure units in the avoirdupois system; we will look at how they are different and how simple it is to convert PSIA to PSIG. To help you out, we have included a simple-to-use PSIA to PSIG calculator (insert PSIA, get PSIG) and a conversion chart spanning from 14.7 PSIA to 1000 PSIA.
Here is the difference between PSIA vs PSIG:
- PSIA or Pound per Square Inch Absolute is a unit that measures pressure relative to vacuum (absolute pressure). At sea level, the PSIA is equal to 14.7 pounds per square inch. Compared to PSIG, PSIA does not account for atmospheric pressure; it is an absolute unit.
- PSIG or Pound per Square Inch Gauge is a unit that measures pressure relative to atmospheric pressure; thus it is a relative unit (depended on atmospheric pressure). At sea level, the PSIG is equal to exactly 0.0 pounds per square inch.
How to convert PSIA to PSIG?
This is a very simple conversion if we are converting PSI absolute to PSI gauge at sea level. Namely, the PSIA to PSIG equation at sea level looks like this:
PSIG = PSIA – 1 atm = PSIA – 14.7 PSI
As you can see, to calculate PSIG from PSIA, we need to subtract atmospheric pressure from PSIA. At sea levels, this atmospheric pressure is equal to 14.7 PSI. Note: At higher altitudes, the atmospheric pressure is a bit lower.
Example: Let’s say that we have an absolute pressure of 80 PSI (that means that PSIA is 80 pounds per square inch). What will our gauge read (what will the PSIG be)? Here is how we can calculate this using PSIG to PSIA formula at sea level:
80 PSIA – 14.7 PSI = 65.3 PSIG
As we can see, at sea level, 80 PSIA is equal to 65.3 PSIG.
Note about wrong conversion: 1 PSIA is equal to -13.7 PSIG. It is wrong to conclude that 2 PSIA is equal to -27.4 PSIG; this is not how the PSIA to PSIG conversion works.
To help you out with this conversion, you can freely use the following PSIA to PSIG calculator that automatically converts pounds per square inch absolute to pounds per square inch gauge at sea level (14.7 PSI difference). Below the calculator, you will also find a handy conversion chart:
PSIA To PSIG Calculator
Here is how you can use this conversion calculator to determine PSIG based on PSIA input:
Let’s say that you want to convert 100 PSIA to PSIG. You just insert ‘100’ in the calculator above and you will get the result: 100 PSIA is equal to 85.3 PSIG.
Here is the full PSIA to PSIG conversion chart:
PSIA To PSIG Conversion Table
| PSIA (Absolute Pressure): | PSIG (Gauge Pressure): |
| 14.7 PSIA | 0.0 PSIG |
| 15 PSIA | 0.3 PSIG |
| 20 PSIA | 5.3 PSIG |
| 25 PSIA | 10.3 PSIG |
| 30 PSIA | 15.3 PSIG |
| 35 PSIA | 20.3 PSIG |
| 40 PSIA | 25.3 PSIG |
| 45 PSIA | 30.3 PSIG |
| 50 PSIA | 35.3 PSIG |
| 60 PSIA | 45.3 PSIG |
| 70 PSIA | 55.3 PSIG |
| 80 PSIA | 65.3 PSIG |
| 90 PSIA | 75.3 PSIG |
| 100 PSIA | 85.3 PSIG |
| 110 PSIA | 95.3 PSIG |
| 120 PSIA | 105.3 PSIG |
| 130 PSIA | 115.3 PSIG |
| 140 PSIA | 125.3 PSIG |
| 150 PSIA | 135.3 PSIG |
| 160 PSIA | 145.3 PSIG |
| 170 PSIA | 155.3 PSIG |
| 180 PSIA | 165.3 PSIG |
| 190 PSIA | 175.3 PSIG |
| 200 PSIA | 185.3 PSIG |
| 250 PSIA | 235.3 PSIG |
| 300 PSIA | 285.3 PSIG |
| 350 PSIA | 335.3 PSIG |
| 400 PSIA | 385.3 PSIG |
| 450 PSIA | 435.3 PSIG |
| 500 PSIA | 485.3 PSIG |
| 600 PSIA | 585.3 PSIG |
| 700 PSIA | 685.3 PSIG |
| 800 PSIA | 785.3 PSIG |
| 900 PSIA | 885.3 PSIG |
| 1000 PSIA | 985.3 PSIG |
Hopefully, you will now be able to convert PSIA to PSIG without much problem (using the calculator and this chart). If you need a bit of help, you can always use the comments below and we will help you out.
You can also consult a similar calculator for PSIG to PSIA conversion here.
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