PSI vs. PSIG: Key Difference Explained in 30 Seconds
PSI stands for pounds per square inch—absolute pressure counted from a perfect vacuum. PSIG is the same unit, but the “G” means it’s gauge pressure, which ignores atmospheric pressure already pushing on everything around us.
People mix them up because tire pumps, pressure washers, and HVAC gauges all display PSIG, so “30 PSI” on a tire actually means 30 PSIG. The missing “G” tricks us into thinking the reading is absolute.
Key Differences
PSI counts every molecule, starting at zero in outer space. PSIG starts at Earth’s sea-level atmosphere (14.7 PSI), so 0 PSIG equals 14.7 PSI. Quick math: add 14.7 to PSIG to get PSI.
Which One Should You Choose?
Use PSIG for daily tools like compressors or bike pumps; they’re pre-calibrated to ignore air. Use PSI when calculating burst ratings, altitude effects, or scientific formulas that need true, absolute pressure.
Is car tire pressure PSI or PSIG?
Always PSIG; gauges are vented to air, so they read 0 at sea level.
How do I convert PSIG to PSI?
Add 14.7 at sea level: 35 PSIG = 49.7 PSI.
Does weather affect PSIG readings?
Slightly; a 1 PSI drop in barometric pressure will raise PSIG by 1 PSI if the tire stays sealed.