2025-07-16 PSI, PSIA, and PSIG represent different ways to measure pressure. PSI stands for pounds per square inch, a common unit of pressure. PSIA means pounds per square inch absolute, where it includes atmospheric pressure. PSIG refers to pounds per square inch gauge, which measures pressure relative to atmospheric pressure. Using the correct measurement matters in fields such as manufacturing, pharmaceuticals, and valve engineering. Accurate pressure readings can improve efficiency and safety.