What is the difference between gauge pressure and absolute pressure?

Absolute pressure is measured relative to a perfect vacuum (zero reference), while gauge pressure is measured relative to local atmospheric pressure. The relationship is: P_absolute = P_gauge + P_atmospheric. At sea level, atmospheric pressure is approximately 101.325 kPa or 14.7 psi. Gauge pressure can be negative (vacuum condition) when below atmospheric pressure. Absolute pressure is used in gas law calculations and compressible flow analysis, while gauge pressure is commonly used in industrial applications for pump discharge, tank pressures, and piping systems.