Gauge pressure is the pressure measured by pressure reading instruments that actually measure the difference between the pressure they are measuring and atmospheric pressure surrounding the instrument. Accordingly, when such a pressure gauge reads 0 psig (pounds per square inch gauage) in a vessel or pipe, the absolute pressure in the vessel/pipe is 0 psig + atmospheric pressure (in psi). A typical value for atmospheric pressure at sea level is 14.7 psi.

Psig (gauge) + psi (atmospheric pressure) = psia (absolute pressure).