To convert a gage reading to psia (pounds per square inch absolute), simply add the atmospheric, or barometric, pressure to the gage reading.
One problem with adding the barometric pressure to the gage reading is that most barometers are not scaled in pounds per square inch. They are scaled in inches of mercury. The chemical symbol for mercury is Hg. When a measurement is in inches of mercury, it's usually labeled with "Hg.
Converting "Hg to psi requires a mathematical conversion. A column of mercury approximately 29.9 inches (759.5 millimeters) tall exerts 14.7 psi (1.01 bar) of pressure on the surface below it. Therefore, 29.9" Hg (759.5 mmHg) is equal to 14.7 psi (1.01 bar). Normal atmospheric pressure at sea level can be expressed as 14.7 psi (1.01 bar) or 29.9" Hg (759.5 mmHg).