Work out gauge pressure instantly with clear inputs, formula shown and shareable results.
Gauges read zero at atmospheric pressure, so an absolute figure needs the ambient pressure added back: 220 kPa gauge is 321 kPa absolute at sea level. This matters for gas law calculations, which always need absolute pressure - using a gauge reading there is one of the commonest errors in thermodynamics problems.
Absolute pressure
P(absolute) = P(gauge) + P(atmospheric)
Vacuum
negative gauge pressure means below ambient
Because the tyre's structural load is the difference across the wall. The atmosphere pushes back on the outside, so only the excess matters.
Absolute, always. PV = nRT counts molecules, and molecules do not know where the gauge was zeroed.