gauge pressure

Gauge pressure is the pressure of something measured relative to the surrounding air, rather than relative to a perfect vacuum. When you check a car tyre and the gauge reads 220 kPa, that is not the total pressure inside the tyre; it is how much the inside beats the outside air. Gauge pressure answers the practical question of how much extra squeeze a container holds compared with the atmosphere around it.

The precise definition subtracts the atmospheric pressure from the true, total pressure. Calling the total pressure the absolute pressure P_abs, and the outside air pressure P_atm, the gauge pressure is P_gauge = P_abs - P_atm. Most everyday instruments, from tyre gauges to blood-pressure cuffs, naturally read gauge pressure because one side of the measuring device is open to the atmosphere, so the air's push is already cancelled out. Gauge pressure can even be negative, which simply means the inside is below atmospheric, a partial vacuum, as inside a drinking straw when you suck.

The distinction matters whenever you plug pressure into a physics equation. Laws like the ideal gas law need the absolute pressure, so you must add the atmosphere back: P_abs = P_gauge + P_atm. One common trap is to forget this and use the gauge reading directly. A tyre reading 0 on the gauge is not empty of air; it is simply at atmospheric pressure, holding about 101 kPa of absolute pressure.