absolute pressure

Meaning: Absolute pressure is the total force per unit area a fluid exerts at a point, with zero defined as a vacuum. In a static fluid it rises with depth because of the weight of fluid above, and it includes any pressure acting on the surface, such as the atmosphere. Gauge pressure is absolute pressure minus atmospheric pressure, which is what a tire gauge reads.

Equation: P = P₀ + ρgh, where P is absolute pressure (Pa), P₀ is the pressure at the surface (Pa), ρ is fluid density (kg/m³), g is 9.8 m/s² (gravitational acceleration), and h is depth below the surface (m). Also Pgauge = P − Patm, with 1 atm ≈ 1.01 × 10⁵ Pa.

On the MCAT: A passage describes a diver, an open tank or a sealed container and asks for the pressure at a depth, or how it changes with depth, fluid density or surface pressure. Doubling the depth doubles ρgh, but not the absolute pressure, because P₀ is added. Gas-law questions use absolute pressure.

Common confusion: Students put gauge pressure into PV = nRT, or treat the pressure at an open water surface as zero. Its gauge pressure is zero, but its absolute pressure is about 1 atm. Pressure at a given depth depends on depth, not on container shape or width.