Abstract: A variable area flow meter for measuring the volumetric flow of a fluid flowing through the flow meter includes a body portion having a fluid inlet, a fluid outlet portion and a channel. A fixed boundary is defined between the channel and the fluid outlet portion. A fluid flow path extends between the fluid inlet and fluid outlet. A piston is arranged within the channel and longitudinally displaceable by the flow of fluid. The cross-sectional area of a portion the fluid flow path is variably determined by the structural shape of the piston and the instantaneous location of the piston relative to the fixed boundary. The longitudinal displacement of the piston in the direction of the fluid flow and consequently the cross-sectional area of the portion of the fluid flow path are dependent on the volumetric flow rate of the fluid.