The ratio of the area of the air piston to the area of the fluid piston in an air-powered pump.

The pump ratio determines the multiplication factor between the air-input pressure of the air motor and the fluid-output pressure of the pump.

For example, if the air-input pressure is 100 psi, a pump ratio of 2:1 will yield a fluid-output pressure of 200 psi. For a pump ratio of 3:1, the same input pressure of 100 psi will yield a fluid-output pressure of 300 psi.

See also PEI/RP700.