A flow meter has a housing having an inlet, an outlet, and a measuring chamber between the inlet and outlet. The measuring chamber has a diameter larger than that of the inlet. A blade section is located between the inlet and the measuring chamber, and the blade section comprises a cylindrical part near the measuring chamber and having a cylindrical inner wall, an expanding part between the cylindrical part and the inlet, and spiral blades which extend from the expanding part to the housing, for imparting the rotational movement to the fluid when the fluid enters the measuring chamber. A ball is positioned in the measuring chamber such that the rotational movement of the fluid moves the ball in a circular motion around the measuring chamber for measuring the flow of the fluid.