A magnetorheological (MR) valve is a device that uses the MR fluid flowing through a cylindrical orifice. In this paper, based on volumetric flow rate equation in the cylindrical MR valve, the necessary active MR fluid volume and the thickness between two cylinders of the valve are carried out. They are the minimum active fluid volume and gap of MR fluid exposed to the magnetic field. The equations for the volume and the gap derived to provide the theoretical foundation for the key geometric design in the valve. The results indicate that the volumetric flow rate can be continuously adjusted by an external magnetic field. The thickness and the volume for the cylindrical MR valve can be calculated based on MR fluid properties, the desired control pressure ratio, and the pressure of the valve.