Comprehensive flow and magnetization data obtained previously on the behavior of ferrofluid based magnetorheological fluids (FF-MRF) [Susan-Resiga and Vekas, Rheol. Acta 55(7), 581-595 (2016)] were correlated using a Mason number (Mn) defined for these nano-micro composite fluids and the corresponding Casson (Ca) number, instead of the Bingham number used for conventional magnetorheological (MR) fluids. Practically, agglomerate-free, high colloidal stability ferrofluids with Newtonian behavior in the absence of a magnetic field were used as carrier to keep the off-state viscosity as low as possible and to ensure the reproducibility of the MR response of the composite fluid. The apparent viscosities and magnetization values for samples with various nanosized magnetites and micrometer range iron particle volume fractions (from 2.75% to 22.90% and from 4% to 44%, respectively) at different values of magnetic induction and shear rate collapsed on a master curve by using the nondimensional Mn and Ca numbers to correlate the measured data. By controlling the volume fraction both at nano-and microlevels, the magnetic and MR properties of the resulting composite fluids can be tailored to offer a high-performance lubricant for semiactive dampers and brakes, as well as for magnetofluidic rotating seals. The Mn and Ca numbers offer a design metric based solely on the properties of FF-MRFs and can be reliably scaled to devices of different sizes and operating conditions. (C) 2017 The Society of Rheology.