The rheology of nanofiber suspensions is studied solving numerically the Population Balance Equations (PBE). To account for the anisotropic nature of nanofibers, a relation is proposed for their hydrodynamic volume. The suspension viscosity is calculated using the computed aggregate size distributions together with the Krieger-Dougherty constitutive equation. The model is fitted to experimental flow curves for Carbon NanoFibers (CNF) and for NanoFibrillated Cellulose (NFC), giving a first estimation of the microscopic anisotropy parameter, and yielding information on the structural properties and rheology of each system.