We report that the intrinsic viscosity [eta] of nanocellulose dispersions can be solely expressed as a function of the aspect ratio p of the nanocellulose. Both short rod-like nanocrystalline and long spaghetti-like nanofibrillated celluloses were prepared as dispersions in water. The influence of the flexibility and dimensions of the nanocelluloses on the flow properties of their dispersions was investigated by experimental and theoretical approaches using seven nanocellulose samples with different widths (2.6-14.4 nm) and aspect ratios (23-376). As the aspect ratio of a nanocellulose increases, it becomes more flexible, and its dispersion has higher viscosity. Irrespective of the flexibility and dimensions of these nanocelluloses, the relationship between [eta] and p was rho[eta] = 0.15 x p(1.9), where p is the density of the nanocellulose.