The objective of this paper is to model the energy consumed in generating cellulose microfibres, 1 μm in diameter, as reinforcing agents, by refining bleached softwood kraft pulp in a PFI mill. An average initial fibre diameter of 13 μm was assumed. 125,000 revolutions in a PFI mill was found to produce a high yield of fibres 1.3 μm in diameter, and the minimum refining energy needed to reduce the fibre diameter to 1.3 μm was estimated as 1875 kJ for each 24 g charge in the PFI mill. Since elastic deformation of the fibres was found to be negligible, the size reduction was assumed to follow Rittinger’s Law. This gave a Rittinger’s constant of 28 J.m/kg for the given system. Using this value of Rittinger’s constant, the energy required to generate microfibres 1 μm in diameter was predicted as 2480 kJ for each 24 g charge in the PFI mill. It was deduced that microfibres generated in this way would cost a minimum of $2.37 per kilogram. Hence even this relatively inefficient method of grinding would not be prohibitively expensive, provided the resulting microfibres can be used as high quality reinforcements.