In materials science and crystallography, the true density is an important derived physical quantity of solids. Here we report the correlation of the true density of nanometer-wide fibrillar crystallites of cellulose with their purity, crystallinity, morphology, and surface functionality. In the single fibrils, all the cellulose molecules are uniaxiallly oriented. Thus, the true density indicates the molecular packing density in the single fibrils and is essential for the precise estimation of the volume fraction of cellulose in fibril-based composites or porous structures. We demonstrate that the true density of fibrillar crystallites of cellulose is approximately 1.60 g/cm(3) irrespective of the biological origins of the cellulose (wood, cotton, or a tunicate) and the crystallinity. The true density is in fact independent of the dimension of the crystallites and the atomic conformation of the uniaxially oriented but noncrystalline molecules at the crystallite surface. In the single fibrils, all the cellulose molecules are densely packed from the crystalline core to the noncrystalline outermost regions. The value of 1.60 g/cm(3) remains unchanged even when the fibrils are dispersed through the wet disintegration process of "nanocellulose" production. In contrast, tailoring the surface functionality of the fibrils by oxidation and/or adsorption results in a substantial change in the true density up to 1.8 g/cm(3) or down to 1.3 g/cm(3). The true density of nanocellulose is indeed governed by the surface functionality and has a strong gradient in the fibril cross-sectional direction.