Over a range of intrinsic viscosity of 0—20 dI/g the relation between the relative viscosity ηr and the concentration C of dilute n-butyl acetate solutions of fractionated cellulose nitrates can be expressed by the equation log10 (ηr—1)/C= log i + aC + bCi + eCi2, where a, b, and care constants, and i is the intrinsic viscosity. The constants a, b, and c have been evaluated, and a table constructed of corresponding values of ηr, and log10iC for a concentration of 0.2 g/dl. The table permits the intrinsic viscosity of a nitrate to be computed from a single measurement of relative viscosity made at about this concentration.