A method for measuring the average viscosity of a test fluid uses calibrated magnetic nanoparticles, with certain chosen hydrodynamic diameters and actual lateral dimensions (e.g. diameters), that are mixed into a small volume of the test fluid and a single magnetic relaxation curve measurement to provide data for viscosity determination. The distribution of hydrodynamic particle sizes of an ensemble of magnetic nanoparticles that are magnetically blocked at room temperature can be determined. Modifications of the method can be used to determine the distribution of viscosities in a complex fluid at the sub-microscopic level providing a novel type of viscosity measurement.