We describe a fast, reproducible method to accurately measure the length of single-walled carbon nanotubes (SWNTs). The method is based on measuring the viscosity of a macroscopic sample of dilute suspended SWNTs. The average length is determined from the difference between the zero-shear viscosity of the suspension and that of the solvent. Using the relationship between viscosity and length, the average length of HiPco SWNTs is found to range from 400 to 700 nm, in agreement with atomic force microscopy (AFM) measurements. Compared to AFM, length determination by viscosity is faster and appears more reproducible.