A method to assess the purity of single-walled carbon nanotube (SWCNT) bulk samples based on Raman spectroscopy is reported. The new method has higher reproducibility and quantitative precision than those previously used. It consists of measuring the G-band intensity of liquid suspensions of SWCNT samples as a function of solid concentration. A simple equation is proposed in which one of the two adjustable parameters is the SWCNT purity. The method has been applied to a series of samples of similar characteristics, but varying quality. The results are compared to those obtained on the same series of samples using standard analytical techniques, including electron microscopy, thermogravimetric analysis and optical absorption.