The diameter of single-walled carbon nanotubes (SWNTs) is an important characteristic to determine their electronic properties and direct further applications in electronics and photonics. A demand currently exists for an accurate and rapid method of evaluating the mean diameter and diameter distribution of bulk SWNTs. Here, we provide an effective means for quantifying the diameter distribution of SWNTs using optical absorption spectroscopy without a strict prior assumption on the form of the diameter distribution. Verification of this assignment protocol is based upon statistical analysis of hundreds of high-resolution transmission electron microscopy (HRTEM) images as well as comparison with Raman measurements on the same SWNT samples. A good agreement among different techniques indicates that this approach enables accurate and rapid assessment of diameter distribution and can be extended to bulk SWNT samples with various diameter distributions.