A reference material is essential to enable and accelerate commercialization of new materials. The National Institute of Standards and Technology (NIST) of the United States recently released the world's first reference material of a single-walled carbon nanotube (SWCNT) dispersion known as RM8281. Although the description of the material have been well documented by NIST, the chirality population, one of the most important properties, is yet unidentified for the RM8281. Here, we present for the first time a quantitative chirality assessment of the RM8281 reference material by using a method based on optical absorption spectroscopy. A universal background model has been established for SWCNT solid film samples, which proves to be the key to the successful chirality assessment. Our results show that approximately 75% of SWCNTs in RM8281 have diameters distributing in a narrow range of 0.7-0.9 nm, and about 69% of SWCNTs have chiral angles ranging from 15° to 30°. For the whole population, semi-conducting SWCNTs (∼74%) prevail significantly over metallic ones (∼26%). Importantly, ∼25% of the total RM8281 SWCNT population was found to be (6,5) nanotube. High-resolution transmission electron microscopy and electron diffraction technique were utilized to complete an adequate statistical analysis of chirality distribution in RM8281, giving a satisfactory agreement with the above absorption spectrum measurements, thus, validating absorption spectroscopy serving as a fast and standard protocol for quantifying the SWCNT chirality population.