The accurate estimation of optical absorption coefficient ( \(\alpha\) ) for powdered nanomaterials to use in Tauc plot analysis remains a challenge due to the lack of explicit methodologies in the literature. This ambiguity leads to inconsistencies in bandgap determination and improper labeling of units on Tauc plot axes, particularly for nanopowdered samples without a measurable thickness. This study provides a detailed methodology for estimating α for nanopowders using UV–Vis-NIR spectroscopy data, and proposes alternative graphing techniques that circumvent the need for thickness measurements. Results demonstrate that, while the conventional Tauc plot ( \(\alpha\) hν)n vs. hν can yield accurate bandgap values regardless of thickness substitution to evaluate \(\alpha\) , the unit of ( \(\alpha\) hν)n must be stated as \({(\text{eV}/\text{cm })}^{n}\) if path length (thickness) was included in α calculation, or \({(\text{eV})}^{n}\) when path length was excluded. It is inappropriate to state the (arb. unit) even in the case of Tauc plot [F(R)hν]n vs. hν derived from diffuse reflectance data. Furthermore, alternative methods (modified Tauc plots), such as (Ahν)n vs. hν and An vs. hν with a common unit \({(\text{eV})}^{n}\) , which do not require thickness measurements were validated as reliable methods for bandgap estimation. Practical demonstrations using SrZrO3, TiO2: Li, Co nanopowders, and ZnO thin film confirmed the robustness of these methods and ensured consistency with literature values. It is hoped that the present work would curtail the widespread of unwarranted units on the Tauc plot. The correct unit must be stated based on how the Tauc’s equation was evaluated.
