The light absorption and scattering play a prominent and often underrated role in the overall photocatalytic process and heavily affect the rate. This is particularly important for the choice of the catalyst in addition to other chemical and physical parameters usually considered for their catalytic role. Here we propose an approximated but easy-to-apply method to evaluate the light harvested by the photocatalyst slurry and its scattering/absorption coefficients, which does not require the use of complex spectrophotometric tools and the complicated radiative transport equation. The optical properties are obtained with the lamp and in the experimental setup employed in the photocatalytic batch tests. Among the four TiO2 specimens considered, we characterized Evonik P25 and Hombikat UV100. The obtained scattering and absorption coefficients helped in rationalizing the experimental results on the degradation of formic acid at low concentration. From the rate dependence on the catalyst concentration, this approach allowed further understanding of the role of catalyst-specific properties affecting the overall catalytic performance. This approach is proposed as a starting point for fixing conditions to compare different photocatalysts.