The EPA reported that Americans spend 90% of their time indoors, where the air quality may be poor. People often use air purifiers to improve air quality; hence, the efficiency of air purifiers is crucial to our health. However, although many air purifier manufacturers claim to use HEPA filters for better performance, as a HEPA filter must filter 99.97% of the 0.3 μm particles, their filter thickness might not allow them to perform at their optimal CADR. Although theoretically achieving optimal CADR filter thickness has been studied, no experiments have been performed. Therefore, this study aims to determine the relationship between filter thickness and optimal CADR experimentally. This study used a homemade air purifier with a fan and filters of varying thicknesses; four filter brands and four fan powers were tested. The filtration efficiency was determined by measuring upstream and downstream of the cleaner using SMPS, and the CADR was calculated using the single-pass method by multiplying the filtration efficiency and flow rate. The results showed that the filtration efficiency increased with increasing filter thickness. Despite this, the optimal CADR occurred when the filter was layered to a specific thickness, providing 20 ~ 40% filtration efficiency for 0.1 μm particles and 10 ~ 35% filtration efficiency for MPPS particles (fan power was 2.7 W). The range of MPPS was 0.243 ~ 0.375 μm. When the purifier was layered using the same filter brand (Filter M), but the fan was turned on to four different powers (2.7 ~ 23W), the highest CADR occurred when the filtration efficiency was about 35% regardless of the fan power. The optimal CADR increased linearly with the fan power, with the MPPS ranging from 0.197 μm to 0.318 μm. We concluded that many air purifiers use filters that do not provide filtration efficiency close to HEPA standards. The air purifier most likely does not use filter thicknesses that provide the best CADR. To make full use of the air purifier and extend its service life, the optimal filter thickness should be slightly smaller than the optimal CADR filter thickness.