Currently, a tool for determining the total suspended solid (TSS) removal efficiency of a bioretention cell is not readily available to designers in Ontario. The lack of such calculation in design reports may lead to difficulties in receiving credit or approval from the reviewing authorities for an appropriately designed bioretention facility. Recognizing that the stormwater runoff quality treatment capacity of a bioretention cell varies from site-to-site, a tool was developed which incorporates each unique site’s characteristics into its calculation of the treatment. Research was carried out to determine which numerical models for runoff TSS treatment had been successfully applied to bioretention cells, and the Ali and O’Melia’s (1978) filtration equations were chosen, due to their prior use (Li and Davis (2008), Liu and Davis (2014)). The equations were adapted to integrate with the widely used EPA SWMM, through its Add-in Tools feature. The created tool was 1) tested to ensure the model’s ability to match the monitored performance of the Toronto and Region Conservation Authority’s (TRCA) Sustainable Technology Evaluation Program’s (STEP) bioretention cell, and 2) benchmarked against the TSS removal of the bioretention cell predicted by other software such as WinSLAMM. The tool’s capacity to accurately simulate the TSS reduction performance of the TRCA’s STEP site, supports its applicability to Ontario based bioretention cells. This research, model development, and verification, are the first steps towards the complete development of a stormwater runoff TSS and Total Phosphorous (TP) reduction model to aid in bioretention cell design in Ontario.