Use of sustainable stormwater technologies such as bioretention cells (BRCs) is gaining in popularity across the world as episodes of severe flooding are becoming more frequent due to increased urbanization, and associated costs are rising due to decaying infrastructure and insufficient flood management. The aim of this study is to use numerical modeling to expand the understanding of BRC systems across the Toronto region. There is no one universally accepted approach to designing BRC systems. Local sensitivity analysis (LSA) with the one-factor-at-a-time method and global sensitivity analysis (GSA) with factorial design were conducted to identify the most influential components of BRC design for overflow reduction. Eight different model scenarios were used in a long term simulation to determine the efficacy of Ontario’s BRC design standards for meeting Toronto’s runoff volume control target (RVCT) of 27 mm. LSA shows that the highest reduction in overflow can be achieved by increasing BRC surface area, the saturated hydraulic conductivity (BSM Ksat) of bioretention soil media, or BRC ponding depth. On the other hand, GSA suggests that the most effective BRC performance can be achieved by simultaneously increasing the area of BRC, BSM Ksat, and BRC storage depth. Continuous simulation results show that Ontario’s minimum BRC design guideline does not meet Toronto’s RVCT. However, small adjustments to the baseline design, such as a 0.4% increase in BRC surface area, a 5 cm increase in ponding depth, or a 3 cm/h increase in BSM Ksat, can reduce the number of storm events causing overflow by up to 50% and meet RVCT.