I built an interactive simulator to help with the rent vs. buy decision. It compares three strategies: (A) buying with a mortgage, (B) renting and investing the down payment in equities, and (C) renting while investing monthly savings (when mortgage > rent). The tool visualizes asset growth, cumulative outflows, and net value over time with breakeven analysis. Built with Streamlit, Plotly, and vectorized NumPy calculations. All formulas and assumptions are documented. Try it at https://rent-or-buy-sim.com or run locally with Docker.