Buy vs Build Business ROI Calculator

For “Buy,” the tool starts with your monthly profit at acquisition and applies your monthly growth rate each month. It sums projected monthly profits until that cumulative total equals (or exceeds) the purchase price—then that month is the buy breakeven point. For “Build,” it assumes profit ramps linearly from $0 to a mature profit over your ramp months, subtracts startup cost from cumulative profit to find build breakeven, and then computes ROI at 12 and 24 months.

The assumptions keep things simple: no taxes, financing/interest, acquisition fees, owner salary, ad spend, or overhead. For “Buy,” it assumes the current profit trajectory continues without major shocks; for “Build,” it assumes a linear ramp and doesn’t model failed launches, pivots, or irregular spikes. Because time value of money is ignored, breakeven timing and ROI can differ from real-world discounted results—especially for longer breakeven scenarios.

If the existing business monthly profit is $0 and/or the growth rate is negative enough to prevent cumulative profits from ever reaching the purchase price, the tool will show “No breakeven under current assumptions” for the buy scenario. If the build ramp months are long, build breakeven can extend beyond 24 months, triggering a “Long path to profitability” flag. If buy breakeven and build breakeven are within 10% of each other, the calculator leans on the higher 12-month ROI to break the tie.

Be careful with the monthly growth rate for an acquired business—small increases can significantly improve ROI in the model, even if real growth depends on traffic sources, platform changes, and competition. If growth is negative, the buy scenario is automatically treated as high risk. For “Build,” choosing an overly short ramp months can make breakeven appear much faster than a realistic execution timeline.