The calculator on this site estimates monthly rental cash flow using one formula:
Net monthly cash flow = Monthly rental income − Monthly mortgage payment (PITI) − Management reserve − Maintenance reserve − Vacancy reserve − Other monthly expenses.
Each reserve (management, maintenance, and vacancy) is calculated as a percentage of the rental income you enter, not as a percentage of the property's value or purchase price. This matches how most property managers actually bill their fee and how most landlords budget for maintenance and vacancy — as a share of what the unit is expected to bring in, applied every month whether or not that specific cost occurs that month.
A simpler calculator might just ask for rent and mortgage and stop there. We don't, because that approach systematically overstates cash flow. Maintenance costs and vacancy periods are likely over a long period of ownership — even though they rarely arrive on a neat monthly schedule. Reserving for them monthly, at a percentage you set, converts an unpredictable lump-sum cost into a predictable monthly line item, which is the same logic professional property managers and experienced landlords use when building their own budgets.
This also means the "management reserve" input applies whether or not you actually hire a property manager. If you self-manage, you can set it to a low number or zero — but be aware that doing so effectively assigns a $0 value to your own time, which understates the true cost of owning and running the property yourself.
The mortgage field asks for your full PITI payment: principal, interest, taxes, and insurance combined into one monthly figure. We ask for the combined number rather than breaking it into four separate fields because most mortgage statements and loan estimates already present it this way, and combining it reduces the chance of a user accidentally leaving taxes or insurance out. If your lender escrows taxes and insurance separately from your loan servicer, add both figures together before entering the total here. If you own the property outright with no mortgage, you can enter just your property tax and insurance costs in this field, or split insurance into the "other expenses" field — either approach produces the same net cash flow total as long as every real cost is entered exactly once.
To keep the first-use experience simple and fast, this version of the calculator does not compute: appreciation or long-term property value growth; loan amortization and the portion of each mortgage payment that goes toward principal paydown (which is a form of equity return, separate from cash flow); depreciation and other tax effects; closing costs, renovation costs, or the total cash invested figure needed for a cash-on-cash return calculation; or multi-year projections. These are all legitimate parts of full underwriting, but they require more inputs and more assumptions than a fast, anonymous first-pass calculator should demand. If a property clears your cash flow bar here, the responsible next step is to run a fuller analysis — including your actual loan amortization schedule and a realistic hold-period projection — before making an offer.
Results are rounded to the nearest whole dollar for readability. The default values pre-filled in the calculator (for example, $2,200 in monthly rent) are illustrative starting points only, meant to show the tool working with a plausible example — they are not a recommendation, a market average, or a claim about what any particular property will rent for. Replace every field with your own numbers before relying on the result for a real decision.
All calculations run locally in your browser using JavaScript. Nothing you type into the calculator — rent, mortgage payment, or any other field — is transmitted to or stored on our servers. Refreshing or closing the page clears every value you entered.
Because the formula is simple by design, you can verify any result by hand in under a minute. Multiply your rent figure by each percentage reserve to get the dollar value of that line item, add the mortgage payment and other expenses, and subtract the total from rent. If the number the calculator shows doesn't match your own arithmetic, double-check that you entered percentages as whole numbers (for example, 8 for 8%, not 0.08) and that your mortgage figure represents a true monthly PITI payment rather than an annual total or a principal-and-interest-only figure. Being able to reproduce a tool's math by hand is one of the simplest ways to build confidence that a calculator is doing what it claims, and we'd rather you check our work than take it on faith.
This calculator is intentionally scoped to a single question: what is a property's estimated monthly operating cash flow, given a set of inputs you control? If you need a 5, 15, or 30-year projection, an internal rate of return calculation, comparison across multiple financing scenarios, or portfolio-level reporting across several properties, you'll eventually want a more advanced underwriting tool built for that purpose. Starting with a fast, transparent cash flow estimate is still a reasonable first step even if you expect to move to deeper analysis later — it tells you quickly whether a property is even worth the time a full underwriting workflow requires.