Rental Property Cash Flow Calculator: Project Monthly Cash Flow
Use a rental property cash flow calculator to project monthly cash flow from rent, vacancy, operating expenses, and the mortgage before you buy.
By David Chen, Buy-and-Hold Investor · Last reviewed: 2026 · 9 min read
Run the numbers while you read
Open the matching calculator and test each assumption against your own deal.
What Cash Flow Really Measures
Cash flow is the money left over each month after a rental property pays every one of its bills, including the mortgage. A rental property cash flow calculator brings rent, vacancy, operating expenses, and debt service into one place so you can see the true number rather than guessing from rent minus mortgage.
Positive cash flow means the property puts money in your pocket each month. Negative cash flow means you feed it. The point of running the numbers before you buy is to find out which one you are signing up for.
The Cash Flow Formula
Monthly Cash Flow = Effective Rent - Operating Expenses - Mortgage Payment
Effective rent is gross rent minus a vacancy allowance. Operating expenses cover taxes, insurance, management, maintenance, repairs, and reserves for big-ticket items. The mortgage payment is principal and interest, plus taxes and insurance if they are escrowed.
Net operating income is effective rent minus operating expenses, before the mortgage. Subtract the mortgage from NOI and you get cash flow. Keeping the two ideas separate makes it easy to see whether a weak result comes from the property or from the financing.
Expenses Investors Forget
- Vacancy: budget for the property sitting empty part of the year, even in a strong market.
- Capital expenditure reserves: roof, HVAC, and water heater replacements do not show up monthly but they are real.
- Property management: include it even if you self-manage, so the number holds if your plans change.
- Maintenance and turnover: repairs between tenants and ongoing upkeep add up over a year.
Worked Example
A rental collects $2,000 in monthly rent. A 5% vacancy allowance drops effective rent to $1,900. Operating expenses such as taxes, insurance, management, maintenance, and reserves total $650. That leaves $1,250 of NOI.
If the mortgage payment is $1,050, monthly cash flow is $200. That is a modest but positive result. Raise the rate or add an expense and the same property can slip into negative cash flow, which is why testing the assumptions matters.
How to Use the Result
Start with monthly cash flow, then look at the return on the cash you invested. A property with thin cash flow can still make sense if the down payment is small, and a property with strong headline rent can disappoint once reserves and vacancy are included.
If cash flow is negative, test a lower purchase price, a larger down payment, higher rent, or lower financing costs. The goal is a property whose income covers its costs with a margin you are comfortable with.
Frequently Asked Questions
How do you calculate cash flow on a rental property?
Subtract operating expenses and the mortgage payment from effective rent. Effective rent is gross rent minus vacancy. Operating expenses include taxes, insurance, management, maintenance, and reserves. What remains each month is your cash flow.
What is good monthly cash flow for a rental property?
There is no universal number. Some investors target a set dollar amount per unit, others focus on cash-on-cash return. What matters is that cash flow is positive after realistic vacancy and reserves, and that the return justifies the cash invested and the risk.
Why is my rental cash flow negative?
Negative cash flow usually comes from a high purchase price, high interest rate, low rent, or underbudgeted expenses. Try a larger down payment, a lower price, higher rent, or better financing, and confirm your expense estimates are realistic.
Should cash flow include reserves for repairs?
Yes. Setting aside money each month for capital expenditures such as roof and HVAC replacement gives a more honest cash flow figure. Skipping reserves makes a property look better than it will perform over time.
David Chen · Buy-and-Hold Investor, Denver, CO
David is a long-term rental investor who manages a portfolio of buy-and-hold properties. He writes from the operator seat about expenses, reserves, and the numbers that decide whether a rental actually performs.
Educational Disclaimer
All calculations are estimates for educational and planning purposes only. PropertyFlowTools.com does not provide financial, tax, legal, lending, or investment advice. Verify calculations and consult qualified professionals before making property or financing decisions.