Cash flow is the most fundamental metric in rental property investing. It tells you whether a property puts money in your pocket each month or takes money out. Positive cash flow means the property pays for itself and generates profit. Negative cash flow means you're subsidizing it out of your own income.
Calculating cash flow accurately requires accounting for every expense, not just the obvious ones. Investors who underestimate costs end up with properties that look profitable on paper but drain their bank accounts in practice.
The Cash Flow Formula
Cash Flow = Gross Rental Income - Vacancy - All Operating Expenses - Debt Service
Each term matters. Here's what goes into each bucket.
Gross Rental Income
This is the total rent you collect if the unit is occupied 100% of the time. Use the actual current rent for an occupied property, or a realistic market rent estimate for a vacant one. Don't inflate this number.
Vacancy
No property is rented 100% of the time. Tenants move out, units need time to be cleaned and re-rented, and sometimes you have an unexpectedly long vacancy. A common estimate is 5-8% of gross rent, but this varies by market and property type. In tight rental markets, vacancy may be lower. In softer markets or with higher-turnover tenants, it can be higher.
Operating Expenses
This is where most new investors undercount. Include all of the following:
- Property taxes: Check the county assessor's actual figure. Don't estimate.
- Insurance: Landlord insurance costs more than standard homeowner's insurance.
- Property management: If you self-manage now, budget for it anyway. Circumstances change. Typical cost is 8-12% of gross collected rent.
- Repairs and maintenance: A conservative estimate is 1% of property value per year for ongoing upkeep.
- Capital expenditures (CapEx): Major items like roofs, HVAC systems, water heaters, and appliances wear out and eventually need replacement. Budget for these separately from routine maintenance. A typical estimate is another 1% of property value per year, though this varies significantly with property age.
- Utilities: If you pay any utilities as the landlord (water, trash, common area electric), include them.
- HOA fees: If applicable, these are fixed monthly costs and must be included.
- Landscaping or snow removal: If you pay for this, account for it.
Debt Service
This is your monthly mortgage payment: principal plus interest. It's the largest single expense for most leveraged investors. Use the actual payment if you have a loan, or model a payment based on your expected down payment and interest rate.
The $100 Per Door Rule of Thumb
A common shortcut is the "$100 per door per month" benchmark. The idea is that a rental property should generate at least $100/month in positive cash flow per unit after all expenses and debt service. A single-family home should cash flow at least $100/month; a four-unit building should cash flow at least $400/month.
This is a rough screen, not a rigorous analysis. It's useful for quickly filtering out deals that clearly won't work before you spend time running full numbers.
Common Mistakes That Kill Cash Flow Projections
Forgetting vacancy. Assuming the property will always be rented is the most common error. Even a single month of vacancy wipes out several months of profit.
Ignoring capital expenditures. Maintenance and CapEx are different. Maintenance covers small recurring repairs. CapEx covers big-ticket items that eventually fail in every property. Ignoring CapEx produces projections that look great until the roof needs replacing.
Using asking rent instead of achievable rent. Verify rental comps in the area before projecting income.
Self-management discount. Some investors subtract property management fees when they plan to self-manage. This hides the true cost of ownership and makes the property look more profitable than it is. Self-management has real time and opportunity costs.
Underestimating insurance. Landlord insurance on a rental property is more expensive than the typical homeowner's policy. Get an actual quote.
Positive vs. Negative Cash Flow
Positive cash flow means the property generates income beyond all its costs. This is what most investors target.
Negative cash flow (sometimes called "alligator properties" because they eat your cash) can still make sense in limited scenarios. If appreciation is strong and you can afford to subsidize the property, some investors accept negative cash flow in high-growth markets. This is a speculative strategy and requires significant reserves.
For most investors, especially those building a portfolio, positive cash flow from day one is the safer, more sustainable approach.
Run the Numbers Before You Buy
Manual calculations work, but a purpose-built tool handles all the variables at once. The Rental Cash Flow Calculator lets you input gross rent, vacancy rate, all expense categories, and your mortgage details to see projected monthly and annual cash flow instantly. Adjust any input and the result updates in real time, making it easy to stress-test different scenarios before committing to a purchase.