Rental Property Cash Flow Calculator

The gap between “cash flows on paper” and cash flow that survives a real year

Ask a landlord whether a rental cash flows and the answer is usually rent minus mortgage payment. That subtraction produces a number, and the number is almost always wrong, because it omits every cost that is real, recurring and simply doesn’t happen to show up in the specific month someone runs the math.

A rental property cash flow calculation that survives contact with an actual year needs four reserves most back-of-envelope estimates skip entirely: vacancy, maintenance, management, and capital expenditure. Each one is a real cost. None of them appears on a mortgage statement.

Vacancy: budgeting for the months nobody is paying rent

Even a well-run rental sits empty between some tenants — during turnover, during a slow rental season, or while a unit is prepped for a new occupant.

Budgeting 5 to 8 percent of gross rent as a vacancy reserve treats this as the predictable cost it is rather than a surprise when it happens. That is not pessimism; it is arithmetic based on typical turnover patterns. A property manager or landlord who has run rentals for years can usually cite their actual historical vacancy rate, which is a better input than a generic percentage once you have a track record.

Using zero percent because the unit happens to be occupied when you run the numbers is the single most common way a cash-flowing property turns into a cash-negative year.

Maintenance: the ongoing cost, separate from the big-ticket items

Maintenance covers the routine repairs every property generates — a leaking faucet, a broken garbage disposal, a patch of drywall, seasonal upkeep. It is distinct from capital expenditure, which covers the large components that fail on a longer cycle.

Budgeting 5 to 10 percent of rent for routine maintenance is a common working range, and older properties or ones with deferred maintenance at purchase should sit toward the higher end. This is spend that happens continuously across ownership, not an occasional surprise — treating it as one is how otherwise profitable rentals bleed cash in small, hard-to-notice amounts.

Property management: budget for it even if you self-manage today

Property management typically runs 8 to 10 percent of collected rent, and it is worth including in every projection even if you intend to manage the property yourself, for two reasons.

First, circumstances change — a move, a growing portfolio, or simply burnout from 2am maintenance calls can make self-management untenable, and a deal that only worked because you personally worked for free is not a deal that actually works. Second, your own time managing a property has a real opportunity cost even when no invoice changes hands for it.

Excluding this line to make a deal look better on paper is one of the more common ways new investors talk themselves into a marginal property.

Capital expenditure: the reserve for things that have not broken yet

This is the reserve most new landlords skip entirely, and it is the one that eventually produces the largest single unbudgeted expense.

Roofs, HVAC systems, water heaters, major appliances and flooring all wear out on a predictable if imprecise schedule. Setting aside 5 to 10 percent of rent every month, whether or not anything breaks that month, means a $12,000 roof replacement in year eight is a withdrawal from a reserve you already built rather than an emergency that wipes out several years of accumulated cash flow at once.

A property with no CapEx reserve is not actually cash-flowing at the rate it appears to — it is deferring a cost to a future date and pretending the deferral is a saving.

What the fully reserved number tells you that the naive one does not

Once all four reserves are subtracted alongside the mortgage, property tax, insurance and HOA dues, what remains is a number that should survive an ordinary bad year — a vacancy between tenants, a maintenance call, a management fee, a portion of a future capital repair — rather than one that only works in a year where nothing goes wrong.

A property that cash flows well after all four reserves is a genuinely strong deal. A property that only cash flows when reserves are set to zero is not cash-flowing at all — it is accumulating risk that shows up eventually, usually at the worst possible time.

Building this into a full underwriting

This calculator assumes you already know the mortgage payment, property tax and insurance figures — if those still need working out, the mortgage payment calculator covers the loan side directly.

Once full cash flow is known, compare it against the cap rate calculator to see how the property performs independent of financing, and the cash-on-cash return calculator to see the return on the actual capital invested. The vacancy cost calculator goes deeper on the vacancy assumption specifically, including turnover costs this calculator treats as a flat percentage.