How to analyze rental property cash flow (the honest version)

Why rent minus mortgage is not analysis

Every listing marketed to investors implies the same arithmetic: the tenant pays rent, the rent covers the mortgage, the difference is yours. That arithmetic omits every cost that makes a rental a business. A roof does not fail monthly, but it fails; a tenant does not leave every year, but tenants leave. Analysis that ignores lumpy, irregular costs is not conservative or aggressive — it is simply wrong, and the error always runs in one direction.

The discipline that fixes this is old and simple: convert every irregular cost into a monthly accrual. You will not write a check for vacancy each month, but the property incurs the cost continuously. Underwriting it that way is how lenders think, how appraisers approach income property, and how experienced landlords decide what to pay.

The expense lines that belong in every model

A defensible pro forma for a Florida rental includes each of the following, every month, whether or not the check goes out that month. The IRS's own guidance on rental property lists most of these as ordinary deductible expenses — which is a useful tell: if the tax code expects the cost, your model should too.

• Vacancy and turnover — an allowance for the weeks a unit sits empty between tenants, plus make-ready costs like paint, cleaning, and re-listing.
• Property management — even if you self-manage, price your time; if the deal only works with free labor, the deal is paying you a wage, not a return.
• Repairs and maintenance — the routine stream of appliance calls, plumbing fixes, and lawn care that never appears in a listing flyer.
• Capital expenditure reserves — an accrual for the roof, HVAC, water heater, and repiping that will eventually come due; Florida sun and humidity shorten those clocks.
• Property taxes and insurance — in Florida, model both with escalation in mind rather than trailing figures; non-homestead assessment rules and the insurance market both move.
• Association dues and assessments — for condos and HOA communities, dues plus a realistic view of special assessment risk.

A hypothetical walk-through

Suppose, purely as an illustration, a house rents for $2,400 a month. A rent-minus-mortgage view compares that $2,400 to a $1,900 payment and calls it $500 of monthly profit. A real model first deducts a vacancy allowance, a management line, a maintenance line, and a capital reserve accrual — hypothetically, several hundred dollars combined — and only then compares net operating income to the debt service. The same property can show positive cash flow under the shortcut and negative cash flow under honest accounting. The property did not change; the honesty did.

The point of the exercise is not pessimism. It is that offers, financing structure, and rent targets should all be set against the honest number. Investors who model correctly can hold through a bad year because the bad year was already in the spreadsheet.

How lenders read your cash flow

When you finance a rental with a conventional loan, the lender does not take your pro forma at face value. Fannie Mae's Selling Guide sets out how rental income is documented and adjusted — generally from lease agreements or the property's tax return history — and applies its own discounting before any of that income supports your qualification. Income you project and income a lender will count are different numbers, and the gap matters when you size a loan.

Wholesale Mortgage Lending is a mortgage broker: we arrange financing and can tell you how a given program will treat a property's income. We are not investment advisers, and nothing here is a recommendation to buy any property — the modeling is yours to own, and a licensed accountant should review the tax treatment.

Run the numbers before you write the offer

The order of operations matters. Model the property's full expense load first, see what income survives, and then shop financing sized to that number. Doing it backwards — falling for a property, then stretching the model until it works — is how rent-minus-mortgage thinking turns into an alligator that eats a paycheck every month. A cash flow calculator that forces every expense line into the open is the cheapest insurance an investor can buy.