Ask a new landlord what their property earns and you will usually get rent minus the mortgage payment. It is a comforting number and it is wrong by a wide margin, because it omits every cost that arrives irregularly: the empty month between tenants, the boiler, the roof, the letting agent's fee, the turnover repaint.
Those costs are not unpredictable in aggregate. They are highly predictable as percentages of rent — which is precisely why professional underwriting includes them as fixed reserve lines rather than hoping for a quiet year.
The correct order of operations
- Gross scheduled rentAnnual rent at full occupancy. This is the only optimistic number allowed in the model.
- Less vacancyGross rent × vacancy rate. Even a great property between tenants sits empty during turnover.
- = Effective gross incomeWhat you realistically collect.
- Less operating expensesTaxes, insurance, HOA, repairs, management, capital reserves. Not the mortgage.
- = Net operating income (NOI)The property's earning power, independent of how you financed it. This drives valuation.
- Less debt serviceAnnual principal and interest.
- = Cash flowThe money that actually reaches your account.
Why NOI excludes the mortgageTwo buyers paying the same price for the same building have the same NOI but wildly different cash flow depending on their deposit and rate. Keeping financing out of NOI is what makes cap rate comparable across properties — and what stops a cheap loan from disguising a bad asset.
Underwrite the deal
The reserves nobody wants to budget
| Reserve | Typical allowance | Why it is not optional |
|---|---|---|
| Vacancy | 5–8% of gross rent | Turnover takes two to six weeks even in tight markets |
| Repairs & maintenance | 5–10% of rent | Appliances, plumbing, decorating between tenancies |
| Capital expenditure | 5–8% of rent | Roof, boiler, windows, kitchen — 15–25 year cycles |
| Management | 8–10% of rent | Your own time has a cost even if you self-manage |
| Bad debt / legal | 1–2% of rent | One eviction can cost three months of income |
Add those up and 25–35% of gross rent is consumed before the mortgage. This is the "50% rule" many investors use as a sanity check: operating expenses plus reserves tend toward half of gross rent on older single-family properties. If your model shows 15%, you have forgotten something.
Reading the four metrics correctly
- Cap rate = NOI ÷ price. Compares assets regardless of financing. Useful for judging whether the price is fair relative to the local market.
- Cash-on-cash = annual cash flow ÷ total capital invested. The only metric that answers "what is my money earning?" Include closing costs and rehab in the denominator or you will flatter the deal.
- DSCR = NOI ÷ annual debt service. A lender's stress test and yours. Below 1.15 there is no room for a vacancy, above 1.35 the deal has genuine resilience.
- The 1% rule — monthly rent ≥ 1% of price. A screening heuristic only, and increasingly rare in appreciating markets. Use it to filter, never to decide.
Negative cash flow is a position, not a strategyAccepting negative cash flow means funding the property from your salary every month while hoping appreciation compensates. That can work in supply-constrained markets, but it is a leveraged directional bet requiring a large cash reserve — and it fails badly if you lose your income. Never confuse it with investing for yield.
Tax treatment changes the answer
Cash flow and taxable income diverge sharply for rental property, because depreciation is a non-cash deduction. A property producing $3,000 of positive cash flow can simultaneously report a taxable loss once depreciation and mortgage interest are deducted — which is why after-tax returns often beat the pre-tax model.
The offsetting cost is depreciation recapture on sale, and passive loss rules that may defer deductions until you dispose of the property. Model the mechanics before assuming a paper loss is usable this year, and read the deduction documentation standards that apply equally to rental schedules.
Limits, errors and borders of the standard advice
Critical reviewRental property advice is dominated by people selling courses, mentorships and deals. The rules of thumb that survived are the ones that make deals look feasible quickly — which is precisely the wrong selection criterion.
"The 1% rule: monthly rent should be 1% of purchase price."
The claimA simple screen separates viable rentals from bad ones.
Where it breaksIt is a screening heuristic calibrated to a specific era of interest rates, property taxes and insurance costs, and it silently assumes all of those are constant. It ignores the two variables that most determine returns — financing cost and operating expense ratio — so a property passing the rule in a high-tax, high-insurance, high-rate environment can still lose money every month. It is also unattainable in most large coastal markets, where investors nonetheless earn returns through appreciation and rent growth, so the rule filters out entire viable strategies while admitting properties in declining areas where the ratio looks excellent because prices are falling for a reason.
The borderUseful only as a first-pass filter on a long list within a single market at a single moment. Never as a buy decision: replace it with an actual pro forma using local taxes, real insurance quotes, current financing and realistic vacancy.
"Cash flow is positive — the deal works."
The claimIf rent exceeds the mortgage, taxes and insurance, the property is profitable.
Where it breaksThat calculation omits most of the cost base. Vacancy is not zero, and turnover carries cleaning, marketing and lost rent. Maintenance is not zero, and capital items — roof, heating, appliances, exterior — do not appear for years and then arrive as five-figure sums, which is why they must be reserved monthly rather than expensed when they happen. Management costs money even when self-managed, because your time is not free. Deals are routinely underwritten without capital reserves, then reported as cash-flow positive right up to the first major failure.
The borderPositive cash flow is meaningful only after vacancy, management, maintenance and a funded capital-expenditure reserve are deducted. Underwrite with conservative assumptions on all four; a property that only works with none of them was never positive.
"Real estate is passive income."
The claimRentals produce income without ongoing work.
Where it breaksIt is a small business with tenants, regulation and physical assets. Self-management is ongoing work with unpredictable timing, and the emergencies do not schedule themselves conveniently. Delegating to a manager costs a meaningful share of rent — often the entire margin on a thin deal — and still requires oversight. The word "passive" is doing marketing, not description, and it is why so many first-time landlords are surprised by the workload rather than the numbers. Concentration is the other omission: one property is one tenant, one roof, one local employment market.
The borderIt becomes closer to passive at scale with professional management, or through pooled vehicles that own property without you operating it. For one or two units, budget for it as a part-time business — and if you want genuinely passive property exposure, that is a different instrument.
"Leverage magnifies returns — buy with as little down as possible."
The claimDebt increases return on equity, so minimum deposits maximise wealth.
Where it breaksLeverage magnifies outcomes in both directions, and the failure mode is not a lower return but insolvency. High leverage means thin cash flow, so a single extended vacancy or one large repair creates a shortfall you must fund from elsewhere. Refinancing risk compounds it: loans mature, and the terms available at maturity may be far worse. The illustrations promoting this almost always assume continuous occupancy, steady rent growth and available refinancing — the three things that fail together in a downturn.
The borderModerate leverage on a property with genuine cash-flow cushion and reserves is a sound way to build equity. The border is the point where a few months of vacancy plus one capital item would force a distressed sale; beyond that, you are not investing with leverage, you are relying on conditions holding.
Our conservative underwriting will reject deals that go on to perform well, particularly in strongly appreciating markets where investors who ignored cash flow entirely did better than those who insisted on it — that is a real cost of our framing, not a hypothetical. We also model the property in isolation and give little weight to the tax treatment of depreciation, deductions and gains, which in some jurisdictions materially changes the outcome. And tenant law, eviction timelines and rent regulation vary so much that operating assumptions transferable in one market are dangerous in another.
Frequently asked questions
Should I self-manage to improve cash flow?
You can, but do not delete the line from your model — replace it with an honest estimate of your hours. If the deal only works because you supply unpaid labour, you have bought a part-time job with a mortgage attached. Model it at market management cost and treat any saving as your wage.
How much cash should I hold per property?
Six months of full operating cost plus debt service, plus the cost of the single largest foreseeable capital item. For a typical single-family rental that is usually $12,000–20,000 held separately from your personal emergency fund.
Is a REIT a better way to get property exposure?
For most people with under six figures to deploy, yes — no leverage risk, no tenants, instant diversification and liquidity. Direct ownership wins on control, leverage and tax treatment. Compare honestly using the REIT guide before committing capital.
How do I handle a property that appraises below contract?
Renegotiate or walk. Bridging the gap with extra cash raises your invested capital and lowers cash-on-cash return immediately, and it removes the equity cushion that protects you if values soften.
The bottom line
Underwrite in the correct order: gross rent, vacancy, operating expenses including reserves, then debt service. Judge the result on cash-on-cash and DSCR together, and require the deal to survive a 10% vacancy year with a $6,000 repair. Properties that clear that bar produce income. Properties that need perfect conditions produce anxiety and a tax loss.
Revision log. Jul 2026: added the full underwriting model with DSCR, opex ratio and the reserve allowance table. Reviewed twice yearly.