Rental Property Income Explained: Cash Flow, Cap Rate, and the 1% Rule

A rental property’s income is not its rent. It is what is left after vacancy, operating expenses, and debt service — and the order matters, because each deduction is computed on the figure before it. This guide covers the cash flow waterfall, the 50% rule for estimating expenses, the cap rate for comparing properties without leverage, cash-on-cash return for comparing them with it, and the screening rules — the 1% rule and the rent-to-value ratio — that decide whether a property is worth analysing at all. The cap rate’s full treatment, with its own worked examples, is in the companion article linked below.

The cash flow waterfall: rent to cash flow, in order

Start with gross scheduled rent: the monthly rent times 12. Deduct vacancy and collection loss — a planning allowance, commonly 5% — to get effective rental income. Deduct operating expenses: property tax, insurance, maintenance and repairs, property management, utilities paid by the owner, and reserves for capital items. What remains is net operating income (NOI). Deduct debt service — the mortgage payment, principal and interest — and what remains is the cash flow before tax. Each line is computed on the figure above it, and skipping a line inflates every line below it.

The 50% rule estimates the operating expenses when they are unknown: over the long run, about half of gross rent goes to operating expenses, excluding debt service. It is a rule of thumb, not a measurement, and it is conservative by design — it exists to prevent optimistic underwriting, not to replace the actual expense figures. When the actual figures are known, they replace the rule, and the rule’s job is done. A property that cash-flows under the 50% rule will usually cash-flow on actuals; a property that cash-flows only by assuming 30% expenses is a property whose expenses have been assumed away.

Worked example. A $200,000 property, $2,000 a month in rent, $100,000 financed at 6.5% over 30 years, 50% expenses. Gross rent: $24,000 a year. Operating expenses at 50%: $12,000. Net operating income: $12,000. Debt service: $632.07 a month, $7,584.84 a year. Cash flow: $12,000 − $7,584.84 = $4,415.16 a year, or $367.93 a month. Cash-on-cash return on the $100,000 down payment: 4.42%. The Rental Property Calculator runs the same waterfall with the figures entered, including vacancy and reserves, so the comparison between properties is apples to apples.

Cap rate: the property, without the loan

The capitalisation rate — cap rate — is net operating income divided by the property’s value, or price. The example property: $12,000 ÷ $200,000 = 6.0%. The cap rate measures the property’s earning power as if it were bought for cash, with no mortgage. That independence from leverage is its purpose: two properties with the same cap rate earn the same on their price, whatever the buyer’s financing, and the cap rate is therefore the right tool for comparing properties, while cash-on-cash is the right tool for comparing financing choices on one property.

A higher cap rate means a higher return on the price, and usually a higher risk or a weaker market — cap rates are higher where prices are low relative to rents, and lower where prices are high relative to rents. A 6% cap rate in a stable market and a 10% cap rate in a declining one are not the same investment at different prices; they are different risks at different prices. Cap rates also move with interest rates: when borrowing costs rise, buyers demand higher cap rates, which means lower prices for the same income. The cap rate is a market price for risk, not a quality score.

The full treatment — cap rate versus cash-on-cash, the cap rate’s relationship to price, worked examples at several prices and incomes, and the mistakes of comparing cap rates across markets — is in the cap rate calculator. This guide keeps the cap rate brief because the waterfall is its subject, and the cap rate deserves its own.

Screening rules: the 1% rule, the 50% rule, and the rent-to-value ratio

The 1% rule screens: monthly rent should be at least 1% of the purchase price. The example property rents for $2,000 on a $200,000 price — exactly 1% — and it passes. A $200,000 property renting for $1,400 is at 0.7% and fails, and it will rarely cash-flow after expenses and debt service, because the rent is too small relative to the capital tied up. The rule is a filter, not a valuation: it rejects quickly, and it does not approve. A property that passes the 1% rule still needs the full waterfall, and a property in a high-appreciation market may be bought despite failing it, for growth rather than cash flow — consciously, with the cash flow shortfall funded from elsewhere.

The rent-to-value ratio is the same test in annual form: annual rent divided by price. The 1% rule is a 12% annual ratio. Markets vary: a 1% property is common in lower-priced markets and rare in expensive coastal ones, where 0.5–0.7% is normal and the return comes from appreciation. Screening rules must be calibrated to the market, and a national rule applied to a local market misleads in both directions — rejecting good properties in cheap markets and approving bad ones in expensive ones.

The 50% rule, the 1% rule, and the cap rate together make a three-minute screen: does the rent clear 1% of the price, do the expenses at 50% still leave a positive NOI, and does the cap rate clear the market’s range. A property that passes all three deserves the full analysis; a property that fails any of them needs a reason — appreciation, a value-add renovation, or a below-market purchase price — that is stated explicitly and costed. An unstated reason is not a reason; it is optimism.

Tax, reserves, and the decision

Cash flow before tax is not profit after tax. Rental income is taxable, mortgage interest and operating expenses are generally deductible, and depreciation — the paper deduction for the building’s wear, over 27.5 years for US residential property — shelters part of the cash flow from tax. Depreciation is recaptured on sale, so the shelter is a deferral, not an exemption. The tax position varies by jurisdiction and by the owner’s other income, and the calculator’s after-tax figures are estimates at the entered rate, not a tax return. Take tax advice for the actual position.

Reserves are the line most often omitted and most often regretted. A roof, a furnace, a water heater, and a turnover’s paint and flooring arrive as lumps, and a property with no reserve funds them from the owner’s pocket in the month they arrive. A reserve of 5–10% of rent, held separately, turns lumps into a smooth cost and keeps a single repair from wiping out a year’s cash flow. The 50% rule includes reserves implicitly; an actual-expense budget must include them explicitly, or the waterfall is incomplete.

Decision checklist. The waterfall is positive after vacancy, actual expenses, reserves, and debt service — not merely after the 50% rule. The cap rate clears the market’s range for the property’s risk. The cash-on-cash return clears the return available elsewhere for the same risk, after tax. And the screening rules were passed, or the reason for failing them is stated and costed. A property that passes all four is an investment; one that passes the rent and fails the waterfall is a liability with a tenant.

Vacancy and turnover: the cost of an empty month

Vacancy is the line that turns a performing property into a losing one, and it arrives as a whole month, not as a percentage. One empty month on a $2,000-a-month property costs $2,000 of rent, plus the turnover costs of preparing it for the next tenant — cleaning, paint, minor repairs, advertising, and the letting fee where an agent is used — which commonly add $500–$1,500 to the empty month. Two empty months in a year cost $4,000–$5,500, which wipes out the $4,415 of annual cash flow in the worked example entirely. The vacancy allowance in the waterfall — 5%, or $1,200 a year on $24,000 of rent — is the average; the actual vacancy arrives in lumps, and the lumps are what the reserve is for.

Turnover is the controllable part. A tenant who stays three years costs one turnover in three years; a tenant who stays one year costs three. The difference, at $1,000 of turnover cost each time, is $2,000 over three years — $667 a year — which is 15% of the worked example’s cash flow. Retention is therefore a financial strategy, not a courtesy: responsive maintenance, fair rent increases, and a good relationship keep tenants, and keeping tenants is cheaper than finding them. A rent increase that drives a good tenant out and leaves the property empty for a month loses more than the increase gains, and the arithmetic should be done before the increase is proposed, not after the tenant leaves.

The vacancy assumption should be set from the market, not from optimism. A market with 3% vacancy and a property in good condition near transport and jobs can be underwritten at 5% with confidence; a market with 10% vacancy, or a property in a weak location, should be underwritten at 10% or more, and the cash flow tested at that figure. If the property does not cash-flow at the market’s vacancy rate, it does not cash-flow — it cash-flows only in the assumption. Run the waterfall at the market rate, at twice it, and at zero, and the range of outcomes is the risk. A property whose cash flow survives twice the market vacancy is robust; one that survives only zero vacancy is a hope.

Frequently asked questions