Rental property cash flow: how to calculate it

Rental property cash flow is the money left over each month or year after you collect the rent and pay every cost of owning the property, including the mortgage. The basic formula is: cash flow = rent collected − operating costs − mortgage payments. A property with positive cash flow pays for itself and puts money in your pocket; a property with negative cash flow needs regular top-ups from you.

The concept is simple. The hard part is being honest about every cost. The worked example below uses euros and a hypothetical 4% interest rate, which is an illustration and not a market rate. The maths works the same in any currency.

The rental property cash flow formula, line by line

Work through the figures in this order:

1. Gross potential rent: monthly rent × 12.
2. Minus vacancy and bad debt: an allowance for empty periods and unpaid rent.
3. = Effective rent: what you realistically collect.
4. Minus operating costs: management, non-recoverable service charges or HOA fees, insurance, property taxes, maintenance reserve, letting fees, accounting, utilities you pay.
5. = Net operating income (NOI): income from the property before financing.
6. Minus debt service: your full mortgage payments, interest and principal.
7. = Cash flow before tax.

Tax comes after that and depends heavily on where you live, how you own the property and how mortgage interest is treated. Ask a tax adviser for your country rather than assuming a rate.

Worked example: a €200,000 flat

Assumptions, all example figures:

• Purchase price €200,000, plus €16,000 purchase costs
• Rent €1,200 a month
• Vacancy allowance 5% of gross rent
• Property management 10% of rent collected
• Loan €120,000 (60% loan to value), 25-year repayment mortgage at a hypothetical 4%
• Monthly mortgage payment €633.40 (from the annuity formula)

Line	Per year
Gross potential rent (€1,200 × 12)	€14,400
Vacancy allowance (5%)	−€720
Effective rent	€13,680
Management (10% of €13,680)	−€1,368
Non-recoverable service charge / HOA fees	−€1,000
Insurance	−€350
Property tax	−€500
Maintenance reserve	−€1,200
Accounting, bank and other costs	−€300
Total operating costs	−€4,718
Net operating income (NOI)	€8,962
Mortgage payments (€633.40 × 12)	−€7,601
Cash flow before tax	€1,361 (about €113 a month)

A €1,200 rent turns into roughly €113 a month of cash. That is a typical surprise for new landlords: the gap between rent and real cash flow is large, and a property can look comfortable on gross figures while being finely balanced in practice.

The rental yield calculator runs this calculation for your own numbers, including vacancy, operating costs and the loan.

Cash flow is not the same as profit

In the example, year-one mortgage payments of €7,601 split into roughly €4,748 interest and €2,853 principal. Interest is a cost. Principal is you paying down your own debt, which increases your equity.

• Cash flow before tax: €8,962 − €7,601 = €1,361
• Profit before tax and depreciation (simplified): €8,962 − €4,748 interest = €4,214

The difference, €2,853, is equity you built in year one. Over time the principal share of each payment grows, as explained in how mortgage amortization works. Keep both numbers in view: cash flow tells you whether the property is sustainable month to month, profit and equity tell you whether it is building wealth.

Cash flow as a return on your cash

To compare properties, express annual cash flow as a percentage of the cash you put in. This is the cash-on-cash return.

• Cash invested: €80,000 deposit + €16,000 purchase costs = €96,000
• Cash-on-cash return: €1,361 ÷ €96,000 = 1.4%

That modest figure is partly a result of the low 60% loan to value. The relationship between NOI-based and cash-based returns is covered in cap rate vs cash on cash return, and gross versus net yield in how to calculate rental yield.

Stress test: what pushes cash flow negative

A single scenario is not enough. Change one assumption at a time and see what happens to the same property.

Scenario (one change from the base case)	Annual cash flow	Monthly
Base case	€1,361	€113
No vacancy	€2,009	€167
Vacancy 10%	€713	€59
Rent €1,100 instead of €1,200	€335	€28
Hypothetical rate 3%	€2,133	€178
Hypothetical rate 5%	€544	€45
Hypothetical rate 6%	−€316	−€26
75% loan to value (€150,000 loan)	−€539	−€45

Lessons from the table:

• Leverage cuts both ways. Borrowing €150,000 instead of €120,000 means €30,000 less of your own money in the deal, but the extra mortgage payments turn cash flow negative.
• Rates matter more than you think. A two-point rise in the hypothetical rate wipes out the monthly surplus.
• Rent is the most sensitive line. €100 less rent per month removes about three-quarters of the cash flow, because vacancy and management costs only fall slightly with it.

Break-even rent

Break-even rent is the rent at which cash flow is exactly zero. In this example it is about €1,068 a month: below that, you subsidise the property. It is a useful number to compare with what similar properties actually let for. If the market rent is only slightly above break-even, you have little margin for error.

Rules of thumb, and their limits

You will hear quick heuristics for screening properties. They are rough filters, not answers.

• The 50% rule: assume operating costs (excluding the mortgage) take about half of the rent. In the example, operating costs plus vacancy are €5,438, about 38% of gross rent. The rule can be far too high or far too low depending on the building, who pays which costs and local taxes.
• The 1% rule: monthly rent should be at least 1% of the purchase price. The example flat rents for 0.6% of its price. The rule was popular in markets with low prices relative to rents and screens out most properties in many cities, without telling you anything about costs or financing.

Use heuristics to decide which properties are worth a proper calculation, then do the full line-by-line version.

How to improve cash flow

• Buy well. The purchase price drives the loan and every return metric. Negotiation does more than most later optimisations.
• Set the right rent. Check comparable lets and review rent periodically within the rules where you are. See how to raise rent.
• Reduce vacancy. Good tenant selection, timely repairs and early renewal conversations shorten empty periods.
• Control operating costs. Compare insurance annually, understand your service charges and fix small issues before they become large.
• Choose financing deliberately. A larger deposit, a longer term or an interest-only loan can all raise monthly cash flow, each with trade-offs. Compare options in interest only vs repayment mortgage and model payments with the mortgage calculator.

Common mistakes

• Ignoring vacancy. Even a well-let property has gaps between tenants. Budget for them.
• No maintenance reserve. Repairs feel optional until the boiler fails. Set a realistic amount; rental property maintenance budget shows how to size it.
• Counting recoverable costs twice, or forgetting non-recoverable ones. Know which service charges you can pass to tenants and which you cannot. Rules differ by country and lease.
• Using a teaser rate for the full period. Model cash flow at the rate you will pay after any fixed period ends, and at a higher stress rate.
• Forgetting your time. If you manage the property yourself, the 10% management fee you save is payment for your work, not free money.
• Leaving out tax. Before-tax cash flow can look healthy and still shrink substantially after tax.

Cash flow checklist

• Gross rent based on real comparable lets, not the asking rent you hope for
• Vacancy and bad-debt allowance
• Management or letting fees, or a value for your own time
• Non-recoverable service charges or HOA fees
• Insurance, property taxes, licences and accounting
• Monthly maintenance and replacement reserve
• Full mortgage payment, at current and stressed rates
• Tax, estimated with a professional
• A cash buffer outside the property for months that go wrong

Key takeaways

• Cash flow = effective rent − operating costs − full mortgage payments.
• In the example, €1,200 of monthly rent becomes about €113 of monthly cash flow.
• Cash flow and profit differ by the principal you repay, which builds equity.
• Stress test vacancy, rent and interest rates; small changes can flip the result.
• For the bigger picture of strategy and risk, read the buy to let investing guide.