Net Operating Income (NOI) is the standard measure of a rental property's profitability before financing and taxes enter the picture. Because it strips out how a specific buyer finances the deal, NOI lets you compare properties on equal footing and is the foundation for both cap rate and debt service coverage ratio (DSCR) — two numbers lenders and investors both rely on.
The formula
NOI = Gross Operating Income − Operating Expenses
Two things trip people up in that simple-looking formula: what counts as "gross operating income" (it's not just rent) and what counts as an "operating expense" (it deliberately excludes some big costs you might expect to see).
Step 1: Calculate Gross Operating Income
- Start with Gross Potential Rent (GPR) — total rent if every unit were occupied all year at market rent.
- Subtract a vacancy and credit loss allowance — a realistic estimate of income lost to vacancy between tenants and to unpaid rent, based on the specific property's and market's actual vacancy history, not a single number that applies everywhere.
- Add other income — laundry, parking, storage, pet fees, or any other recurring income the property generates.
Gross Operating Income = GPR − Vacancy/Credit Loss + Other Income
Step 2: Subtract Operating Expenses
Operating expenses are the real, recurring costs of running the property. Common line items include:
- Property taxes
- Property insurance
- Property management fees
- Repairs and maintenance
- Utilities paid by the owner (common in multifamily)
- HOA or condo fees, if applicable
- Landscaping, pest control, and other routine service contracts
- A reserve for future replacements (roof, HVAC, appliances) — many investors budget this as a line item for planning purposes, even though it isn't a cash expense in the month it's set aside
What NOI deliberately excludes
This is the part people get wrong most often. NOI does not subtract:
- Mortgage principal and interest (debt service) — NOI is calculated before financing so you can compare properties regardless of how they're financed.
- Capital expenditures (a new roof, a full HVAC replacement) — these are treated as capital improvements, not operating expenses, even though the reserve line above budgets for them separately.
- Income taxes and depreciation — these depend on the owner's individual tax situation, not the property's operating performance.
A simple illustrative example (not a real property)
- 10 units at $1,200/month market rent = $144,000 Gross Potential Rent
- Vacancy/credit loss allowance at 5%: −$7,200
- Laundry income: +$3,000
- Gross Operating Income: $139,800
Operating expenses:
- Property taxes: $14,000
- Insurance: $6,500
- Property management (8% of collected rent): $11,184
- Repairs and maintenance: $9,000
- Landscaping/pest control: $2,400
- Reserve for replacements: $4,000
- Total operating expenses: $47,084
NOI = $139,800 − $47,084 = $92,716 (illustrative figure, made-up numbers for demonstration only.)
What you do with NOI once you have it
- Cap Rate = NOI ÷ Property Value (or purchase price). Using the example above, on a $1,200,000 purchase price, cap rate = $92,716 ÷ $1,200,000 ≈ 7.7%.
- DSCR = NOI ÷ Annual Debt Service. If annual mortgage payments (principal + interest) are $60,000, DSCR = $92,716 ÷ $60,000 ≈ 1.55, meaning the property generates about 1.55 times the cash needed to cover its debt payments. Many lenders look for a DSCR above a minimum threshold that varies by lender and loan program.
Common mistakes when calculating NOI
- Including mortgage payments as an expense. This is the most common error and it makes NOI (and cap rate) meaningless for comparison purposes.
- Using an unrealistically low vacancy assumption to make a deal's numbers look better than the market actually supports.
- Forgetting property management costs even when you plan to self-manage, since your own time has real value and future buyers or lenders will often add this line back in anyway.
- Skipping a reserve for capital items in your underwriting, then getting surprised when the roof needs replacing in year three.
Frequently Asked Questions
Q: Is NOI the same as cash flow?
A: No. Cash flow to the owner is NOI minus debt service (and sometimes minus capital expenditures actually spent that year). NOI ignores financing; cash flow doesn't.
Q: Should I include a vacancy allowance even on a property that's fully occupied right now?
A: Yes. Underwriting NOI with a realistic vacancy assumption protects you from overpaying based on a temporarily perfect occupancy snapshot.
Q: Do I include capital expenditure reserves in NOI?
A: Practices vary. Many investors track a reserve as a planning line item outside the strict NOI calculation, while appraisers and lenders sometimes include a replacement reserve as an operating expense. Be clear about which convention you're using when comparing numbers with someone else.
Q: Why do lenders care about DSCR instead of just NOI?
A: DSCR tells the lender whether the property's income can comfortably cover the specific loan being underwritten, which is why it's calculated after NOI rather than replacing it.