NOI explained: The most important number in rental property investing
Net Operating Income (NOI) is the foundation of rental property analysis. Learn how to calculate it correctly and why it matters.
If you only learn one number in real estate investing, make it NOI. Net Operating Income is the foundation that almost every other metric builds on.
What is NOI?
NOI = Gross Rental Income − Operating Expenses
That's it. But the simplicity hides some important nuances.
What's included in NOI
Income (add these up)
- Gross scheduled rent
- Other income (laundry, parking, storage, pet fees)
- Minus vacancy allowance (typically 5-8%)
- Minus credit loss (tenants who don't pay)
Operating Expenses (subtract these)
- Property taxes
- Insurance
- Property management (8-10% of rent)
- Repairs and maintenance
- Utilities (if owner-paid)
- Landscaping / snow removal
- Reserves for capital expenditures
What's NOT in NOI
- Debt service (mortgage payments)
- Capital expenditures (roof, HVAC replacement)
- Depreciation (accounting concept)
- Income taxes (personal)
NOI is "above the financing line"—it tells you how the property performs regardless of how you finance it.
Why NOI matters
NOI is the input for most major metrics:
| Metric | Formula |
|---|---|
| Cap Rate | NOI ÷ Price |
| DSCR | NOI ÷ Annual Debt Service |
| Cash Flow | NOI − Debt Service |
Get NOI wrong, and everything downstream is wrong too.
Common NOI mistakes
1. Using seller's "pro forma" numbers
Sellers often show best-case rent with understated expenses. Always verify:
- Actual lease rates (not "market" rates)
- Real expense history (not estimates)
2. Forgetting property management
Even if you self-manage, include 8-10% for management. Your time has value, and you might hire help later.
3. Ignoring vacancy
A property is never 100% occupied forever. Budget 5-8% for vacancy and turnover.
4. Skipping reserves
Capital expenditures are real. Budget $200-400/unit/year for reserves (roof, HVAC, appliances).
How to calculate NOI step-by-step
Example: 4-unit property
- Gross scheduled rent: $1,200 × 4 = $4,800/month = $57,600/year
- Vacancy (5%): −$2,880
- Effective gross income: $54,720
Operating expenses:
- Property taxes: $4,800
- Insurance: $1,800
- Management (8%): $4,378
- Repairs (5%): $2,736
- Reserves: $1,200
- Total expenses: $14,914
NOI = $54,720 − $14,914 = $39,806
Quick NOI sanity checks
- Expense ratio should be 35-50% for residential rentals
- If seller shows 25% expenses, dig deeper
- If NOI seems too good, it probably is
Use NOI to make decisions
Once you have accurate NOI:
- Price the deal: What cap rate makes sense for this market?
- Check debt coverage: Can NOI cover the mortgage with room to spare?
- Estimate cash flow: What's left after debt service?
Use the Rental Deal Analyzer to calculate NOI and run these scenarios automatically.