How to Calculate Net Operating Income (NOI) for Rental Property

Net Operating Income — NOI — is the single most important number in rental property investing. It drives cap rate calculations, determines property value in commercial real estate, and tells you whether a deal pencils out before you ever talk to a lender. Here's exactly how to calculate it and what pitfalls to avoid.

The NOI Formula

NOI = Gross Rental Income − Vacancy Loss − Operating Expenses

That's it. Two subtractions. The complexity is in knowing what belongs in each bucket — and what doesn't.

Step 1: Calculate Gross Rental Income

Start with the total rent you'd collect if every unit were occupied and paying full market rent. For a 4-unit building with units renting at $1,400, $1,450, $1,400, and $1,500 per month, gross scheduled income is:

($1,400 + $1,450 + $1,400 + $1,500) × 12 = $69,000/year

Add any other recurring income: laundry machines, parking fees, storage unit rentals, pet fees. These belong in gross income too.

Step 2: Subtract Vacancy and Credit Loss

No property is 100% occupied 100% of the time. A standard allowance is 5–8% of gross income for residential properties in stable markets. In high-turnover or transitional markets, use 10%.

For our example at 6%: $69,000 × 0.06 = $4,140 vacancy allowance

Effective Gross Income = $69,000 − $4,140 = $64,860

Step 3: Subtract Operating Expenses

Operating expenses are the costs to run the property — not the financing costs. This distinction matters enormously. Common operating expenses include:

• Property taxes: Use the actual annual bill, not the seller's number (reassessment at purchase often raises taxes)
• Insurance: Landlord/dwelling policy, liability umbrella if applicable
• Property management: Typically 8–10% of collected rent if you hire a PM; your time cost if self-managing
• Maintenance and repairs: Budget 1–1.5% of property value per year for older properties, 0.5–1% for newer
• Capital expenditure reserve: Set aside 5–10% of gross income for roof, HVAC, appliances — even if you don't spend it every year
• Utilities: Only if landlord-paid (water/sewer for multi-family is common)
• Landscaping, snow removal, pest control
• Accounting and legal fees

What Does NOT Go in Operating Expenses

• Mortgage payments (principal + interest) — financing is not an operating expense
• Depreciation — a non-cash tax deduction, not a real operating cost
• Income taxes
• Capital improvements (a new roof is not a repair; it's capitalized)

Worked Example: 4-Unit Building

• Effective Gross Income: $64,860
• Property taxes: $7,200
• Insurance: $2,400
• Property management (9%): $5,837
• Maintenance reserve (1% of $380,000 value): $3,800
• CapEx reserve (7% of EGI): $4,540
• Water/sewer (landlord-paid): $2,400
• Total Operating Expenses: $26,177

NOI = $64,860 − $26,177 = $38,683

How NOI Connects to Cap Rate

Once you know your NOI, you can calculate your cap rate. The formula is simple: Cap Rate = NOI ÷ Property Value

If the same building is listed at $540,000: Cap Rate = $38,683 ÷ $540,000 = 7.2%

You can also run it backwards to value a property: if market cap rates for this asset class and neighborhood are 6.5%, the property's market value implied by its NOI is $38,683 ÷ 0.065 = $595,123. This is how commercial appraisers work.

Common Mistakes That Inflate NOI

• No vacancy allowance: Sellers often present "pro forma" numbers with 100% occupancy. Always apply a vacancy factor.
• Using asking rent, not market rent: Long-term tenants paying below-market artificially suppress NOI. But also beware sellers inflating rent rolls.
• Ignoring CapEx reserves: The building will need a roof eventually. If you don't budget for it, you're borrowing from future-you.
• Using current tax bill: Property taxes often reset at sale in many states. Get an estimate of post-purchase taxes from your county assessor.

Track NOI Automatically

RentrIQ calculates NOI by property and across your entire portfolio in real time — pulling income from recorded payments and expenses from your accounting ledger. You can see monthly and trailing-12-month NOI without touching a spreadsheet.

Bottom Line

NOI is the foundation of every real estate investment decision. Calculate it conservatively — use real vacancy rates, full maintenance reserves, and realistic management costs. A deal that only works at 100% occupancy with no repairs is not a good deal. Build your NOI on assumptions you can defend, and you'll make better investment decisions every time.