Net Operating Income: How to Build and Audit NOI on a Rental

Net operating income (NOI) is what a rental property earns in a year after you collect the rent and pay the costs of running it, but before mortgage payments, depreciation and income taxes. The formula is short: gross potential rent, minus vacancy and credit loss, plus other income, gives you effective gross income (EGI), and EGI minus operating expenses is your net operating income.

The formula is easy. The inputs are the hard part, because most listing-sheet numbers come from someone who wants a higher price. This guide shows how to build NOI, audit a seller’s version, and price a small rental deal for buyers. NOI is the top of the cap rate fraction, and most cap rate mistakes start here.

The Net Operating Income Formula, Line by Line

Line	What it means
Gross potential rent (GPR)	Every unit leased all year at market rent
– Vacancy and credit loss	Empty months, plus rent billed but never collected
+ Other income	Laundry, parking, storage, pet rent, late fees
= Effective gross income (EGI)	What actually lands in the bank account
– Operating expenses	Taxes, insurance, utilities, repairs, management, reserves
= Net operating income (NOI)	What the property earns, regardless of who owns it or how it’s financed

NOI describes the property, not the owner. Two buyers with different loans and tax situations should reach the same NOI for the same building, which is why anything tied to a particular owner gets pulled out.

Why vacancy belongs above the line

Beginners skip vacancy because the building is “full right now.” But even a fully leased fourplex turns over units, and each turnover costs weeks of rent plus balances you never collect. Apply a market-appropriate vacancy and credit loss rate even at 100% occupancy. The example below uses a hypothetical 6%.

What Goes Into NOI, and What Never Does

Operating expenses are the recurring costs of keeping the building running and rented. IRS Publication 527 lists the kinds of rental expenses landlords usually deduct, including advertising, cleaning, insurance, repairs, taxes, utilities and management fees. NOI and tax rules part ways on financing, depreciation and improvements.

Include in operating expenses	Exclude from NOI
Property taxes	Mortgage principal and interest (debt service)
Insurance (hazard, liability, flood if required)	Depreciation
Owner-paid utilities (water, sewer, trash, common electric)	The owner’s income taxes
Repairs and routine maintenance	Capital improvements (new roof, HVAC replacement, additions)
Property management fee	Owner’s personal car, phone, meals, travel
Landscaping, snow removal, pest control	Loan fees, points, and refinance costs
Turnover, cleaning, advertising, leasing	One-time legal costs from buying or selling
Accounting, licenses, rental registration	Asset-management fees tied to a specific owner or fund
Replacement reserves (an annual allowance)	The actual capital spend those reserves will fund

Debt service, depreciation and taxes

Your mortgage payment depends on your down payment, your rate and your credit, not on the building, so it stays out. Depreciation is a tax deduction, not cash leaving your account. Publication 527 has residential rental buildings depreciated over 27.5 years under the general depreciation system, which shows up on your tax return but has nothing to do with how the property performs. Income taxes depend on the owner’s whole financial picture, so they’re excluded too.

Capital expenditures vs. reserves

Publication 527 says you must capitalize what you spend to improve a rental property rather than deduct it. A $14,000 roof doesn’t belong in one year’s operating expenses. Careful buyers subtract an annual replacement reserve, a per-unit allowance that spreads future capital needs across the years. Fannie Mae’s small multifamily term sheet says replacement reserve escrows are typically required on higher-leverage loans. If a seller’s NOI leaves reserves out, add them before you compare.

The owner’s personal expenses

Small landlords often run their truck, cell phone and family dinners through the rental’s books. Those costs come out of NOI, but watch the other direction too: the owner who does the repairs, mows the lawn and shows units is giving the building free labor. Any buyer who isn’t that owner will have to pay for it, so that labor has to come back in as an expense.

Building NOI From Scratch: A Hypothetical Fourplex

Here’s a fully worked, hypothetical fourplex. Four two-bedroom units rent for $1,250 a month each. The tenants pay their own electric and gas, the owner pays water, sewer, trash and the common-area lights, and there’s a coin laundry in the basement. Every figure here is illustrative.

Line item (hypothetical)	Annual	How it was estimated
Gross potential rent	$60,000	4 units x $1,250 x 12
Vacancy and credit loss (6%)	-$3,600	Local market assumption
Laundry income	$1,440	12 months of coin receipts
Late and pet fees	$360	Trailing 12 months, actual
Effective gross income	$58,200
Property taxes	$6,100	Re-estimated at the purchase price, not the seller’s bill
Insurance	$3,400	Written quote from an agent
Water, sewer, trash	$3,800	12 months of utility bills
Common-area electric	$600	12 months of utility bills
Repairs and maintenance	$4,200	About $1,050 per unit
Property management (8% of EGI)	$4,656	Quote from a local manager
Lawn and snow	$900	Vendor quote
Turnover and leasing	$1,000	Paint, cleaning, listing fees
Accounting, licenses, admin	$500	Tax prep and rental registration
Replacement reserves	$1,000	$250 per unit per year
Total operating expenses	$26,156	About 45% of EGI
Net operating income	$32,044	$58,200 – $26,156

No mortgage, no depreciation. Management is included even if you plan to self-manage, because the next buyer won’t work for free.

From NOI to a Price: The Cap Rate Bridge

For income property, value is roughly NOI divided by the market cap rate. At a hypothetical 7.5% cap rate, $32,044 / 0.075 = about $427,000. The cap rate guide covers where that rate comes from and how it shifts by market and asset class. What matters here is the leverage running the other way: because NOI sits on top of the fraction, every dollar of NOI is multiplied when it turns into value.

Sensitivity: what $1 of expense is worth

At a 7.5% cap rate, every $1 of annual NOI is worth about $13.33 of value. Miss a $1,900 tax increase and you’ve overpaid by roughly $25,000. This table shows how much value a change in annual expenses (or income) moves:

Annual NOI change	At 6.5% cap	At 7.5% cap	At 8.5% cap
$500	$7,692	$6,667	$5,882
$1,000	$15,385	$13,333	$11,765
$1,900 (tax reassessment in our example)	$29,231	$25,333	$22,353
$2,500	$38,462	$33,333	$29,412
$5,000	$76,923	$66,667	$58,824

The lower the cap rate, the more each dollar matters. In low-cap markets, a sloppy insurance estimate can erase your whole assignment fee.

NOI and the Lender: Debt Service Coverage

Lenders turn NOI into a second test: the debt service coverage ratio, or DSCR = NOI / annual debt service. At 1.0 the property just covers its loan payments. Fannie Mae’s Small Mortgage Loan program term sheet, checked October 22, 2026, lists a minimum DSCR of 1.25x and a maximum 80% LTV for properties with five or more units. Our fourplex is too small for that program. Fourplexes are usually financed with residential loans or the investor products covered in our DSCR loan guide, and DSCR minimums and calculation methods vary from lender to lender.

To see how the test bites, assume a hypothetical $320,000 loan at 7.75% over 30 years. The payment is about $2,293 a month, or $27,510 a year. DSCR = $32,044 / $27,510 = 1.16. Against a 1.25x requirement, the most annual debt service the NOI supports is $32,044 / 1.25 = $25,635, which at the same rate and term caps the loan around $298,000. The buyer needs about $22,000 more cash, or a lower price. For a market benchmark, Freddie Mac’s Primary Mortgage Market Survey put the average 30-year fixed rate at 7.32% for the week of October 22, 2026. That survey tracks conventional loans, so don’t assume an investor loan will price at that number.

One more point for 1-4 unit deals: conventional residential underwriting doesn’t use NOI the way commercial lenders do. Fannie Mae’s Selling Guide section on subject-property rental income tells lenders to multiply monthly gross rent by 75% to get net rental income on a purchase. If your buyer is using a conventional loan on a fourplex, that is the number their lender will qualify them on, not your NOI.

Pro Forma vs. T12: How Listing Sheets Inflate NOI

A T12 (trailing twelve months) is the property’s actual income and expenses for the last year. A pro forma is a projection of what the seller or broker thinks it could earn. Only the T12 is evidence. Here’s the same hypothetical fourplex as a marketing package might present it:

Item (hypothetical)	Seller pro forma	Underwritten	The trick
Rent	$1,350/unit	$1,250/unit	“Market rent” that no current lease supports
Vacancy	3%	6%	Best-year occupancy treated as normal
Property taxes	$4,200	$6,100	Seller’s current bill, before any reassessment at sale
Insurance	$2,100	$3,400	An old policy with low coverage
Management	$0	$4,656	“Owner self-manages”
Utilities and lawn	$4,700	$5,300	Common-area electric left out
Repairs	$2,000	$4,200	Owner does the repair work himself
Turnover, admin, reserves	$0	$2,500	Simply left off
NOI	$51,656	$32,044
Value at 7.5% cap	~$689,000	~$427,000	A gap of about $261,000

No single line is an outright lie, yet together they add more than 60% to the price. Also watch for one-time income (an insurance payout, a back-rent catch-up) shown as recurring, and “potential” parking or storage income nobody pays yet. Ask for the T12 and rent roll, and treat any pro forma as an opening argument.

Auditing Every Expense Line Before You Trust It

1. Property taxes: Pull the current bill from the county tax collector or assessor website, then find out how that jurisdiction treats a sale. In many places a sale can trigger reassessment toward the price paid, but the rules vary a lot by state and county, so check the local assessor’s rules before you assume either way.
2. Insurance: Get a written quote from an agent for a landlord or dwelling-fire policy that matches the coverage a lender will require. Don’t rely on the seller’s premium.
3. Utilities: Ask for 12 months of water, sewer, trash and house-meter bills, or call the utility directly.
4. Rents: Match the rent roll to signed leases and to bank deposits. Check for concessions, tenants who are behind, and family members paying below market. Our guide to buying tenant-occupied property covers estoppel letters and lease review.
5. Repairs and capital needs: Walk the property and estimate the roof, HVAC, water heaters and plumbing ages. Big items go into reserves or into a capex deduction from the price. Our repair cost estimating guide helps here.
6. Management: Call two local property managers and ask their fee for a small multifamily. Use that rate whether or not the buyer plans to hire one.
7. Other income: Ask for laundry collection logs or the vendor’s revenue-share statements. If a fee isn’t in the bank deposits, don’t count it.

How a Wholesaler Prices a Rental Deal With NOI

Rental buyers pay for income, not ARV. When wholesaling a fourplex or rental package, work your contract price backward from the NOI your buyer will underwrite:

1. Rebuild NOI yourself. Use the audited figures above: $32,044.
2. Apply your buyer’s cap rate. If your rental buyers want at least 8% on stabilized small multifamily in that area (know their buy box), the price that works for them is $32,044 / 0.08 = about $400,500.
3. Subtract near-term capex. If the roof needs replacing within two years at a hypothetical $15,000, a buyer will take that off the price: about $385,500.
4. Subtract your fee. To keep a hypothetical $15,000 assignment fee, your contract price with the seller needs to be about $370,000 or less.
5. Stress-test the cash flow. Run the buyer’s likely loan through the cash-on-cash return calculator to confirm the deal still pays them something after debt service. Our cash-on-cash return guide explains the metric.

Send buyers your NOI worksheet, with the source for each line, alongside the deal. Quoted insurance and reassessed taxes build more trust than a forwarded pro forma. If you’re learning to put these packages together, this kind of underwriting is part of what’s covered in the Real Estate Wholesaling Mastery course. The same NOI-first approach is how deals are priced when wholesaling commercial real estate, where every buyer starts from income rather than comps.

NOI vs. Cash Flow vs. EBITDA

Metric	Starts from	Subtracts	Answers
NOI	Effective gross income	Operating expenses (incl. reserves)	What does the property earn?
Cash flow (before tax)	NOI	Debt service, actual capex beyond reserves	What does this owner keep?
EBITDA	Business revenue	Operating costs of a business	How profitable is an operating company?

In our fourplex, NOI of $32,044 minus $27,510 of debt service leaves about $4,534 of pre-tax cash flow. EBITDA is a corporate metric for operating businesses; if someone quotes it on a fourplex, ask for NOI. And the gross rent multiplier skips expenses entirely, which makes it a quick screen but never a substitute for this work.

When a Clean NOI Still Misleads You

• Deferred maintenance. An owner who stopped fixing things shows low repair costs and a strong NOI. The bill comes due for the next buyer. Reserves don’t cover a backlog.
• Below-market leases. Long-term tenants paying well under market make NOI look weak, but raising rents means turnover, vacancy and sometimes legal limits. Rent control and notice rules vary by state and city, so check local law.
• Very small buildings. On a duplex, a single vacancy is 50% of the income. One bad year can swing NOI more than any expense table predicts.
• Value-add deals. If the plan is to renovate and re-lease, today’s NOI is the floor, not the answer. Price off current NOI and treat the upside as the buyer’s reward for doing the work.

The most expensive beginner mistake is taking the seller’s NOI, applying a market cap rate, and sending the result to buyers as fact. If the expenses were understated, the deal falls apart at the buyer’s due diligence and your credibility goes with it.

Net Operating Income Questions Investors Ask

Can NOI be negative?

Yes. Mostly vacant buildings, properties going through heavy renovation, and some distressed or REO assets lose money before any debt service. Those deals get priced on land value, replacement cost or the after-stabilization NOI, minus the cost and risk of getting there.

What is a good expense ratio?

It depends on who pays the utilities, the age of the building, local taxes and insurance costs, so be careful with any national rule of thumb. Compare your number with recent T12s from similar local buildings and with what property managers in that market tell you. If a seller’s expense ratio is far below theirs, find out why.

Should a wholesaler use the seller’s T12 at all?

Yes, as the starting point. Keep the actual income lines you can confirm, replace taxes and insurance with your own estimates at the new price, add management and reserves, and note every change so buyers can see your reasoning.

Keep underwriting income property