July 30, 2026
Ask a buyer, a lender, and a seller to independently calculate the NOI on the same property and you'll frequently get three different numbers. Not because the math is hard, but because everyone has an incentive to define "operating expense" a little differently. Sellers want a high NOI because it supports a higher asking price. Lenders want a conservative NOI because it determines how much they'll lend. Buyers should want the real number, whatever it turns out to be.
Net Operating Income is the single most important figure in commercial real estate. It's the numerator in every cap rate calculation, the basis for most valuation approaches, and the number lenders use to size a loan. Get it wrong and every decision downstream of it — your offer price, your financing, your return projections — is built on a bad foundation.
The Definition, Precisely
NOI is calculated as: Gross Operating Income minus Operating Expenses. That's the entire formula. The nuance is entirely in what belongs in each bucket.
Gross Operating Income starts with potential rental income at full occupancy, subtracts a vacancy and credit loss allowance, and adds back other income the property generates — parking fees, laundry income, storage fees, application fees, and similar line items. What's left is the income the property actually collects, or should collect under normal, stabilized operation.
Operating Expenses are the costs of running the property: property taxes, insurance, utilities (to the extent not billed back to tenants), repairs and maintenance, property management fees, landscaping, pest control, and general administrative costs. These are the recurring costs required to keep the building operating and the income flowing.
What Does NOT Belong in the Calculation
This is where most confusion, and most manipulation, happens. NOI is calculated before two categories of cost:
- Debt service. Mortgage principal and interest payments are a financing decision, not an operating cost. NOI is meant to represent the income-producing capacity of the asset itself, independent of how any particular buyer chooses to finance it. Two buyers with different loan structures on the identical property should calculate the identical NOI.
- Capital expenditures. A new roof, a parking lot repave, HVAC replacement, or major system upgrades are capital improvements, not operating expenses. They extend the useful life of the asset rather than simply keeping it running day to day. They matter enormously to your investment return, but they belong in your capital budget and your overall return calculation, not in the NOI line.
- Depreciation and income taxes. These are accounting and tax concepts tied to ownership structure, not to the property's operating performance. They don't belong in NOI either.
The reason this distinction matters so much: NOI is supposed to be a clean, comparable, ownership-agnostic measure of what the real estate itself produces. The moment you fold in financing costs or capital spending, you've built a number that only applies to one specific buyer under one specific set of assumptions, and it's no longer useful for comparing properties or calculating a cap rate that means anything.
Common Mistakes When Calculating NOI
- Accidentally including debt service. This sounds basic, but it happens constantly with less experienced sellers or with financial statements pulled straight from a bookkeeping system that wasn't set up with commercial real estate conventions in mind. Mortgage interest sometimes gets buried in an "expenses" tab without a clear label. Always ask directly: does this expense schedule include any debt service?
- Treating capital expenditures as operating expenses, or vice versa. A seller trying to inflate NOI will sometimes reclassify a capital expense as a one-time, non-recurring item and strip it out entirely, even if it's the kind of expense that recurs every few years (a roof, a resurfaced lot, unit turns). Ask for the trailing three to five years of actual expenses, not just the most recent twelve months, so you can see the pattern.
- Underwriting to pro forma instead of trailing, actual numbers. A pro forma NOI reflects what the property could earn under an assumed set of improvements: rents pushed to market, expenses trimmed, occupancy stabilized. That's a legitimate way to think about upside, but it is not the property's current, in-place NOI, and it should never be the basis for your initial offer. Underwrite to trailing twelve months of actual, verified income and expenses, and treat any pro forma upside as exactly that — upside you have to go execute on, not income you're buying today.
- Using a management fee that doesn't reflect reality. If the seller is self-managing, the expense statement often shows no management fee line item at all. But a buyer who isn't going to self-manage needs to underwrite a market-rate management fee (typically a percentage of collected rent) into the expense side, even though the seller never paid it. Skipping this step overstates the NOI you'll actually realize as a new owner.
- Ignoring below-market or above-market rents without adjusting. If in-place rents are meaningfully below market, current NOI understates the property's real income potential — that's a legitimate upside case, but it's still upside, not current income. If rents are above market and unsustainable (perhaps propped up by a tenant concession structure that's about to expire), current NOI overstates what you'll actually collect going forward. Either way, don't take the trailing number at face value without understanding why it is what it is.
How to Sanity-Check a Seller's NOI Claim
When I'm representing a buyer, or advising a seller on how to present a property credibly, here's the process I run every time:
- Request the trailing 12-month profit and loss statement, ideally with three prior years attached, not just a one-page summary. A single-year snapshot hides patterns.
- Cross-reference against the rent roll. Add up actual, in-place rent across all units or tenants and compare it to the gross income line on the P&L. Discrepancies need an explanation.
- Verify property tax and insurance figures independently. County tax records and a quick insurance quote will tell you whether the seller's numbers reflect current reality, especially if the property is changing hands and tax basis or insurance costs are about to reset.
- Ask what's excluded, and why. If a seller has stripped out a category of expense as "non-recurring" or "one-time," ask for the underlying justification and check whether that category shows up in prior years.
- Rebuild the management fee assumption to reflect what you'll actually pay, not what the seller happened to pay (or not pay) themselves.
- Get a property condition assessment before you finalize any offer built on the trailing NOI, so you understand what capital expenditures are coming that the NOI calculation intentionally excludes.
None of this is exotic. It's just discipline, applied consistently, before you let a single number drive a seven-figure decision.
How I Help
On acquisitions, I walk the trailing financials line by line with clients before we ever talk about price, so the offer is built on real, defensible NOI rather than a seller's marketing number. On dispositions, I help owners present financials in a way buyers and their lenders can verify quickly, which shortens the path to a clean close.
If you're evaluating a property and want a second set of eyes on the seller's numbers, or you're preparing to list and want your financials in shape before buyers start asking questions, you can schedule a call here, call (919) 797-5900, or reach me at sebastian@mullarkeycre.com.
This article is for informational purposes only and does not constitute investment, tax, or legal advice. Consult with a qualified financial advisor, CPA, and real estate attorney before making an investment decision.