Net operating income (NOI) is a property's rental income minus its operating expenses, measured before mortgage payments, capital expenditures, and depreciation. It answers one question: how much does the building itself earn, stripped of how you financed it and how the tax code lets you write it down. Because it ignores the loan, NOI lets you compare a paid-off fourplex against a heavily mortgaged one on equal footing, and it is the number that sits on top of a cap rate.
In practice
Say you own a duplex that rents for $1,500 a unit, so $3,000 a month, or $36,000 a year if both units stay full. Knock off a 5% vacancy and credit-loss factor and your collectible income is about $34,200. That figure, gross rent reduced to what you actually expect to collect, is your effective gross income.
Now subtract the operating expenses: property taxes of $4,200, insurance of $1,400, water and trash of $1,800, repairs and maintenance of $2,600, and a management allowance of $2,700. That is $12,700 in operating expenses. Effective gross income of $34,200 minus $12,700 leaves an NOI of $21,500. Notice what did not appear in that math: no mortgage payment, no new roof, no depreciation. Those are real costs, but they belong to the financing and tax conversations, not to the operating one.
Why it matters to a small landlord
NOI is the hinge between two numbers you care about. Divide it by the purchase price and you get the cap rate, which is how you sanity-check what a property is worth before you buy. On that $21,500, a $310,000 price is a 6.9% cap rate; a $430,000 price is a 5.0% cap rate on the identical building. You can run that quickly with the cap rate calculator, but the answer is only as honest as your expense list. The single most common way landlords flatter NOI is by leaving out vacancy, management, and a repair reserve, which makes a tired building look like a bargain.
The other reason to keep NOI clean is that it forces you to categorize every dollar correctly all year, not at tax time. A torn-out kitchen is a capital expenditure that should not touch NOI; a fixed garbage disposal is a repair that should. Sorting those as you go is the heart of a tight monthly close, and it is the same discipline that keeps your rental books ready for your CPA. I built rents.ai to do that sorting for me, and it correctly excludes principal, capex, and depreciation from NOI, though it will not pull your numbers from a bank feed, so you still import the transactions yourself.
NOI rarely travels alone. It is built from your effective gross income, it is reduced by your operating expenses, and it feeds directly into your cap rate. Learn those three terms alongside it and most rental underwriting stops looking like a foreign language.