NOI (Net Operating Income): How to Calculate It

Net operating income is the number every other real estate metric is built on. Before you can talk about cap rate, DSCR, or what a building is worth, you have to get NOI right — and it is also the figure beginners most often calculate wrong, usually by subtracting the mortgage. This is what NOI is, how to compute it, and the one rule that trips people up.

What NOI actually measures

NOI tells you how much income a property produces from operations, after the costs of running it but before the cost of financing it. It answers a single question: how much money does this building throw off on its own, regardless of who owns it or how they paid for it?

The formula is short:

NOI = Effective Gross Income − Operating Expenses

Two pieces, and each has a precise definition. Get the inputs right and NOI falls out cleanly.

Effective gross income

You do not start with the rent on the lease. You start with what you actually collect.

• Gross potential rent — every unit rented at full asking rent, all year, with no gaps.
• Plus other income — laundry, parking, pet fees, storage, late fees. Real money the property earns beyond base rent.
• Minus vacancy and credit loss — the rent you will not collect because units sit empty between tenants or someone stops paying.

That last line matters. No building runs at 100% occupancy forever, so underwriting full rent is fantasy. A vacancy allowance — often 5% to 10% of gross rent depending on the market — is what turns a wish into effective gross income, the rent you can actually bank.

Operating expenses

Operating expenses are the recurring costs of keeping the property running and rentable. The usual line items:

• Property taxes
• Insurance
• Property management (whether you pay a manager or do it yourself, count it)
• Repairs and maintenance
• Utilities that the owner pays rather than the tenant
• A vacancy allowance, if you did not already net it out of income above

These are the costs that recur whether the building is financed or owned free and clear. They are the true cost of operations.

The one rule that trips everyone up

Here is the part that sinks beginner underwriting. Operating expenses exclude:

• Mortgage and debt service — principal and interest are financing costs, not operating costs
• Capital expenditures (capex) — a new roof, an HVAC unit, a repaved lot are improvements, not operating expenses
• Depreciation — an accounting entry, not cash leaving the building
• Income taxes — those are yours, not the property's

The mortgage exclusion is the big one. NOI is deliberately unlevered — it ignores financing entirely. That is the whole point: it lets you compare two buildings, or compare your offer against another buyer's, without one person's loan terms muddying the picture. The day you subtract the mortgage from NOI, you have stopped calculating NOI and started calculating something closer to cash flow.

A worked example

Take a small rental building:

• Gross annual rent: $48,000
• Vacancy and credit loss: $3,000
• Effective gross income: $48,000 − $3,000 = $45,000
• Operating expenses (taxes, insurance, management, maintenance, vacancy): $15,000

Run it:

NOI = $45,000 − $15,000 = $30,000

Thirty thousand dollars of net operating income. Notice what never entered the math: no loan payment, no roof reserve, no depreciation. If this owner has a $300,000 mortgage and the next buyer pays all cash, both still compute the same $30,000 NOI. That consistency is exactly why lenders and appraisers lean on it.

Why NOI drives everything else

Once you have NOI, two of the most important numbers in real estate fall right out of it.

Cap rate is NOI divided by value. At a 6% cap rate, our $30,000 NOI implies a value of $30,000 ÷ 0.06 = $500,000. Flip it around and value is what most people care about.

DSCR is NOI divided by annual debt service. If the loan on this building costs $24,000 a year, DSCR is $30,000 ÷ $24,000 = 1.25 — right where most lenders want it. NOI is the income figure feeding that ratio, which is why a bad NOI estimate quietly breaks your financing math too.

This is also why NOI deserves real attention rather than a rough guess. Because value equals NOI divided by cap rate, NOI changes get multiplied into value. At a 6% cap rate, every extra $1 of durable NOI adds about $16.67 of value. Cut $2,000 a year off your insurance bill and you have not saved $2,000 — you have added roughly $33,000 to what the building is worth. Small operating wins compound into large valuation swings.

That leverage cuts both ways. Underwrite vacancy too low or forget an expense line, and you have not just overstated income by a few thousand dollars — you have overpaid by sixteen times that. NOI is where careful operators separate themselves from optimistic ones.

Increasing NOI

Since NOI is income minus operating expenses, there are only two levers:

• Raise effective gross income — push rents to market, cut vacancy, add fee or other income (parking, storage, pet rent).
• Lower operating expenses — re-shop insurance, appeal an inflated tax assessment, tighten management and maintenance contracts.

Either way, the gain flows straight through to value at your cap rate. That is the operator's edge: you cannot control where cap rates go, but you can control NOI.

Watching NOI across a portfolio

For one property, NOI is a quick subtraction. Across a dozen assets — each with its own rent roll, expense creep, and seasonal vacancy — it becomes a moving number, and a quiet slide in NOI on one building is an early warning you want to catch before it shows up in a lower valuation or a tighter DSCR. Rolling NOI up into portfolio-level metrics — read alongside a condition measure like FCI — is what keeps a single building's slide from hiding inside the blend.

That is what Portfoliq's asset breakdown is built to surface: income and operating expenses broken out per property, with NOI tracked over time so a drifting expense line is visible the month it starts, not the year you refinance. You see the foundation number clearly, continuously, instead of rebuilding it in a spreadsheet every quarter.

The takeaway

NOI is effective gross income minus operating expenses — and nothing else. Exclude the mortgage, exclude capex, exclude depreciation and income taxes; those are financing, capital, and accounting items, not operations. Get NOI right and the rest of your underwriting follows, because it is the number cap rate, DSCR, and value all stand on. Get it wrong and every metric downstream inherits the error.