Two identical fourplexes sit on the same street. One owner paid cash; the other carries a large mortgage at a high rate. Ask each owner what the building earns and you get two wildly different answers — yet the buildings are the same asset producing the same rent. Net operating income is the number that sees through that difference: the property's income after operating expenses, before financing, taxes, and depreciation enter the picture.
If you buy, sell, or borrow against rental property, NOI is the figure every other calculation starts from. It sets the price through the cap rate, sizes the loan through the coverage ratio, and lets you compare two buildings as though both were bought with cash. This post walks through the formula step by step, shows exactly what stays out and why, and covers how lenders use your NOI to decide how much they will lend you.
What NOI Measures
Net operating income answers one question: how much cash does the property itself generate in a year, regardless of who owns it and how they financed it? It is the building's operating profit — rent and other income minus the day-to-day cost of running it.
That "regardless of financing" clause is the whole point. Mortgage payments reflect the owner's choices — down payment size, rate, term — not the property's earning power. By excluding debt service, NOI lets a buyer compare a free-and-clear duplex against a leveraged eight-unit on equal footing, and lets a lender judge whether the building can support a new loan rather than the seller's old one.
NOI is an annual figure by convention. Monthly rents get annualized, expenses get tallied for twelve months, and the result is one number: the property's yearly operating earnings.
The Formula, Step by Step
The full computation runs through effective gross income before it reaches NOI:
Gross Potential Income (GPI)
− Vacancy and credit loss
+ Other income (parking, laundry, fees)
= Effective Gross Income (EGI)
− Operating expenses
= Net Operating Income (NOI)Each line deserves a closer look.
Gross potential income is the rent roll at full occupancy and full collection — every unit rented at its current contract rent for all twelve months. It is a ceiling, not a forecast; no building collects 100 percent of it.
Vacancy and credit loss brings that ceiling down to reality. Units sit empty between tenants, and some rent billed is never collected. Underwriters typically allow roughly 5 to 10 percent of gross potential income here, depending on the local market and the building's history. A stabilized apartment building in a tight market might justify 5 percent; a property with high turnover might warrant 8 to 10 percent. Setting this line too low is the single most common way a seller's numbers flatter a building, as discussed below.
Other income captures everything the property earns besides base rent: parking fees, storage rentals, laundry machines, vending, pet fees, late fees, application fees. On some buildings this line is trivial; on others — garage-heavy urban properties, for instance — it is material. Count only income tied to operating the property, not one-time windfalls like an insurance payout.
Operating expenses are the recurring costs of keeping the building running and rented: property taxes, hazard insurance, property management fees, routine maintenance and repairs, utilities the owner pays, janitorial and landscaping, HOA dues on a condo rental, and licenses or permits. Subtract them from effective gross income and you have NOI.
What Stays Out — and Why
The exclusions define NOI as much as the inclusions. Four categories never appear, each for a deliberate reason.
Mortgage principal and interest. Debt service is a financing choice, not an operating cost. Two buyers paying different prices with different down payments would compute different "incomes" if loan payments counted — and the number would be useless for comparing buildings or sizing a new loan. NOI deliberately measures the property unlevered, as if bought with cash.
Capital expenditures. A new roof, a repaved lot, a full HVAC replacement — these are investments in the asset's future, not costs of this year's operations. They are lumpy (zero most years, five figures in one), so including them would make NOI swing wildly and hide the building's steady earning power. CapEx still matters enormously to your cash flow — you pay for that roof out of pocket — but it lives below the NOI line, in cash-flow-after-CapEx measures and reserve budgets. Folding CapEx into operating expenses is a real underwriting error: it depresses NOI, which tanks both the cap rate valuation and the coverage ratio, sometimes enough to make a fundable deal look unqualified.
Depreciation and amortization. These are non-cash accounting charges that allocate an asset's cost across years. They reduce taxable income but no money leaves your account when you book them, so they have no place in a measure of operating cash generation. (Property taxes — the actual bill from the county — absolutely count as an operating expense. It is income taxes and depreciation that stay out.)
Income taxes. Your tax bill depends on your bracket, your entity, your depreciation elections, and a dozen other owner-specific facts. Like financing, it says nothing about the building. NOI is a before-income-tax figure.
Also excluded, though rarely stated: owner draws, personal expenses run through the building, and any expense the next owner will not inherit. When you underwrite someone else's building, normalize first — strip the owner's car insurance out of the P&L before you trust the NOI.
A Worked Example
Consider a four-unit building where each unit rents for $1,500 a month:
Gross potential income: 4 × \$1,500 × 12 = \$72,000
Vacancy/credit loss (7%): − \$5,040
Other income (parking + laundry): + \$3,600
Effective gross income: \$70,560
Operating expenses:
Property taxes \$8,400
Insurance \$3,200
Management (8% of EGI) \$5,645
Maintenance and repairs \$6,000
Owner-paid utilities \$4,800
Landscaping and misc \$1,200
Total operating expenses: − \$29,245
Net operating income: \$41,315Note everything absent from this page: the $2,100 monthly mortgage payment, the $14,000 roof scheduled for next year, the depreciation schedule, and the owner's tax return. Change any owner's financing and the $41,315 does not move. That stability is what makes it the input to everything that follows.
From NOI to Value: The Cap Rate
Investors convert NOI into a price with the capitalization rate:
Cap Rate = NOI / Property Value
Value = NOI / Cap RateA building with $41,315 of NOI in a market where similar fourplexes trade at a 6 percent cap rate is worth roughly $688,600. The cap rate itself comes from comparable sales — what yield are buyers actually accepting for this property type in this area — which is why the same NOI produces different values in different markets. An 8 percent cap market prices that building at $516,400; a 5 percent coastal market prices it near $826,300.
The cap rate is best read as an all-cash yield: buy the building free and clear at that price, and NOI is your first-year return before reserves. It lets you rank opportunities fast — a 7.5 percent cap building against a 5.5 percent one — but it is a snapshot, not a forecast. It says nothing about rent growth, coming CapEx, or financing, which is why serious underwriting layers a multi-year cash-flow projection on top.
Small NOI differences swing value more than most buyers expect. At a 5 percent cap rate, every $1,000 of annual NOI is worth $20,000 of price. That arithmetic is worth keeping in mind when a seller's pro forma assumes rents 8 percent above the current roll — the optimism flows straight through to the asking price.
How Lenders Use NOI to Size Your Loan
Where buyers divide NOI by a cap rate, lenders divide it by debt service. The debt-service coverage ratio is the gatekeeper of investment-property lending:
DSCR = NOI / Annual Debt ServiceA DSCR of 1.25 means the property's NOI covers its yearly mortgage payments 1.25 times — 25 percent of cushion. Most commercial and investment-property lenders require at least 1.20 to 1.25, and they underwrite it on their NOI, not yours: trailing-twelve-months income, market vacancy (not your optimistic 3 percent), and often a management-fee floor even if you self-manage.
Lenders frequently work the formula backwards. Rather than asking how much you want and checking coverage, they start from the coverage they require and solve for the loan: maximum annual debt service equals NOI divided by 1.25, and the loan amount follows from that payment at the offered rate and amortization. A $41,315 NOI supports at most about $33,050 of annual debt service at 1.25x — at 7 percent over 30 years, that caps the loan near $414,000 no matter what the appraisal says. When the appraisal-based ceiling (value times max loan-to-value) and the income-based ceiling disagree, the lender funds the smaller of the two.
A related metric, debt yield (NOI divided by loan amount), gives lenders a second lens that ignores the interest rate entirely. On this building, a $414,000 loan against $41,315 of NOI is a 10 percent debt yield — the lender's all-cash return if it had to take the building back. Minimum debt yields, often 8 to 10 percent, bind hardest when rates are low and payments look deceptively affordable.
Mistakes That Distort NOI
Most NOI errors inflate it, and most inflated NOIs come from sellers' marketing packages rather than buyers' spreadsheets. Watch for these four.
Pro forma presented as fact. A seller's package blends real numbers with projections: current rents bumped to "market," vacancy set at 5 percent against a history of 12 percent, concessions quietly omitted. Always rebuild NOI from the trailing twelve months of actuals — bank statements, tax returns, the current rent roll — and treat the seller's forward-looking version as a hypothesis to verify, not a baseline to trust.
Vacancy below reality. A 2 or 3 percent vacancy allowance assumes almost no turnover and no collection loss. Compare the assumption against the building's own history and the submarket average; if in-place rents exceed market rates, expect the loss to arrive when leases roll and tenants renew lower or leave.
Loss to lease ignored. When current contract rents sit below market, the gap — loss to lease — means today's NOI understates the building's stabilized potential. The reverse flatters it. Model both in-place and stabilized income, and know that lenders fund on the conservative one.
CapEx and one-timers mixed into operations. Repairs that are really improvements inflate expenses and depress NOI; deferred maintenance omitted entirely does the opposite. Either way the number stops describing steady-state operations. Keep a bright line between the maintenance line in NOI and the capital budget below it.
Tracking NOI in Your Books
NOI falls out of clean books almost for free — provided operating costs and capital costs live in different accounts from the start. The habit that matters is booking every property expense to an operating account by default and moving improvements to the balance sheet as assets, so your expense total is the NOI expense line with no year-end archaeology.
A Beancount structure for the fourplex above might look like this:
2026-01-01 open Income:Property:Fourplex:Rent
2026-01-01 open Income:Property:Fourplex:Parking-Laundry
2026-01-01 open Expenses:Property:Fourplex:Taxes
2026-01-01 open Expenses:Property:Fourplex:Insurance
2026-01-01 open Expenses:Property:Fourplex:Management
2026-01-01 open Expenses:Property:Fourplex:Maintenance
2026-01-01 open Expenses:Property:Fourplex:Utilities
2026-01-01 open Assets:Property:Fourplex "Fourplex building"
2026-01-01 open Liabilities:Mortgage:Fourplex
2026-03-05 * "March rents, units A-C (unit D vacant)"
Assets:Checking 4500.00 USD
Income:Property:Fourplex:Rent -4500.00 USD
2026-03-05 * "Property management, March"
Expenses:Property:Fourplex:Management 396.00 USD
Assets:Checking -396.00 USD
2026-03-12 * "Water heater replacement, unit B"
Assets:Property:Fourplex 1850.00 USD
Assets:Checking -1850.00 USDThe water heater booking is the one to study: a $1,850 improvement capitalized to the building asset, not expensed to maintenance. If you have not kept books this way before, the docs walk through opening accounts and recording transactions in plain-text Beancount. Next March, when you sum Expenses:Property:Fourplex:* for the NOI worksheet, the roof-class spending is already out of the total — and the mortgage payment, booked against Liabilities:Mortgage:Fourplex, never touched expenses at all. Your chart of accounts is doing the NOI exclusions for you, every month, automatically.
Your Next Step
Pull the trailing twelve months on a building you own or are eyeing, rebuild the waterfall from gross potential income to NOI using market vacancy instead of anyone's assumption, and divide by the going cap rate for the submarket. If the value you get and the price on the table disagree by more than your renovation budget can explain, the gap is telling you something — listen to it before you wire the earnest money.
