Net Operating Income Explained: The Number Every Other Metric Is Built On

The Formula

Effective Gross Income = Gross Scheduled Rent − Vacancy and Credit Loss
NOI = Effective Gross Income − Operating Expenses

Gross scheduled rent is what the unit would collect if it were rented every day of the year at full price. Vacancy and credit loss is what that number loses to empty days and unpaid rent. What's left is effective gross income, the rent the property actually collects. Subtract the cost of running the building and what remains is NOI.

The Duplex, Worked

I use the same $425,000 duplex across this site so the numbers agree wherever a reader lands. It rents for $3,800 a month.

$3,800 × 12 = $45,600 gross scheduled rent
$45,600 × 5% vacancy = $2,280
$45,600 − $2,280 = $43,320 effective gross income
$43,320 − $17,100 operating expenses = $26,220 NOI

$26,220 is the number that feeds every other calculation on this duplex. Divide it by the $425,000 price and the cap rate is 6.2%. Divide it by the annual debt service on a typical loan and you get DSCR. Nothing about the mortgage, the down payment, or who owns the building enters the calculation.

What Counts as an Operating Expense, and What Doesn't

Operating expenses are the recurring costs of running the property: property taxes, insurance, management fees, repairs and maintenance, owner-paid utilities, and turnover costs between tenants. Anything that keeps the building operating and would exist under any owner belongs here.

Four costs stay out of NOI by convention, and all four are things that depend on who owns the building rather than on the building itself:

• Mortgage principal and interest. Debt service sits below NOI, not inside it, because two buyers financing the same building differently would otherwise get two different NOI figures for the identical property.
• Depreciation. It's a tax deduction, not a cash cost, so it never touches an operating income calculation.
• Capital expenditures. A new roof or a replaced HVAC system is a capital cost, not an operating one. Patching the existing roof is a repair and does count.
• Income taxes. These depend on the owner's tax situation, not the property's operations.

That exclusion list is the whole reason NOI is comparable between buyers. Strip out financing, depreciation, and taxes, and what's left describes the building, not the owner.

Why NOI Is the Number Everything Else Is Built On

Once NOI exists, three other calculations fall out of it directly.

Cap rate is NOI divided by price: $26,220 ÷ $425,000 = 6.2%. It measures the building's unlevered yield, the return before anyone's loan enters the picture. Try the cap rate calculator against your own NOI to see it.

DSCR is NOI divided by annual debt service. On the duplex, a $340,000 loan at 7% over 30 years runs about $2,262 a month, or $27,144 a year. $26,220 ÷ $27,144 is a DSCR of 0.97, meaning the property's operating income falls just short of covering the loan payment on its own.

Value by income capitalization runs the cap rate formula backward: Value = NOI ÷ Cap Rate. If a lender or appraiser applies a 6% cap rate to this duplex's $26,220 NOI, the implied value is $437,000. Move the assumed cap rate to 6.5% and the same NOI implies a value of $403,385. The NOI didn't change; the assumed cap rate did, and the value estimate moved by more than $30,000 because of it.

One NOI number, three different uses, and an error anywhere in that $26,220 propagates into the cap rate, the DSCR, and the appraised value all at once. Run the full breakdown in the NOI calculator.

The Vacancy Assumption Does Quiet Work

Vacancy is a percentage typed into one line of the formula, which makes it easy to treat as an afterthought. It isn't. Move the same duplex from 5% vacancy to 10% and effective gross income drops from $43,320 to $41,040.

$45,600 × 10% = $4,560 vacancy
$45,600 − $4,560 = $41,040 effective gross income
$41,040 − $17,100 = $23,940 NOI

That's a $2,280 drop in NOI from a five-point change in one assumption, and it pulls the cap rate on an unchanged $425,000 price from 6.2% down to 5.6%. A seller-supplied vacancy figure that understates how often the unit actually sits empty inflates NOI, and every metric built on top of it inherits the inflation.

The Trap: NOI Excludes Capital Expenditures, and It Isn't Cash Flow

NOI's exclusion list is also its blind spot. Roofs, furnaces, parking lots, and other capital items are real cash a landlord eventually spends, and NOI never sees any of it. A property can post a clean NOI for years and still absorb a $15,000 roof replacement that no NOI calculation ever flagged as coming.

The other trap is confusing NOI with cash flow. NOI is what the property earns before debt service. Cash flow is what's left after the mortgage payment, and the two numbers can point in opposite directions on the same building. This duplex's NOI is a positive $26,220. Subtract the $27,144 of annual debt service on the loan above and cash flow is negative $924. The building operates profitably by NOI's definition and still costs its owner money every month, because NOI was never designed to answer the cash flow question in the first place.

The Owner-Managed Blind Spot

A common way NOI understates the true cost of running a property: an owner who self-manages often reports an operating expense list with no management fee in it, because nobody wrote themselves a check. That NOI looks stronger than a professionally managed building's NOI would, for the identical property, purely because one owner's labor isn't priced into their own numbers. Adding back whatever a third-party manager would charge in that market is the adjustment that makes the two buildings comparable, and it's worth doing before comparing a self-managed listing's NOI to anyone else's.

FAQ

What is the formula for net operating income?

NOI = Effective Gross Income − Operating Expenses, where effective gross income is gross scheduled rent minus vacancy and credit loss. On the $425,000 duplex: $45,600 of scheduled rent, less $2,280 of vacancy, less $17,100 of operating expenses, equals $26,220 of NOI.

Is the mortgage payment part of NOI?

No. Principal and interest are debt service, which is subtracted after NOI, not as part of it. Leaving the loan out of NOI is what lets two buyers with different financing compare the same building on the same terms.

What's the difference between NOI and cash flow?

NOI comes before debt service; cash flow comes after. On the duplex, NOI is $26,220 and cash flow after a $340,000 loan at 7% is negative $924, because annual debt service runs $27,144. Same building, two different answers, because they're answering two different questions.

Do capital expenditures count as operating expenses?

Not under the standard definition. A roof replacement is a capital expenditure and sits outside NOI; patching the same roof is a repair and counts as an operating expense. That line moves real money, which is why a seller-prepared NOI is worth rebuilding from the actual expense detail rather than taking on faith.

How much does the vacancy assumption change NOI?

On this duplex, moving from a 5% to a 10% vacancy assumption drops NOI by $2,280, from $26,220 to $23,940, which pulls the cap rate from 6.2% to 5.6% on an unchanged price. A vacancy rate that doesn't match the submarket's actual experience distorts NOI and everything calculated from it.

Related Reading

• Cap Rate vs Cash-on-Cash Return: the two metrics NOI feeds directly, and why they can disagree on the same building
• Cap Rate Explained: Complete Guide: how NOI turns into a cap rate and what published benchmark surveys say about current ranges
• DSCR Loans Explained: how lenders divide this same NOI by debt service to decide what they'll lend