How to Calculate NOI: A Practitioner’s Guide

Net operating income is the most load-bearing number in real estate. Value is derived from it. Cap rate is derived from it. Lenders size debt against it. Get NOI wrong by 10% and every downstream number is wrong by at least that much — and at a 6.5% cap rate, a 10% NOI error moves implied value by roughly the same 10%, which on a $600,000 building is $60,000 of price you either overpaid or left on the table.

So it is worth doing carefully.

The formula

NOI = Effective Gross Income − Operating Expenses

Where:

Effective Gross Income = Gross Potential Rent
                       + Other Income
                       − Vacancy & Credit Loss

Two things are excluded from NOI by definition, and this trips people up constantly:

• Debt service is not an operating expense. NOI describes the property. Your loan describes you.
• Depreciation is not an operating expense. It is a tax construct, not cash.

Also excluded: capital expenditures, income taxes, and one-time items. We will come back to capex, because it is the interesting one.

Line by line

Gross potential rent

What the property would collect at full occupancy at market rent.

Use market rent, not in-place rent, when in-place rents are meaningfully below market — but then be honest that capturing it requires turnover, which costs money and takes time. If you underwrite market rent from day one on a building with long-term tenants paying 20% under market, you have assumed away the entire value-add execution.

Conversely, if in-place rents are above market — which happens after a hot leasing period — underwrite the market rent, not what the current tenants happen to be paying. Those tenants will eventually leave.

Other income

Laundry, parking, storage, pet rent, application fees. Real, but generally small and generally overstated in offering memoranda. If it is more than about 5% of gross rent in a small residential property, ask where it is coming from.

Vacancy and credit loss

The line brokers most often set to zero.

Physical vacancy plus non-payment. In a stable residential market, 5–8% is a normal underwrite. Class C properties and short-term rentals run higher. If a seller shows you 0% vacancy over three years, the correct response is not “great, I’ll use zero” — it is to underwrite market vacancy anyway, because you are buying the next ten years, not the last three.

Operating expenses

Everything required to keep the property producing income:

Three of these deserve more attention.

Property taxes after transfer. In reassessment states, the seller’s tax bill is not your tax bill. California’s Proposition 13 means a long-held property can be assessed at a fraction of market value, and that assessment resets when you buy. Florida’s Save Our Homes cap works differently but produces the same trap. Using the seller’s number here is not a small error — it can be several thousand dollars a year, permanently.

Insurance. Premiums in Florida, Texas, Louisiana, and increasingly California have moved sharply enough that a two-year-old comparable is not a reliable guide. Get a quote on the actual property before you commit to a number.

Management at 8–10%, even when self-managing. This is the one investors argue with most. The logic: if the property only works because you provide unpaid labor, it does not work — it is a job you bought. Underwriting a management fee tells you whether the asset stands on its own. If you then self-manage, that fee becomes your compensation for the work, not a phantom saving.

Where capital reserves belong

Strictly, capital expenditures sit below NOI. A roof replacement is a capital event, not an operating expense, and institutional NOI excludes it.

But that convention exists because institutional owners hold reserve accounts and model capex separately in the cash flow. Retail investors typically do neither — they exclude capex from NOI, never model it anywhere, and then discover in year four that the roof was always going to cost $18,000.

The practical fix is to carry a reserve line explicitly:

• Residential, newer: $200–300 per unit per year
• Residential, 1970s–80s: $300–500 per unit per year
• Residential, pre-1960 or with known deferred maintenance: $500+ per unit per year, plus a specific capex schedule for the items you can already see

Where you put that line is a presentation choice. If you are computing a cap rate to compare against market cap rates, keep it below NOI so you are comparing like with like — market cap rates are quoted on NOI excluding reserves. If you are deciding whether you can actually afford the building, put it in and look at what is left.

The mistake is not choosing one convention over the other. The mistake is having the number appear in neither place.

Worked example

An eight-unit building, 1978 vintage, purchase price $1,250,000. Six units at $1,250, two at $1,150.

Note the expense ratio: 50% of effective gross income. For small residential with owner-paid water and third-party management, that is normal. If someone shows you a small multifamily deal running at a 30% expense ratio, something has been left out — and it is usually management, reserves, or a tax line that has not been reassessed yet.

Note also that a 4.5% cap rate on a 1978 building is thin in most rate environments. Whether that is acceptable depends on the spread over Treasuries and the market’s trajectory, but this is the moment where you find out — and you only find out if the NOI was built honestly.

NOI is not cash flow

A distinction worth being precise about, because the two get used interchangeably and they are not the same thing.

NOI                        $55,856
− Debt service            –$44,000     (assume 75% LTV, 6.75%, 30-yr am)
= Cash flow before tax     $11,856
− Capital reserve          –$3,200
= Cash available            $8,656

NOI is a property-level number that lets you compare this building against any other building. Cash flow is what actually reaches your account, and it depends entirely on how you financed it. Both matter. They answer different questions, and the same distinction runs through cap rate versus cash-on-cash return.

The reason this gets skipped

Building an honest NOI takes fifteen to twenty minutes per property. Screening thirty properties is a full working day, most of which is spent re-keying the same expense assumptions into a fresh spreadsheet.

That friction is why investors end up running the broker’s numbers. Not because they believe them, but because building their own version thirty times is genuinely tedious. Deal analysis tools exist to remove exactly that step — you set your assumptions once and every property gets underwritten the same way. We compared the options in best real estate deal analysis software.

Whatever you use, the assumptions above are the ones to check first. A tool with a default 0% vacancy and no reserve line will produce the broker’s number just as cheerfully as the broker did.

FAQ

Does NOI include the mortgage payment? No. NOI is calculated before debt service. This is what allows two buyers with different financing to agree on what a property is worth.

Does NOI include property taxes? Yes. Property taxes are an operating expense. Income taxes are not.

Should capital expenditures be in NOI? By convention, no — capex sits below NOI. But you must model it somewhere. Excluding it from NOI and then never modelling it at all is the actual error, and it is extremely common.

What is a normal expense ratio? For small residential, 35–50% of effective gross income is typical, with the higher end reflecting owner-paid utilities, third-party management, and older buildings. Anything materially below 30% deserves scrutiny.