How to Calculate NOI on Commercial Real Estate (And Why Most Investors Get It Wrong)

NOI Is the Foundation of Every Commercial Valuation

Net Operating Income is the single input that drives cap rate valuation, debt service coverage ratio analysis, and lender underwriting. Get it wrong and every downstream number, including the price you offer or accept, is wrong.

The formula is straightforward: NOI = Effective Gross Income (EGI) - Total Operating Expenses.

The errors happen inside each component, not in the formula itself.

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Step 1: Calculate Potential Gross Income

Potential Gross Income (PGI) is the maximum rent the property would collect if every unit were occupied and every tenant paid on time for a full 12 months.

For a 12-unit apartment building with market rents of $2,200 per month per unit, PGI equals:

12 units x $2,200/month x 12 months = $316,800 per year.

PGI does not appear in the final NOI formula. It is a starting point, not the income figure you use.

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Step 2: Arrive at Effective Gross Income

Effective Gross Income (EGI) adjusts PGI downward for vacancy and credit loss, then adds any ancillary income the property generates.

EGI = PGI - Vacancy and Credit Loss + Other Income

Using the same 12-unit building, assume a 6% vacancy and credit loss rate, plus $4,800 per year in laundry and parking income:

• PGI: $316,800
• Less vacancy and credit loss (6%): $316,800 x 0.06 = $19,008
• Plus other income: $4,800
• EGI: $302,592

Investors who skip the vacancy adjustment and feed PGI directly into the NOI formula overstate income by $19,008 annually. At a 6% cap rate, that error overstates the implied property value by $316,800.

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Step 3: Identify and Subtract Operating Expenses

Operating expenses are costs required to keep the property functional and tenanted. They do not include debt service, depreciation, income taxes, or capital expenditures.

A clean operating expense line for the same 12-unit building might look like this:

NOI = $302,592 - $87,807 = $214,785

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What Must Never Appear in Operating Expenses

Three items corrupt NOI when included, and sellers include them more often than buyers realize.

Mortgage payments. The mortgage is a financing decision, not a property characteristic. NOI must be pre-debt so that investors with different capital structures can compare properties on equal footing.

Depreciation. Depreciation is a non-cash accounting entry governed by IRS Schedule E rules. It has no place in an operational cash flow calculation.

Income taxes. Tax liability varies by ownership structure, whether a C-corporation, LLC, or individual Schedule E filer. NOI strips that out entirely.

Capital expenditures. Replacing a roof or an HVAC system is a capital event, not an operating expense. Buyers account for CapEx separately, often through a reserves-for-replacement line that estimates future costs, not through including actual replacement costs in the current NOI.

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Worked Example 1: Retail Strip Center

A four-tenant retail strip center in a secondary market generates $480,000 in base rent annually. One 1,200 sq ft suite sits vacant. The vacant suite's pro-rata annual rent is $19,200. The landlord collects $12,000 per year in NNN reimbursements from the occupied tenants.

• PGI: $480,000
• Less vacancy (the vacant suite): $19,200
• Plus NNN reimbursements: $12,000
• EGI: $472,800

Operating expenses for the landlord-controlled items total $98,400 annually (the NNN tenants cover their pro-rata share of taxes, insurance, and CAM directly).

NOI = $472,800 - $98,400 = $374,400

At a 7.25% cap rate, the implied value is $374,400 / 0.0725 = $5,164,138.

If the seller had omitted the vacancy deduction, the inflated EGI of $492,000 would produce a false NOI of $393,600, implying a value of $5,429,655. The difference is $265,517. That is not a rounding error. That is a pricing dispute.

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Worked Example 2: Medical Office Building

A 10,000 sq ft single-tenant medical office building leases at $28 per sq ft, triple-net. The tenant pays all operating expenses directly. The landlord collects base rent only.

• Annual base rent: 10,000 x $28 = $280,000
• Vacancy: $0 (tenant is in place with 7 years remaining on the lease)
• Landlord operating expenses: $14,200 (management fee and reserves only)

NOI = $280,000 - $14,200 = $265,800

At a 5.75% cap rate reflecting the long-term lease and credit tenant, the implied value is $265,800 / 0.0575 = $4,623,478.

A buyer who mistakenly added the tenant's NNN obligations ($62,000 per year) to the income side while also subtracting them as expenses would net to the same NOI. But a buyer who only added the NNN income without subtracting operating expenses would report a false NOI of $327,800, implying a $5.7M valuation. Lenders will not accept that underwriting.

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How Lenders Use NOI: The DSCR Test

Lenders do not approve commercial loans on gut feel. They divide NOI by annual debt service to produce the Debt Service Coverage Ratio (DSCR). Most commercial lenders require a minimum DSCR of 1.25x.

For the retail strip center above, NOI is $374,400. If the proposed loan carries $280,000 in annual principal and interest payments:

DSCR = $374,400 / $280,000 = 1.337x

That clears the 1.25x threshold. If the buyer had used the inflated false NOI of $393,600, they might have structured a larger loan that the real NOI cannot support. The lender's appraisal would catch it. A savvy seller will not wait for that moment.

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Run Your Own Numbers Before You Make an Offer

Every deal has a different rent roll, vacancy assumption, and expense structure. The only way to stress-test a seller's pro forma is to rebuild NOI from scratch using your own inputs.

The CalcMoney mortgage calculator lets you model the debt side of the equation once you have a confirmed NOI figure. Enter the loan amount, rate, and term to see whether the property's cash flow survives at your target leverage level. Use the NOI methodology above to build the income side, then use the calculator to confirm the financing works.

Institutional buyers run this analysis before every letter of intent. There is no reason retail investors should operate with less precision.

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Results are estimates for informational purposes only. Consult a licensed financial professional before making financial decisions.