Net operating income is collected property income minus operating expenses, before debt service and income taxes.
Which numbers should support the NOI estimate?
Rent rolls, collection records, tax bills, insurance quotes, and expense invoices should support the estimate. Keep replacement reserves explicit so a favorable NOI does not hide future cash needs.
Why Multifamily Math Looks Simple Until You Try to Close
Every multifamily deal is underwritten on two numbers: NOI and cap rate. The formula is simple — divide NOI by cap rate to get value, or divide NOI by value to get cap rate. But the deals that go sideways almost always trace back to errors in how NOI was calculated or how the cap rate was benchmarked.
This guide covers the most common places where multifamily underwriting breaks, and how to build an NOI model that holds up under due diligence.
NOI: The Most Abused Number in Multifamily
Net Operating Income should be simple:
NOI = Effective Gross Income - Operating Expenses
In practice, sellers, brokers, and optimistic buyers all make the same category of errors. Here is where NOI gets inflated:
1. Gross Rent Instead of Effective Gross Income
Gross Potential Rent (GPR) is what you would collect if every unit were rented at market rate with no vacancy. No property operates at GPR. Effective Gross Income (EGI) adjusts for vacancy and credit loss:
EGI = GPR × (1 - Vacancy Rate) - Credit Loss
A common manipulation: sellers present trailing 12-month rent roll figures during a period of unusually low vacancy, or use "stabilized" vacancy assumptions of 3–4% in a market where 8–10% is realistic.
Always use a vacancy rate you can defend with local market data. For most B and C markets, 7–10% is realistic. For A markets in supply-constrained cities, 4–6% may be defensible.
2. Missing or Underestimated Expense Line Items
The second most common error: the broker's OM (Offering Memorandum) excludes expense categories that are real costs. Common omissions:
Property management fee. Even if you self-manage, underwrite the 8–12% management fee as a real expense. The moment you hire a manager (or want to sell to someone who will), it is a cost. Excluding it inflates NOI and sets a buyer up for negative surprises.
Capital expenditure (CapEx) reserve. Roof, HVAC, plumbing, electrical — these are not operating expenses (they hit the balance sheet), but they are cash expenditures that impact your actual return. Underwriting without a CapEx reserve treats the building as if it never ages.
Standard CapEx reserve rule: $150–$400/unit/year depending on property age and condition. Use the higher end for 25+ year buildings.
Vacancy during turns. Unit turns (paint, carpet, appliance repair) take 2–4 weeks and cost $1,000–$5,000+ per unit. A 20-unit building with 30% annual turnover is losing meaningful income and cash that is invisible in the "5% vacancy" line item.
Insurance cost increases. Insurance premiums on multifamily have risen 20–40% in many markets since 2021. A deal underwritten on 2022 insurance costs is using a number that may be 30% stale.
3. Below-Market Expenses from an Owner-Operator
Mom-and-pop operators often underreport expenses by doing their own maintenance, landscaping, and management. The T12 (trailing 12-month actual financials) may show a 25% expense ratio when the true stabilized expense ratio for a professionally managed property of similar type is 40–45%.
Always normalize the expense ratio. If the T12 shows expenses below 35% of EGI for a 10+ unit building, build your own line-item expense model rather than using their actuals.
Expense Ratio Benchmarks by Property Type
| Property Type | Typical Operating Expense Ratio (% of EGI) |
|---|
| SFR rental | 35–45% |
| Small multifamily (2–4 units) | 35–40% |
| Small apartment (5–20 units) | 40–50% |
| Mid-size apartment (21–100 units) | 45–55% |
| Large apartment (100+ units) | 45–60% |
Higher unit counts come with economies of scale but also higher management complexity, insurance requirements, and regulatory overhead. If an OM is showing expenses below the low end of these ranges for your property type, the number is wrong.
Cap Rate: Benchmarking and the Compression Risk
Once you have a defensible NOI, the cap rate benchmark determines whether the asking price is reasonable. But cap rate benchmarking has its own failure modes.
Using Trailing Sales Instead of Forward-Looking Rates
Brokers typically cite cap rates from recent comparable sales. In a rising-rate environment, comps from 12–18 months ago may be priced at rates that no longer reflect market conditions. The relevant cap rate is what a buyer would pay *today* — which may be 50–150 basis points higher than the trailing comp set.
Always ask: "What would this asset trade for if listed today, at current financing costs?" If DSCR rates are 8% and the asset trades at a 5.5% cap, every buyer is facing negative leverage. Prices need to move.
Assuming Cap Rate Stays Flat at Exit
The 10-year IRR on a multifamily deal is exquisitely sensitive to the exit cap rate assumption. A building bought at a 6% cap rate and underwritten to sell at a 6% cap rate in year 10 gets all its return from NOI growth and debt paydown. The same deal with exit cap rate expansion to 7% could lose money despite 30% rent growth.
Run your exit at three cap rate scenarios:
- Bear: Entry cap + 0.75% (cap rate expansion)
- Base: Entry cap flat
- Bull: Entry cap - 0.25% (cap rate compression)
If the bear scenario produces an acceptable IRR, you have margin of safety.
The Value-Add Premium Trap
In value-add multifamily, buyers pay a premium for the upside — the spread between in-place NOI and stabilized NOI after renovations. This is legitimate. The error is paying too much of the premium upfront.
The formula:
Stabilized Value = Stabilized NOI / Market Cap Rate
Maximum Entry Price = Stabilized Value - (Rehab Cost + Profit Margin + Risk Premium)
If the asking price is $2.1M and stabilized value is $2.5M, the implied profit before rehab costs and risk is $400K. Is that enough to justify the execution risk, carry, and capital? Only the full pro forma math answers that — not the broker's summary page.
Building a Defensible NOI Model: A Step-by-Step Approach
Step 1: Start with the current rent roll, not the OM. Get the actual lease-by-lease rent roll. Calculate GPR from current leases. Note units that are vacant, month-to-month, or under short-term leases.
Step 2: Calculate market rent independently. Pull 3–5 comparable units rented in the last 90 days. Build your own market rent estimate rather than accepting the seller's.
Step 3: Apply your vacancy rate. Use market vacancy data. In markets with rising inventory or supply deliveries coming, underwrite to the higher end of the range.
Step 4: Build expenses line by line.
- Property management: 8–10% of EGI
- Property taxes: verify with the assessor, not the OM
- Insurance: get a quote if possible; otherwise use $400–$800/unit/year depending on market and property age
- Repairs and maintenance: $600–$1,200/unit/year
- Landscaping, trash, utilities (common area): vary by property
- CapEx reserve: $150–$400/unit/year
Step 5: Calculate your NOI and apply the market cap rate.
Value = NOI / Cap Rate
If your NOI and the market cap rate imply a value meaningfully below asking price, you have discovered a real problem — or a real negotiating point.
Connecting NOI to Cash Flow
After NOI, you still need to model the leveraged return. Divide annual debt service out of NOI and you have cash flow. Divide cash flow by cash invested and you have CoC return.
For multifamily deals, also calculate DSCR:
DSCR = NOI / Annual Debt Service
Most agency lenders (Fannie Mae, Freddie Mac) require 1.25 DSCR for multifamily loans. DSCR lenders on smaller multifamily typically require 1.20. If your NOI supports less than a 1.20 DSCR, the loan will be limited — which means you need more equity or a smaller purchase price.
The multifamily investor tools and real estate deal analyzer model all of this: NOI, DSCR, cap rate, CoC return, and 5-year projections with rent growth and exit cap rate sensitivity. Build the full pro forma before you submit your LOI.
Published 2026-07-31. Content by FlipCalculatorPro.