What the Operating Expense Ratio Actually Measures
The operating expense ratio (OER) is calculated by dividing a property's total operating expenses by its total operating revenue. If a six-unit building collects $120,000 in gross rents and spends $54,000 on operating expenses, the OER is 45 percent.
That definition sounds simple, but the ratio is only as reliable as the inputs. Two common errors distort it from the start.
First, some owners mix capital expenditures into their operating expense line. A roof replacement or HVAC system is not an operating expense. It is a capital item. When those costs are buried in maintenance or repairs, the OER looks artificially high, and a buyer who does not catch it will either overprice risk or walk away from a deal that is actually performing well.
Second, the revenue denominator matters. Some sources calculate OER against gross potential rent (what the building would collect at 100 percent occupancy). Others use effective gross income, which accounts for vacancy and concessions. A building with a 10 percent vacancy rate will show a lower OER when calculated against gross potential rent than when calculated against what it actually collected. Always confirm which revenue figure is being used before comparing ratios across properties.
For underwriting purposes, using effective gross income gives you a more conservative and honest picture of how the property is actually performing. When you are reviewing a seller's financials, it is worth checking the rent roll for red flags that might inflate the revenue figure and compress the ratio artificially.
The 35 to 50 Percent Benchmark: Where It Comes From and When It Breaks Down
The most widely cited range for a "healthy" apartment OER is 35 to 50 percent. That range comes from institutional underwriting norms and is repeated across investor education resources, lender guidelines, and property management benchmarks.
RealPage, which tracks professionally managed apartment data across large portfolios, has reported average expense ratios around 0.43 (43 percent) for institutional-grade product. Their data also shows that properties with more than 150 units run roughly 58 basis points lower than smaller properties over a comparable period. That scale advantage is real, and it matters when you are buying a six-unit building rather than a 200-unit complex.
Some broader educational guides cite 60 to 80 percent as a typical expense share, particularly when the author includes debt service, reserves, or management fees that others exclude. That discrepancy is not an error. It reflects different definitions of what counts as an "operating expense." Before you benchmark a deal, confirm exactly what is included in the seller's expense figure.
Here is where the 35 to 50 percent range breaks down in practice.
It was built largely on stabilized, professionally managed properties in markets with moderate taxes and insurance costs. If you are buying a 1960s-era fourplex in a high-tax New York submarket, or a building with deferred maintenance and below-market rents, the ratio will likely land above 50 percent even if the property is being managed competently. That does not automatically make it a bad deal. It means the benchmark needs to be adjusted for the specific conditions of the asset.
A low OER is not always a good sign either. A ratio of 28 percent on an older building often means the owner has deferred maintenance, skipped routine inspections, or underinsured the property. Those savings show up in the ratio today and show up in your capital budget tomorrow. The small multifamily inspection red flags that surface during due diligence are frequently the physical evidence of a ratio that was kept artificially low.
How Property Class, Age, and Unit Count Shift the Range
Once you understand the baseline benchmark, the practical work is adjusting it for the asset in front of you. Four variables move the range most consistently.
Property class. A Class A building, newer construction with modern systems and low deferred maintenance, can underwrite in the 30 to 35 percent range. Class B properties (built in the 1980s to early 2000s, average condition) typically land in the 38 to 45 percent range. Class C assets, older buildings with aging systems and lower rents, often run 45 to 55 percent or higher. The class designation is not just about aesthetics. It reflects the cost structure of the building.
Age and system condition. Older buildings carry higher maintenance costs, higher insurance premiums in some markets, and more frequent capital needs that can bleed into operating lines if the owner is not disciplined about separating CapEx. A building with original 1970s plumbing and a roof that is 18 years old should be underwritten with a higher expected OER than the seller's trailing 12 months might show.
Unit count. Smaller buildings lose the economies of scale that institutional operators enjoy. A triplex or fourplex has fixed costs (insurance, property taxes, management overhead) spread across fewer units. A single vacancy or one large repair represents a much larger share of annual revenue than it would in a 50-unit building. For small multifamily, comparing expenses on a per-unit basis alongside the percentage ratio gives you a clearer picture of the cost structure.
Local taxes and insurance. This is the variable that surprises buyers most often. In New York, property taxes are a significant line item that can push an otherwise well-run building's OER well above the national benchmark range. Insurance costs have also risen sharply in markets exposed to weather risk. When you are underwriting a deal in a high-tax or high-insurance submarket, build those local conditions into your benchmark adjustment before you evaluate the seller's numbers. Understanding how to appeal property taxes after acquisition is one way to address an OER that is inflated by an outdated assessment.
Common Line Items That Distort the Ratio in Small Buildings
Small multifamily properties are particularly vulnerable to ratio distortion because the numbers are less stable year to year. A few specific line items account for most of the noise.
Owner-performed labor. When a self-managing owner handles repairs personally, those costs do not appear on the income statement. The OER looks lean. When you take over and hire a property manager or pay contractors for the same work, the ratio rises immediately. Always add a market-rate management fee (typically 8 to 10 percent of collected rents) to your underwriting even if the current owner is self-managing.
Insurance gaps. Some owners carry minimum coverage to reduce premiums. The OER looks better, but the risk is shifted to you as the buyer. Review the actual policy, not just the premium line, during due diligence.
Vacancy treatment. If the seller is calculating OER against gross potential rent and the building has been running at 85 percent occupancy, the ratio is understated. Recalculate using actual collected revenue.
Irregular maintenance years. A building that had no major repairs in the trailing 12 months may show a 38 percent OER. The same building in a year with an HVAC replacement and a roof repair might show 55 percent. Look at two to three years of expense history when possible, and normalize for one-time items.
These distortions are part of why serious buyers treat the OER as a starting point for questions rather than a final answer. The due diligence process for small multifamily is where those questions get answered.
Applying the Benchmark When Underwriting a Real Deal
Here is a practical sequence for using the OER in your underwriting without over-relying on it.
Start with the seller's stated OER and identify the revenue denominator. Is it gross potential rent or effective gross income? Restate it using effective gross income if needed.
Next, strip out any capital expenditures that appear in the operating expense lines. Ask specifically about roof, HVAC, water heater, and major appliance replacements in the past three years. If any of those costs are buried in maintenance, remove them from the OER calculation and note them separately as capital items.
Then add back any expenses the owner is not showing. Market-rate management fees, a normalized maintenance reserve (commonly estimated at $500 to $1,000 per unit per year for older buildings), and any insurance gaps should be reflected in your version of the ratio.
Once you have a clean OER, compare it against the adjusted benchmark for the property class, age, and local cost environment. A 47 percent OER on a 1975-built six-unit building in a high-tax submarket is not alarming. The same ratio on a 2015-built building with newer systems and moderate taxes warrants more investigation.
Finally, run a per-unit expense analysis alongside the percentage. For a small building, knowing that you are spending $4,200 per unit per year on operating expenses (excluding taxes and insurance) gives you a concrete number to compare against local management company data and your own experience with similar assets.
The OER is most useful when it raises the right questions. A ratio that looks too clean often means something is missing. A ratio that looks too high might reflect a property that is being stabilized, a tax assessment that can be challenged, or an insurance cost that can be restructured after closing.
If you are actively evaluating small multifamily deals and want to connect with sellers who have prepared their financials for serious buyer review, FlowExit works specifically in this space, connecting owners of small apartment buildings with investors who are ready to underwrite real numbers.