Two rental properties can collect the same rent and produce very different income, because one costs much more to operate than the other. The operating expense ratio (OER) captures that difference in a single percentage: how much of the property’s income goes to running it.
The formula
- Operating expenses are the recurring costs of operating the property: taxes, insurance, utilities the owner pays, management, repairs and maintenance, a CapEx reserve, and administrative costs. They exclude the mortgage, depreciation and income taxes. (See what goes into NOI for the full list.)
- Effective gross income (EGI) is scheduled rent plus other income, minus vacancy and credit loss.
Some analysts divide by gross scheduled income instead of EGI. Either works if it’s applied consistently, but the two give slightly different results, so it’s worth noting which one is used.
The OER is the complement of the NOI margin: if OER is 40%, then NOI is 60% of EGI.
A worked example
A six-unit building at $1,100 per unit
The owner pays common-area electric, water, sewer and trash.
| Line item | Annual |
|---|---|
| Gross scheduled rent | $79,200 |
| Vacancy (5%) | −$3,960 |
| Effective gross income | $75,240 |
| Property taxes | $9,800 |
| Insurance | $4,200 |
| Owner-paid utilities | $7,600 |
| Management (8%) | $6,019 |
| Repairs & maintenance | $5,100 |
| CapEx reserve | $3,000 |
| Landscaping & snow | $2,400 |
| Admin & accounting | $900 |
| Total operating expenses | $39,019 |
| Operating expense ratio | 51.9% |
| NOI | $36,221 |
Hypothetical numbers for illustration.
About 52% of every dollar this building collects goes to operating it; the remaining 48% is net operating income.
What drives the ratio
Who pays utilities. This is often the biggest single swing factor in multifamily. In the example, if tenants paid all utilities directly, the OER would drop to 41.8%.
Property taxes. Tax rates vary enormously by location, and a reassessment after a sale can change them significantly.
Insurance. Location, building age, construction type and claims history all affect premiums.
Age and condition. Older buildings generally need more repairs and larger CapEx reserves.
Rent level. Many costs — insurance, a water bill, a furnace replacement — don’t scale with rent. In lower-rent areas, the same costs take a larger share of income, so OERs tend to be higher.
Property size. Larger buildings can spread some fixed costs over more units, though they may add costs like on-site staff or elevators.
Using OER to check an analysis
The ratio is especially useful as a reasonableness check:
- An unusually low OER often signals something is missing — commonly vacancy, management (when an owner self-manages), CapEx or a utility.
- An unusually high OER points to a specific cost worth investigating: an outsized tax bill, a high insurance quote, or owner-paid heat.
- Comparing actual vs. projected OER on an existing property shows whether the original budget held up.
It’s also the basis of quick rules like the 50% rule, which is simply an assumed OER of about 50% applied to gross rent.
Why small changes in OER matter for value
Because value is often estimated as NOI ÷ cap rate, anything that changes operating expenses changes value too. In the example, removing the $7,600 utility cost would raise NOI from $36,221 to $43,821. At a 7% cap rate, that’s the difference between an implied value of about $517,440 and about $626,011.
This is why expense reductions — such as tenants taking over utility billing — can have an outsized effect on value for income properties.
Benchmarks: use with care
People often quote typical OER ranges for different property types, and those ranges can be a useful starting point. But the ratio depends so heavily on local taxes, insurance, utility arrangements and rent levels that a building’s own history — or the actual statements of similar nearby properties — are far better benchmarks than any general figure.
Common mistakes
Including the mortgage. Debt service isn’t an operating expense. Including it inflates OER and misstates NOI.
Mixing bases. One property’s OER on gross income and another’s on EGI aren’t directly comparable.
Using a single year that isn’t typical. A year with a major repair or no turnover can distort the ratio. Several years of data give a steadier picture.
Ignoring reserves. Leaving out CapEx makes the ratio look better than the long-run reality.
Frequently asked questions
Does OER include vacancy? Usually not. When OER is calculated on effective gross income, vacancy has already been subtracted from income, so it isn’t counted again as an expense. If OER is calculated on gross scheduled income, some analysts include vacancy as an expense line instead — another reason to note which base is being used.
Should a CapEx reserve be in the ratio? Practices differ. Including it gives a more complete picture of the long-run cost of ownership; excluding it matches how some lenders and appraisers present NOI. Consistency across the properties being compared matters most.
How often should OER be recalculated? At least annually for owned properties, using actual expenses. Tracking it over several years shows trends — rising insurance or taxes, for example — that a single year can hide.
The takeaway
The operating expense ratio shows what share of a property’s income goes to running it. It’s a quick way to compare properties, to sanity-check a budget for missing items, and to see how a change in a single cost — like utilities or taxes — flows through to NOI and value. It’s most reliable when based on actual expenses and compared against genuinely similar properties.