Reading the number against the market
The ratio is only meaningful in context. A 30% ratio in a low-tax state on a recently renovated house is plausible; the same 30% on a 1950s fourplex with a shared boiler in a high-tax county is fiction. Compare against similar properties in the same market rather than a national rule of thumb, and remember the ratio moves with rent as well as with costs: raising rents lowers it without a single expense changing, which is why a well-managed building in a rising submarket looks efficient on this measure whether or not it actually is.
What counts as an operating expense
Property taxes, insurance, management, routine maintenance and repairs, owner-paid utilities, landscaping and snow removal, HOA dues, licensing, turnover costs, and the administrative overhead of running the property. Excluded are the mortgage payment, capital expenditures, and depreciation. Vacancy is handled on the income side by reducing gross potential rent to gross operating income rather than being listed as a cost. Investors disagree about where to put capex and both conventions are defensible — what matters is using the same one on every deal you compare, and never simply dropping it.
OER = operating expenses ÷ gross operating income × 100. Expenses exclude mortgage principal and interest, capital expenditures, and depreciation. $8,400 of expenses against $21,000 of collected rent is 40%.
The two ways it gets faked
Sellers shave the numerator by leaving out management because the owner self-manages, using a grandfathered insurance premium instead of a current quote, and quoting property tax at the pre-sale assessment rather than what the county will bill you after transfer. They inflate the denominator by presenting market rents on units currently leased below market. A ratio far below local norms is almost never a bargain — it is a missing line item. Rebuild the expense stack yourself from quotes and public records, then recompute before the number influences your offer.
Improving it
The ratio moves for good reasons and bad ones. Genuine improvement comes from lowering controllable costs — appealing an over-assessment, re-shopping insurance at renewal, submetering utilities where the law allows, cutting turnover through retention — or from raising rents to market on a building that has been under-managed. False improvement comes from deferring maintenance, which lowers this year’s expenses and raises next year’s capital bill. Track the ratio across years rather than judging it in a snapshot, because a number that falls while work orders pile up is telling you something other than efficiency.
Put it to work
Related terms
- NOI (net operating income) — Annual rental income minus all operating expenses, but before mortgage payments and income taxes.
- Capex (capital expenditures) — Money spent on big-ticket components that outlast a year — roof, HVAC, water heater, windows, siding, flooring, kitchens — as opposed to routine repairs.
- 50% rule — A rule of thumb estimating that operating expenses — taxes, insurance, maintenance, vacancy, management, and capex — consume roughly 50% of gross rent over time, excluding the mortgage.
- Break-even occupancy — The occupancy level at which rent exactly covers operating expenses plus debt service — the line between cash flow and a cash call.
- Pro forma — A projected income and expense statement for a property — what it should do, not what it has done.