Multifamily Operating Expense Ratio (OER)

The multifamily operating expense ratio (OER) is total operating expenses divided by effective gross income — it tells you what share of collected revenue is eaten by operating costs before debt service. Stabilized multifamily typically runs 40–50%, and a ratio far below that range is usually a sign of understated expenses, not a genuinely efficient property.

OER is a fast triage tool. When you receive an offering memorandum, computing the expense ratio in seconds tells you whether the seller's expense load is plausible. An OER that lands inside the normal band is consistent with a real operating history; one that sits well below it deserves a line-by-line audit of what was left out.

The most common way pro formas overstate value is by understating expenses. A sub-30% OER almost always hides something — no replacement reserves, owner-supplied management or maintenance labor that a buyer will have to pay for, or deferred repairs booked nowhere. Each omission inflates NOI, and because value is NOI divided by cap rate, even a small expense understatement compounds into a large overstatement of price.

Worked example — OER from a stabilized property

OER = $502,200 ÷ $1,116,000 = 45%.

The discipline: an OER inside 40–50% is plausible; a sub-30% OER is a red flag for expenses the seller left off the page.

OER works alongside the underwriting fundamentals: it is the bridge between a property's effective gross income and its net operating income. Use it as the first filter when you evaluate any deal in the multifamily playbook, and pair a clean expense read with your value-add plan so you are not capitalizing costs that were never disclosed.