How to calculate operating expense ratio, step by step
Operating expense ratio is one division that answers one question: how much of the rent does it cost to run the property? A landlord who knows the answer can decide whether a rental pays its way or quietly loses money. The calculation is the operating expenses divided by the effective gross income — two numbers from any rental’s records, divided into each other. The answer is a plain number: an OER of 0.40 means forty cents of every dollar of income goes on operating expenses.
The formula is OER = operating expenses ÷ effective gross income. Operating expenses are the day-to-day costs of running the property — insurance, property taxes, maintenance and repairs, property management, utilities the landlord pays and advertising for tenants. The mortgage payment is not an operating expense, and neither are capital improvements like a new roof. Effective gross income is the gross potential rent minus vacancy and credit losses — the rent you actually expect to collect, not the rent on paper. Both figures come from the same records, in the same currency, over the same period, or the OER is wrong before it starts.
The worked example, line by line. Step 1 — the operating expenses: $120,000. Step 2 — the effective gross income, the gross potential rent minus vacancy and credit losses: $300,000. Step 3 — the division: $120,000.00 ÷ $300,000.00 = 0.40. Step 4 — the percent: 0.40 × 100 = 40.00%. Step 5 — the meaning: 40.00% of the income goes on operating expenses. Work it yourself in the operating expense ratio calculator.
What the number means
An OER of 0.40 means forty cents of every dollar of income goes on operating expenses, and sixty cents is left before the mortgage. A higher OER means the property costs more to run, so less is left for the mortgage and the landlord. A lower OER means the opposite: the property runs cheaply, so more is left over. An OER of 1.00 means the expenses eat the whole income and nothing is left. An OER above 1.00 means the property loses money before the mortgage is even paid — the expenses are bigger than the income. Most rentals sit between 0.35 and 0.60, though the right number depends on the property, the market and the landlord’s own costs.
What moves the operating expense ratio
Two things move the OER: the expenses and the income. Raise the operating expenses and the OER rises: at $150,000 of operating expenses the OER is 0.50, and at $225,000 it is 0.75 — three-quarters of the income gone before the mortgage. Cut the expenses and the OER falls. Raise the effective gross income and the OER falls: more rent collected against the same expenses means a smaller share goes on running the place. Let the income fall — more vacancy, more unpaid rent — and the OER rises, because the same expenses now eat a bigger share of a smaller income.
When the ratio is undefined
If the effective gross income is $0, the OER is undefined — dividing by $0 has no answer. A property with no rent coming in has no ratio to calculate, only a warning. If the operating expenses are $0, the OER is 0.00 — a real figure, not undefined: nothing spent, nothing eaten. A negative OER is possible when the expenses are negative, a refund or a bookkeeping credit, but a landlord who sees one should check the books before trusting it.
Common mistakes
The first mistake is using the gross potential rent instead of the effective gross income — the rent on paper is bigger than the rent collected, so the OER comes out too low and the property looks cheaper to run than it is. The second is putting the mortgage payment in the operating expenses — the mortgage is a financing cost, not a running cost, and it belongs in the debt service coverage ratio instead. The third is comparing properties with different vacancy — a property that is empty half the year has a smaller effective gross income and a higher OER, and the two numbers are not talking about the same thing. The fourth is forgetting that the OER says nothing about the mortgage at all — a property with a low OER can still lose money after the debt is paid. Work it out in the operating expense ratio calculator, and check the debt service coverage ratio calculator and the cash flow adequacy ratio calculator before you decide the property pays.