Operating expenses are the recurring costs of running a rental property: property taxes, insurance, repairs and maintenance, utilities you cover, and management. They are the day-to-day cost of keeping a building rented and habitable, and they deliberately leave out two big-ticket items: your mortgage payment and your capital improvements. That exclusion is the whole point. Operating expenses describe what the property costs to operate, separate from how you financed it and separate from the occasional large upgrades that get spread over many years.
In practice
Say you own a duplex that collects $34,200 a year after a vacancy allowance. In a normal year you might pay property taxes of $4,200, landlord insurance of $1,400, water and trash of $1,800, repairs and maintenance of $2,600, and a management allowance of $2,700. Add those up and you have $12,700 in operating expenses, which is about 37% of collected rent.
Now watch what does not belong in that list. The $1,650 a month you send the bank is a financing cost, not an operating one, so it stays out. The $7,000 you spent tearing out the kitchen is a capital improvement that gets depreciated over years, so it stays out too. Subtract the $12,700 of operating expenses from the $34,200 of income and you are left with $21,500, which is your net operating income. Drop the mortgage or the kitchen into that subtraction by accident and the number stops meaning anything.
Why it matters to a small landlord
Getting this line right decides whether your returns are real or imagined. Operating expenses feed directly into NOI, and NOI feeds your cap rate, so an expense list that quietly omits management, vacancy, or a repair reserve makes a tired building look like a bargain. The fix is to categorize every dollar as it lands instead of guessing at tax time. A repair that keeps the property in service is deductible the year you pay it; an improvement that betters or extends the building is not. Sorting that as you go is the core of a tight monthly close, and the same categories line up with the deductions you will report on Schedule E at year end.
Operating expenses make the most sense next to the terms they touch. They are subtracted from income to produce net operating income, they are the line that the 50% rule tries to estimate with a single shortcut, and they are defined partly by what they are not, namely capital expenditures. Keep those four straight and the rest of rental math falls into line.