Why NOI is the most important real-estate number
Cap rate uses NOI. DSCR uses NOI. The income approach to valuation uses NOI. Improving operations by $1,000/yr in a 7% cap-rate market adds about $14,000 to the property's market value. Get NOI right, and every other underwriting number falls into place. Get it wrong, and your cap rate, DSCR, and valuation are all built on sand.
The formula
What counts as an operating expense
- Property tax
- Insurance
- Maintenance & repairs (the recurring stuff)
- Property management fees
- HOA dues
- Owner-paid utilities (water, sewer, trash, sometimes gas)
- Vacancy & credit loss (subtracted from gross rent, not added as opex)
- CapEx reserves (shown below NOI under the TrueCap standard)
- Make-ready / leasing fees (annualized)
What does not belong in operating expenses: mortgage P&I, depreciation, owner income tax, your personal time, or one-time capital improvements that increase asset value (those go on the balance sheet, not the operating P&L). For the full plain-English definition (and edge cases like CapEx vs. operating repairs), see the NOI glossary entry.
Operating expense ratio
The flip side of NOI margin. OER = operating expenses ÷ effective gross rent. A 40% OER means 40 cents of every dollar of rent goes to running the property; the remaining 60 cents is NOI. Residential rentals typically land between 35% (newer, professionally managed) and 50% (older, self-managed, deferred maintenance).
Common NOI mistakes
1. Using gross rent instead of effective rent
Asking rent is fiction until a tenant pays it. Always subtract vacancy + credit loss to get effective gross rent before computing NOI. Most investors should use 5-8% vacancy as the floor; turnover-heavy markets need more.
2. Forgetting CapEx reserves
The roof, HVAC, water heater, and flooring all wear out. Setting aside 5-10% of rent monthly as a CapEx reserve keeps cash-flow expectations honest. TrueCap shows that reserve below lender-style NOI, then subtracts it before cash-on-cash return — otherwise a strong NOI can still hide a $15k replacement in year 7.
3. Excluding management fees on self-managed properties
If you self-manage, your time is still a real cost. Most underwriters include 8-10% of rent as management fee even if you're doing the work yourself — because the day you hand it off to a PM, the NOI shouldn't suddenly drop. Be honest about the true cost of operating the property. The 50% rule for rentals is a useful sanity check — if your assumed operating expenses are far below 50% of rent on a small residential property, verify each expense line against current property evidence rather than assuming the shortcut proves an omission.