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 (some investors include, some don't — be consistent)
- Make-ready / leasing fees (annualized)
What does notbelong 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).
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 NOI honest — otherwise you're showing a great NOI this year and getting hit with $15k of replacements 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.