Every real estate metric you care about — cap rate, cash-on-cash return, DSCR, property valuation — starts with Net Operating Income (NOI). If your NOI is wrong, everything downstream is wrong. And the most common mistakes aren't in the math — they're in what you include and exclude.
The NOI Formula
Simple formula, but the devil is in the details. Let's break down each component.
Step 1: Calculate Effective Gross Income
Start with Gross Potential Rent — the total rent if every unit is occupied and every tenant pays on time for 12 months. For a single-family rental at $2,000/month, that's $24,000/year.
Then subtract a vacancy and credit loss allowance. No property maintains 100% occupancy with zero missed payments. A 5-8% vacancy allowance is standard for most markets, though high-demand urban areas might justify 3-5% and rougher areas might need 10-15%.
Add any other income: laundry, parking, pet fees, storage, late fees. These can add meaningful revenue on multifamily properties.
Gross Potential Rent: 4 units × $1,200/mo × 12 = $57,600
Vacancy Allowance (7%): −$4,032
Laundry Income: +$1,200
Effective Gross Income: $54,768
Step 2: Calculate Operating Expenses
This is where most investors go wrong. Operating expenses include every recurring cost of running the property — but specifically exclude a few things.
What to INCLUDE in operating expenses:
| Expense | Typical Range | Notes |
|---|---|---|
| Property Taxes | Varies by location | Use actual tax records, not estimates |
| Insurance | $800–$3,000+/yr | Get an actual quote for the property |
| Property Management | 8–10% of rent | Include even if self-managing (your time has value) |
| Maintenance & Repairs | 5–10% of rent | Older properties need more; newer need less |
| Utilities (landlord-paid) | Varies | Water/sewer, trash, common area electric |
| Landscaping / Snow | $1,200–$3,600/yr | Climate dependent |
| Legal / Accounting | $500–$1,500/yr | Evictions, tax prep, entity maintenance |
| Advertising / Leasing | $200–$800/yr | Listing fees, signage, turnover costs |
What to EXCLUDE from operating expenses:
Mortgage payments — NOI is before debt service. This is the #1 mistake beginners make.
Capital expenditures (CapEx) — a new roof or HVAC replacement is not an operating expense. Budget for it separately.
Depreciation — this is a tax concept, not a cash expense.
Income taxes — NOI is a pre-tax, pre-financing measure.
Property Taxes: $4,800
Insurance: $2,400
Management (8%): $4,381
Maintenance (8%): $4,381
Water/Sewer/Trash: $3,600
Snow/Lawn: $1,800
Total Operating Expenses: $21,362
Step 3: Calculate NOI
Effective Gross Income: $54,768
Operating Expenses: −$21,362
NOI: $54,768 − $21,362 = $33,406
With this NOI, if the property is listed at $450,000, your cap rate would be $33,406 ÷ $450,000 = 7.4%. Now you have a meaningful number to evaluate.
The 50% Rule: Quick Sanity Check
A common rule of thumb says operating expenses run about 50% of gross rent for typical residential rental properties. So if a property collects $60,000/year in rent, expect roughly $30,000 in operating expenses and $30,000 in NOI.
This isn't precise enough for actual deal analysis, but it's a fast way to check if a seller's "pro forma" numbers are realistic. If they claim only 25% expenses on an older property, they're almost certainly leaving things out.
Never trust a seller's NOI. Always build your own using actual tax records, insurance quotes, and realistic expense estimates. Sellers have every incentive to understate expenses and overstate income.
Common NOI Mistakes to Avoid
Using the seller's pro forma rents. Sellers often project rents above market rate. Verify with current comps on Rentometer, Zillow, or local listings.
Forgetting property management. Even if you manage the property yourself, include a management fee (typically 8-10%). Your time isn't free, and if you ever want to step away, you'll need to hire a manager. Including it keeps your analysis honest.
Ignoring vacancy. Using 0% vacancy means assuming perfect tenants who never leave and always pay. That doesn't exist. Budget at least 5% for vacancy and credit loss.
Using last year's taxes. Property taxes often increase after a sale because the assessed value resets to the purchase price. Check with the county assessor to estimate post-purchase taxes.
Mixing up NOI and cash flow. NOI does not include mortgage payments. Cash flow does. If someone quotes "NOI" but has subtracted the mortgage, they're actually quoting cash flow.
Calculate NOI and Every Metric That Depends On It
Enter your property's income and expenses into CapRateKit and get NOI, cap rate, cash-on-cash, DSCR, and break-even analysis automatically.
Try CapRateKit Free →Frequently Asked Questions
What is a good NOI for rental property?
NOI by itself doesn't tell you if a deal is good — you need to compare it to the purchase price (which gives you cap rate). A $300,000 property with $24,000 NOI (8% cap) is a better deal than a $1,000,000 property with $50,000 NOI (5% cap), assuming similar risk profiles. Focus on cap rate rather than raw NOI.
What is the operating expense ratio?
The operating expense ratio is total operating expenses divided by effective gross income. For residential rentals, this typically ranges from 35% to 55%. If your OER is significantly outside this range, double-check your expense estimates. A very low OER (under 30%) usually means you're missing expenses.
Should I include a CapEx reserve in NOI?
Technically no — capital expenditures are not operating expenses. However, many investors set aside a separate CapEx reserve (typically 5-10% of rent) in their cash flow analysis even though it doesn't appear in the formal NOI calculation. This is smart budgeting — just don't confuse it with NOI.