21 Questions in This Hub
- What is NOI and how is it calculated?
- What counts as operating expense in NOI and what does not?
- What is the difference between NOI and cash flow?
- What is cap rate and how is it calculated?
- What is the difference between cap rate and cash on cash return?
- What is a good cash on cash return on a rental?
- What is the 50 percent rule and is it useful?
- What is the operating expense ratio?
- What is the gross rent multiplier?
- Should I include vacancy in NOI?
- What is the difference between capex and operating expense?
- How do I calculate NOI per property in a portfolio?
- What are the most common NOI calculation mistakes?
- Is NOI the same as taxable income?
- What counts as cash invested for the cash on cash calculation?
- How do I roll up NOI across a portfolio?
- What does a cash flow waterfall look like for a rental?
- How much does vacancy actually affect NOI?
- How should I handle PM fees in NOI?
- How does software calculate NOI automatically?
- How do brokers and buyers use cap rate to price a deal?
1. What is NOI and how is it calculated? #
Net Operating Income (NOI) equals gross rental income minus operating expenses, before debt service, depreciation, and income taxes. The formula: NOI = Effective Gross Income (gross rent minus vacancy and credit loss plus other income) minus Operating Expenses. NOI measures what the property earns from operations, independent of how the owner financed it. The same property has the same NOI whether it is paid off or 90 percent leveraged.
Worked example: a four-unit building grosses $60,000 in scheduled rent, loses $3,000 to vacancy, earns $1,200 in laundry and parking other income, and runs $22,000 in operating expenses (insurance, taxes, repairs, management, utilities, capex reserve). Effective Gross Income equals $58,200; NOI equals $58,200 minus $22,000, or $36,200. NOI is the input to cap rate (NOI divided by purchase price), to lender DSCR (NOI divided by debt service), and to cash flow (NOI minus debt service).
2. What counts as operating expense in NOI and what does not? #
Included operating expenses: property taxes, insurance, property management fees, repairs and routine maintenance, landscaping, pest control, HOA fees, utilities the owner pays, accounting and legal for ongoing operations, marketing and leasing costs, payroll for property staff, and a reserve for capex (commonly 5 to 10 percent of gross rent for residential). All of these recur, all of them keep the property running.
Excluded from NOI: mortgage principal and interest (debt service is a financing decision, not an operating one), depreciation (a non-cash tax accounting item), capital improvements that increase basis (separate from the capex reserve which approximates them), income tax on the owner, and any owner-personal expenses run through the property by accident. Including any of these inflates or deflates the number compared to industry standard and breaks comparability with lender, buyer, and broker NOIs.
3. What is the difference between NOI and cash flow? #
NOI is operating performance: rent minus operating expenses, before financing. Cash flow is what arrives in the owner's account: NOI minus debt service (principal plus interest plus mortgage insurance) minus any capital expenditures actually paid (as opposed to the capex reserve held inside NOI) minus owner draws or transfers. Two properties with the same NOI can have very different cash flows depending on leverage, age, and capex timing.
A leveraged-up new construction with high NOI can run negative cash flow for the first year because debt service eats it; a paid-off older property with modest NOI can produce strong cash flow but face large capex outflows in any given year that reset cash flow back to NOI minus capex. Underwrite NOI for the asset thesis (does this property earn well from operations?), and underwrite cash flow for the personal finance thesis (does this deal feed me month to month?).
4. What is cap rate and how is it calculated? #
Capitalization rate (cap rate) equals NOI divided by property value (or purchase price for an acquisition). A property with $36,200 NOI and a $500,000 price has a 7.24 percent cap rate. Cap rate measures unleveraged annual return; it is the yield the property would deliver if you bought it cash. Cap rate is the language brokers, lenders, and buyers all use to describe how a property prices relative to its operations.
Cap rate is comparison fuel, not a complete return metric. It excludes financing entirely, so a high-cap rate property that does not cash-flow at typical leverage and a low-cap rate property in a strong appreciation market can both be the right buy depending on the strategy. Use cap rate to compare similar properties in similar markets at a similar moment in time; do not compare a 2019 cap rate to a 2026 cap rate or a Cleveland cap rate to a Phoenix cap rate without context.
5. What is the difference between cap rate and cash on cash return? #
Cap rate measures unleveraged return: NOI divided by property value, ignoring financing. Cash on cash return (CoC) measures leveraged return: annual pre-tax cash flow divided by total cash invested (down payment plus closing costs plus rehab plus initial reserves). Cap rate describes the asset; CoC describes your personal experience after writing the down payment check.
Worked example: a property with $36,200 NOI bought at $500,000 has a 7.24 percent cap rate. Bought with 25 percent down ($125,000) plus $10,000 closing plus $5,000 reserves ($140,000 total cash in), with $24,000 annual debt service, cash flow is $36,200 minus $24,000 or $12,200, and CoC is $12,200 divided by $140,000 or 8.7 percent. Same asset, two different return metrics, both useful. Cap rate compares deals; CoC tells you what your dollar earned this year.
6. What is a good cash on cash return on a rental? #
There is no single answer; the right CoC depends on market, asset class, leverage, and strategy. Common rules of thumb in 2026: 6 to 8 percent on a stabilized turnkey single-family in an appreciating market, 8 to 12 percent on a stabilized property in a cash-flow market, 12 percent or higher on a value-add or BRRR refi where rehab unlocked rent. Properties pricing below 6 percent CoC at typical leverage are usually appreciation plays where the owner accepts thin cash for capital gain.
Beware of CoC that compares poorly across markets and time. A 12 percent CoC at 8 percent interest in 2026 is not the same as a 12 percent CoC at 4 percent interest in 2021; lower rates inflated the same return calculation. Always pair CoC with cap rate, debt cost, expected appreciation, and reserve depth before declaring a deal good or bad.
7. What is the 50 percent rule and is it useful? #
The 50 percent rule is a back-of-envelope shortcut that says, on average, operating expenses (including vacancy and capex reserve, excluding debt service) on a residential rental run about 50 percent of gross rent over the long run. To estimate NOI quickly: NOI roughly equals 50 percent of gross rent. To estimate cash flow: cash flow roughly equals 50 percent of gross rent minus debt service.
The rule is useful for fast first-pass screening when you cannot yet see the actuals. It is misleading anywhere actual expense ratios diverge from 50 percent: low-tax states with newer construction can run 30 to 40 percent expense ratios; high-tax states with older small multifamily can run 55 to 65 percent. Once you have an actual P&L, drop the rule and use real numbers. For the related 1 percent and 2 percent rules and full underwriting, see the Property Analysis hub.
8. What is the operating expense ratio? #
Operating Expense Ratio (OER) equals operating expenses divided by effective gross income, expressed as a percent. A property with $58,200 EGI and $22,000 operating expenses has a 37.8 percent OER. OER measures operational efficiency: lower is better, all else equal, and a property that creeps higher over time is signaling rising expenses, falling rent, or both.
Useful benchmarks vary by asset class. Single-family rentals typically run 35 to 50 percent OER. Small multifamily (2 to 4 units) typically runs 40 to 55 percent. Older properties, properties in high-tax jurisdictions, and properties with deferred maintenance run higher. OER is most useful for tracking the same property year-over-year (any 5+ percentage point swing demands explanation) and for comparing similar properties in similar markets, not as an absolute pass-fail.
9. What is the gross rent multiplier? #
Gross Rent Multiplier (GRM) equals property price divided by annual gross rent. A property priced at $500,000 with $60,000 annual gross rent has a GRM of 8.33. GRM is a fast comparison tool that ignores expenses entirely; brokers use it as a screening number, especially in markets where expense data is hard to verify across listings.
Lower GRM means cheaper relative to rent. Single-family in cash-flow markets often trades at GRM 6 to 9; coastal and high-appreciation markets often trade at GRM 12 to 20 or higher. The big limitation: GRM rewards properties with high rent regardless of how expensive they are to operate, so a property with low GRM can still have weak NOI if expenses are abnormally high. Always validate a low-GRM acquisition with an actual operating P&L.
10. Should I include vacancy in NOI? #
Yes. Industry-standard NOI uses Effective Gross Income (EGI), which is gross scheduled rent minus vacancy and credit loss plus other income. Modeling NOI on 100 percent occupied gross rent inflates the number compared to what lenders, buyers, and brokers will use for the same property, and it understates risk. Standard vacancy assumption: 5 to 10 percent for residential in a stable market, higher in soft markets, lower in tight markets with strong tenant demand.
When backing into NOI from a trailing-12-month P&L, vacancy is already baked in because you only collected what you collected. When forecasting forward NOI, apply an explicit vacancy rate to gross potential rent. Lenders typically require an explicit vacancy line item even on properties that have been 100 percent occupied for years, because they underwrite for sustainable cash flow, not best-case.
11. What is the difference between capex and operating expense? #
An operating expense is a recurring cost that keeps the property running at its current condition: insurance, taxes, repairs that restore working order, routine maintenance, management fees, utilities. Capital expenditures (capex) are larger investments that extend the property's useful life, add value, or adapt it to new use: a new roof, HVAC system, kitchen remodel, exterior siding, parking lot repave. The IRS distinguishes them at tax time (operating deducts now, capex depreciates), and asset analysis distinguishes them in NOI (operating in, capex out).
The complication: NOI excludes the actual capex spend in any given year but includes a capex reserve approximating long-run capex (commonly 5 to 10 percent of gross rent for residential). This is to keep NOI a stable comparable number rather than a lumpy actual that swings huge in a roof-replacement year. When reading someone else's NOI, confirm whether the reserve is included; many casual P&Ls forget it, which makes NOI look better than it sustainably is.
12. How do I calculate NOI per property in a portfolio? #
Per-property NOI requires per-property income and expense tracking, not a pooled bookkeeping setup. The minimum data: gross rent collected per property, every operating expense categorized to a specific property, vacancy days per property, and per-property capex separated from operating repairs. Anything pooled at the portfolio level must be allocated back to individual properties before per-property NOI is calculable.
Common allocation approaches: insurance and umbrella by either premium-as-billed or by replacement cost percent of total; portfolio-wide management fee by gross rent percent; software and bookkeeping by property count or by gross rent percent. The allocation method should be documented and consistent year-over-year. Portfolios that never separate expenses report only a portfolio-wide NOI, which hides the underperformers and overperformers among individual properties.
13. What are the most common NOI calculation mistakes? #
Five recur. First, leaving out vacancy and credit loss, which inflates EGI and the resulting NOI by 5 to 10 percent. Second, leaving out a capex reserve, which makes NOI look healthier than it sustainably is. Third, including mortgage interest as an operating expense, which is wrong by definition (debt service is below the NOI line). Fourth, mixing capital improvements into repairs, which understates NOI in capex years and breaks year-over-year comparability. Fifth, omitting management fees on owner-managed properties, which inflates NOI compared to a properly compared PM-managed portfolio.
A subtle sixth: including HOA fees that the tenant reimburses (so the same dollar lives in income and expense and nets to zero) or excluding utilities the owner pays but recovers via separate billing. The remedy on all six is consistency: define what is in and what is out, document it once, apply it identically every period and every property. Half the apparent NOI volatility in casual P&Ls disappears with consistent definitions.
14. Is NOI the same as taxable income? #
No. NOI is a property-level operating metric; taxable income is a tax-return concept. They differ in three ways. Mortgage interest is excluded from NOI but deducted on Schedule E. Depreciation is excluded from NOI (it is a non-cash item) but is one of the largest deductions on Schedule E. Capex is excluded from NOI in the actual-spend sense (replaced by the reserve) but capitalizes and depreciates on the tax return. Net result: a property can show positive NOI and report a paper Schedule E loss because of mortgage interest plus depreciation.
This is one of the structural advantages of rental real estate: cash-positive operations can generate paper losses that defer or shelter income, subject to the passive activity loss rules. For Schedule E mechanics, depreciation calculation, and passive loss rules see the Schedule E and Taxes FAQ hub.
15. What counts as cash invested for the cash on cash calculation? #
Total cash invested for CoC purposes is every dollar you put in to acquire and stabilize the property: down payment, closing costs (everything on the buyer side of the closing disclosure that came out of pocket), purchase-related rehab and improvements, and an initial operating reserve to cover the ramp-up period. Loan amounts are excluded; this is the equity you wrote.
Worked example: a $500,000 property purchased with $125,000 down, $8,500 closing, $20,000 of immediate rehab, and $4,000 of opening reserves has $157,500 total cash invested. CoC = annual cash flow divided by $157,500. After a refinance, recompute CoC against the post-refi cash invested (initial cash in minus cash pulled out at refi); a successful BRRR with full equity recovery can produce an undefined or negative cash invested (infinite return), which is the math behind why the strategy is appealing.
16. How do I roll up NOI across a portfolio? #
Portfolio NOI is the sum of each property's NOI plus any portfolio-level income (rare) minus any portfolio-level operating expenses not allocated to individual properties. The math is additive, but the analytic value is in the comparison between properties: which are over-performing, which are under-performing, and which deserve more or less capital. Portfolio NOI alone is a vanity number; per-property NOI is the operating signal.
Useful portfolio-level views: NOI per door (portfolio NOI divided by total units), NOI per dollar invested (portfolio NOI divided by total equity outstanding), and trend NOI year-over-year. A portfolio whose total NOI is growing while NOI per door is flat or shrinking is growing through acquisition, not through operational improvement; the opposite trend is operational compounding, which usually beats acquisition-led growth at scale.
17. What does a cash flow waterfall look like for a rental? #
A clean waterfall: start with gross potential rent, subtract vacancy and credit loss, add other income to get EGI. Subtract operating expenses to get NOI. Subtract debt service (P+I, mortgage insurance) to get cash flow before capex. Subtract actual capital expenditures spent in the period (or the capex reserve transfer) to get cash flow available for distribution. Subtract any owner draws or transfers to get the period-end change in operating account balance.
Reading the waterfall top-down explains every variance. A cash flow miss can be a rent miss (top), a vacancy issue (next), an operating expense overrun (middle), a debt service change (lower), or a capex shock (bottom). Most operators jump straight to the bottom number and miss the diagnosis; the waterfall makes the source of any change obvious.
18. How much does vacancy actually affect NOI? #
More than most operators model. A single-family rental at $1,500 monthly rent vacant for one month loses $1,500 of EGI directly, plus typically $300 to $800 in turnover costs (cleaning, painting, repairs to make rent-ready, leasing fee), so the all-in cost of one turnover often runs $1,800 to $2,300 against an annual gross rent of $18,000 (10 to 13 percent of EGI). On NOI of about $9,000 (assuming 50 percent OER), that turnover removes 20 to 25 percent of the year's NOI.
Tenant retention is therefore one of the highest-leverage NOI drivers in any small portfolio. A modest renewal-incentive (capping the renewal increase below market, fixing a long-deferred fix, repainting) that keeps a tenant another year usually pays back many times over compared to chasing the next dollar of rent and triggering a turnover. Modeling vacancy explicitly in NOI keeps the trade-off visible.
19. How should I handle PM fees in NOI? #
Always include them in operating expenses, even if you self-manage. Industry-standard NOI assumes a market-rate management fee (typically 6 to 10 percent of collected rent for single-family) so that NOI is comparable across managed and self-managed portfolios. A self-managed property reports artificially-inflated NOI if you skip the line; that distorts cap rate and trade comparisons.
When the property is professionally managed, use the actual fee from the management agreement and any leasing or renewal fees actually billed in the period. When self-managed, impute a market-rate management fee at the line item. This also makes the personal economics honest: the difference between actual NOI and as-if-managed NOI is the dollar value of the time the owner spends managing, which is real even if it does not appear in the bank account.
20. How does software calculate NOI automatically? #
Operations software calculates NOI per property automatically by ingesting the underlying transactions (rent collected, vendor invoices, PM statements, bank deposits) and categorizing each one to a Schedule-E-aligned chart of accounts. Once categorized, NOI is mechanical: sum the income, sum the operating expenses, subtract. The owner does not touch a spreadsheet; the software produces month-end and trailing-12-month NOI per property as a derived view.
DoorVault's Knox AI categorizes transactions, reconciles PM statements, and surfaces NOI per property in a portfolio dashboard. Because every dollar is categorized at ingest, NOI is always current within hours of the latest deposit or invoice. The same data drives cap rate, CoC, OER, and the year-end Schedule E package without re-entry.
21. How do brokers and buyers use cap rate to price a deal? #
Brokers price commercial and small multifamily by applying a market cap rate to a stabilized NOI: price equals NOI divided by cap rate. A property with $36,200 NOI in a market trading at a 7 percent cap rate prices at about $517,000. The same property in a 6 percent cap market prices at about $603,000; in an 8 percent cap market about $452,000. Cap rate compression (going lower) lifts prices; cap rate expansion (going higher) drops them.
Buyers underwrite the same way in reverse: take their target return cap rate, apply to the underwrote NOI, derive an offer price. The trick is that NOI and cap rate are negotiated together. Sellers present aggressive pro-forma NOI; buyers underwrite a tighter NOI; both apply the local cap rate. Outcome divergence is usually about whose NOI got accepted, not about cap rate disagreement. Underwrite expenses honestly and cap rate becomes the comparable, not the manipulable, lever.