How to Calculate Cash Flow on a Rental Property (The Right Way)

Most rental property cash flow analyses are wrong. Not by a little — by $400–$600/month. The errors are predictable: missing the CapEx reserve, skipping the management fee (because "I'll manage it myself"), and using 0% vacancy. Here's the framework that professional investors use, and why each line item matters.

The Cash Flow Formula

Monthly Cash Flow = Gross Rental Income − Total Operating Expenses − Debt Service

That's it. The work is in making sure every expense category is included.

Gross Rental Income

Start with Effective Gross Income (EGI), not advertised rent:

Gross Rent:        $2,200/month
Vacancy (6%):     -$132/month
Credit Loss (1%):  -$22/month
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Effective Gross:   $2,046/month

Vacancy rate depends on your market. Use 5% for tight markets (sub-3% vacancy), 8% for average markets, 10%+ for tertiary markets or properties that are harder to rent. Using 0% vacancy is how investors fool themselves.

Credit loss is the allowance for late/partial payments and eviction-related income loss. 1% of gross is a conservative assumption — some markets warrant higher.

The 8 Expense Categories

1. Mortgage Payment (P&I)

Principal and interest — your actual payment, not interest-only. If you're running a pro forma before closing, use the actual rate lock or current 30-year fixed rates.

2. Property Taxes

Use the actual assessed tax bill, not the prior owner's. In many states, property reassessment occurs at sale. Call the county assessor's office before closing to get the post-sale estimate. Tax bill surprises are among the most common cash flow killers.

3. Insurance

Landlord insurance (dwelling policy + liability) runs $800–$2,000/year for a single-family home. Don't use homeowner's insurance rates — landlord policies are different products and are usually 15–25% more expensive.

4. Property Management

Even if you plan to self-manage, budget 8–10% of gross rents. Why? First, circumstances change — you may not be able to self-manage indefinitely. Second, it's not actually "free" — your time has value. Self-managing a $2,000/month rental yourself saves $200/month but costs 5–10 hours/month of your time — not a great trade for most investors.

5. Maintenance & Repairs Reserve

Budget 5–8% of gross rents for routine maintenance: appliances, HVAC filters, plumbing leaks, painting, landscaping. On a $2,200/month rental, that's $110–$176/month. This isn't a line item you pay every month — you're building a reserve for the months it costs $800 to fix a leak or replace an appliance.

6. CapEx Reserve

Capital expenditures — big-ticket items with a multi-year lifespan: roof ($10,000–$20,000), HVAC ($5,000–$12,000), water heater ($1,000–$2,500), appliances ($500–$2,000 each), flooring ($3,000–$10,000). Divide the estimated replacement cost by the expected remaining life to get a monthly reserve.

A simple CapEx model for a typical SFR:

Most investors budget 8–10% of gross rents for CapEx — this works as a shortcut if you don't want to model each item.

7. Vacancy Allowance

Already accounted for above in EGI, but some investors model it as an expense instead. Either approach works — just don't double-count it.

8. Utilities (if landlord-paid)

Water/sewer is landlord-paid in many markets. Gas if there are common areas. Budget based on actual utility bills, not estimates.

Full Example: $2,200/Month Rental

This property barely breaks even. Many investors would still buy it if the location has strong appreciation potential, the rent is trending up, or they're using it as a 1031 exchange vehicle. But they know the real number — they're not buying it for cash flow.

Now run the same numbers skipping CapEx and management (a common mistake):

$635 vs. $7. The gap — $628/month — is the amount investors routinely underestimate.

The Cash-on-Cash Calculation

Once you have monthly cash flow, calculate annual cash-on-cash return:

Annual Cash Flow: $84 (at $7/month)
Cash Invested: $55,000 (down payment + closing)
Cash-on-Cash: 0.15%

This property has essentially zero cash-on-cash return — acceptable only if you have a strong appreciation thesis. Compare against a property that generates $300/month cash flow on the same investment:

Annual Cash Flow: $3,600
Cash Invested: $55,000
Cash-on-Cash: 6.5%

Both are real deals. Knowing which one you own changes how you think about holding, selling, and refinancing.

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Related: Cap Rate vs. Cash-on-Cash · Rental Property Depreciation · Real Estate Investment Analyzer