Cash Flow Calculator Guide: Modeling a Rental Property Honestly

What cash flow actually is

Cash flow is what's left after every bill is paid. For a rental property, the calculation is: gross rent collected, minus vacancy, minus all operating expenses, minus debt service (principal + interest), equals monthly cash flow. Positive cash flow means the property pays you to own it. Negative cash flow means you're feeding it.

The trap most beginners fall into is computing cash flow against rent and mortgage payment only — forgetting taxes, insurance, vacancy, maintenance, CapEx, and management. The result looks great on paper and disappoints in real life.

The full cash flow formula

Monthly cash flow = Gross rent − Vacancy − Operating expenses − Mortgage payment

Gross rent: what the property charges. Use comparable rents from Rentometer, Zillow Rent Estimator, and local MLS — average the three and round down.

Vacancy: the share of the year the unit sits empty. 5% (~2.5 weeks/yr) is a reasonable baseline for stable markets; 8–10% for unstable.

Operating expenses: property tax, insurance, utilities (landlord-paid only), HOA, lawn, snow, pest, routine maintenance, leasing fees, management.

Mortgage payment: principal + interest only. Tax and insurance are already in operating expenses; don't double count.

Honest vacancy assumptions

5% vacancy is one tenant turnover per year that takes ~2.5 weeks to refill, plus minor unrented days. That's an optimistic-but-defensible assumption for solid B-class properties in stable markets.

8% vacancy reflects one turnover per year that takes a full month plus some bad-luck rentals. Use this for C-class properties, harder markets, or properties without a property manager.

10%+ vacancy reflects deep value-add work, college-town volatility, or unfamiliar markets where you're guessing at demand.

Don't use 0% vacancy. Even fully leased properties experience turnover. The math on 0% vacancy will turn out to be wrong in year one or two.

Maintenance and CapEx reserves

Routine maintenance — calls for plumbing, appliance repairs, paint touch-up — runs roughly 5–8% of gross rent for solid properties. Older properties trend higher.

Capital expenditure reserve — for big-ticket replacements like roof, HVAC, water heater, kitchen, flooring — is the line item beginners most often skip. Long-run CapEx averages 5–10% of gross rent. The roof and HVAC don't fail every year, but when they do they cost $8,000–$20,000.

Reserve aggressively in year one, deplete the reserve when big items hit, and refill from cash flow. If your deal only cash-flows because you're skipping CapEx, the deal doesn't actually cash-flow.

Property management fees

If you self-manage, you save the fee but spend the time. If you hire a property manager, expect 8–10% of collected rent, plus a leasing fee (usually one month's rent) per new tenant.

Always model the management fee even if you self-manage. If you eventually outgrow self-management — or you move out of state, or you simply don't have the energy — you need to know whether the property still cash-flows after the fee. A deal that requires you to self-manage forever is fragile.

A worked example

A duplex purchased for $325,000. Each unit rents for $1,400 → $2,800/month gross rent ($33,600/yr).

• Vacancy at 6%: $168/month
• Property tax (1.2%/yr): $325/month
• Insurance: $125/month
• Repairs and maintenance (7% of rent): $196/month
• CapEx reserve (7% of rent): $196/month
• Property management (10% of collected rent): $263/month
• Operating expenses subtotal: $1,273/month
• Mortgage (assume 25% down, $244K loan at 7.5% over 30 years): $1,706/month
• Total monthly cost: $2,979

Cash flow: $2,800 − $168 − $2,979 = −$347/month. The deal is upside down. This is the brutal reality most novice spreadsheets hide.

Making it cash flow

Tactics that move the needle: increase down payment to lower the mortgage payment (improves cash flow but hurts cash-on-cash return); negotiate price 5–10% below ask; add cheap value-add to lift rents 10–15%; self-manage to save the property management fee.

Each of these has trade-offs. The honest path is to model with conservative assumptions, then see whether the deal pencils at the assumptions you trust. If it doesn't, pass — there's no virtue in forcing a bad deal.

Tools

Use the payment calculator to nail down PITI on the financing side. Use the house hacking calculator for owner-occupied multi-unit deals. Build the operating expense layer in a spreadsheet so you can vary assumptions and stress-test.