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What is cash flow?

LearningWhat is cash flow?

Cash flow is what is left each month after rent (and other income) covers operating expenses and the mortgage. Positive cash flow means the property is estimated to pay its own way under your assumptions; negative means you may need to cover a shortfall out of pocket.

It is one of the first metrics investors check because it answers a simple question: after the bills and the loan payment, does this property put money in your pocket or take money out?

Monthly cash flow, step by stepIllustrative example — rent in, costs out, what remains.
  1. Gross rent$2,500
    Running total $2,500
  2. − Vacancy−$125
    Running total $2,375
  3. − Operating expenses−$800
    Running total $1,575
  4. − Mortgage (P&I)−$1,200
    Running total $375
  5. Cash flow+$375
    Left in your pocket each month

Sample only: $2,500 rent − $125 vacancy − $800 expenses − $1,200 mortgage = $375 monthly cash flow.

What goes into cash flow

Start with gross income—rent plus any other income you model. Subtract vacancy, then operating expenses such as taxes, insurance, maintenance, and management. Finally subtract debt service (principal and interest, and PMI when it applies). What remains is monthly cash flow.

Small changes in rent, vacancy, taxes, or interest rate can swing the result. Treat listing rents and tax figures as starting points until you verify them locally.

How to use it

Compare cash flow next to DSCR and cash-on-cash return. Thin or negative cash flow with weak debt coverage is a warning even when cap rate looks attractive on paper. In RentStack, open the analysis outputs and stress the offer with price sensitivity and break-even views before you rely on a single cash-flow number.

Key considerations

Cash flow is only as good as the assumptions behind it. Keep these points in mind when you read the number:

  • Optimistic rent or understated vacancy can make cash flow look healthy on paper and fail once the property is leased.
  • Taxes, insurance, and maintenance often rise over time—model a cushion, not a razor-thin surplus.
  • Interest rate, loan term, and PMI change debt service and can wipe out cash flow even when NOI is solid.
  • One strong month does not equal a durable deal; stress rent down and expenses up before you rely on the estimate.
  • Positive cash flow still is not advice—verify local comps, costs, and financing with your own research and professionals.

FAQ

Is cash flow the same as profit?
Not exactly. Cash flow is money left after operating costs and the mortgage in a given period. Profit (or taxable income) can differ because of depreciation, principal paydown, and other accounting items that are not the same as cash in your pocket.
Does positive cash flow mean the deal is good?
It is a strong signal that the property covers its costs under your assumptions, but it is not enough alone. Check DSCR, cash-on-cash return, how sensitive the result is to rent and expenses, and whether your inputs match local reality.
Why can cash flow be negative when cap rate looks fine?
Cap rate is based on NOI before debt. Heavy leverage, a high interest rate, or PMI can leave little or nothing after the mortgage even when the unlevered income yield looks acceptable.
Should I include vacancy in cash flow?
Yes. Modeling full occupancy every month usually overstates income. A vacancy (or credit-loss) assumption makes the cash flow estimate more realistic before you underwrite a purchase.