The REI Vault Pro Cash Flow Calculator: How to Find What a Rental Property Actually Puts in Your Pocket

Cash flow is the most misunderstood metric in buy-and-hold real estate investing. Most new investors calculate it as rent minus mortgage. That number is almost always wrong — and being wrong on cash flow projections is the most direct path from "I thought this was a cash-flowing rental" to "I am feeding this property out of my own pocket every month."

The REI Vault Pro Cash Flow Calculator runs the real formula — every expense category, properly modeled — so you know the true number before you close.

Why the Simple Formula Fails

Rent minus mortgage ignores everything that actually makes rentals expensive to operate: vacancy periods when the unit sits empty between tenants, property management fees if you ever stop self-managing, maintenance and repairs that occur every year, capital expenditure reserves for the roof and HVAC that will eventually need replacement, property taxes that increase, and insurance premiums that are rarely negligible.

A $1,800/month rental with a $1,200 mortgage payment does not cash flow $600/month. After a realistic 7% vacancy allowance, 10% property management, 1% annual maintenance, 8% CapEx reserve, taxes, and insurance, actual net cash flow on that property is often $150–$250/month — or zero. The Cash Flow Calculator shows you the real number.

What the Calculator Takes As Input

Financing Inputs

Purchase Price — what you are paying for the property. Down Payment % — your equity contribution, which determines the loan amount and monthly mortgage payment. Interest Rate — the rate on your investment property loan. These three inputs generate your monthly debt service, which is the largest single expense in most rental property models.

Income Inputs

Monthly Gross Rent — what the market will bear for the unit in its post-renovation condition. Pull comparable rentals within half a mile, same bed/bath count, similar condition. Be accurate, not optimistic. Vacancy Rate % — the percentage of the year the unit will sit empty. Use 7–10% for most stable rental markets. Do not use zero unless you have a long-term tenant in place with a strong renewal history.

Expense Inputs

Property Taxes (monthly) — use the actual current tax bill divided by 12, not an estimate. Tax bills are on the public record for every property. Insurance (monthly) — landlord policies for single-family homes typically run $100–$200/month. Multifamily is higher. Get a real quote if you are serious about the deal. Maintenance (monthly) — budget 1% of property value per year, divided by 12. A $200,000 property needs $167/month in maintenance reserves. Property Management % — even if you self-manage today, budget 8–10% of gross rent. If you ever stop managing yourself — due to scale, burnout, relocation, or any other reason — you need this line item already in your model. CapEx Reserves (monthly) — capital expenditure reserves for large replacement items: roof, HVAC, water heater, appliances, windows. Budget 8–10% of gross rents monthly. This is money you set aside now for the expense you will incur later.

What the Calculator Produces

Monthly and Annual Cash Flow

The bottom-line output: what the property actually puts in your pocket every month and every year after every expense and debt service payment. This is the number that determines whether the deal makes financial sense.

Cash-on-Cash Return

Your annual net cash flow divided by your total cash invested (down payment plus closing costs plus any initial repairs). Cash-on-cash return is the most meaningful performance metric for comparing buy-and-hold deals because it accounts for your leverage and actual capital deployed. Target 8–12% CoC in most markets.

Cap Rate

Net Operating Income divided by purchase price — the property's income performance independent of financing. Cap rate lets you compare this asset against any other investment, and it is the primary metric used by institutional buyers to value income-producing property.

Monthly NOI, Effective Rent, and Expense Breakdown

The calculator also returns your monthly Net Operating Income (NOI) — the income the property generates before debt service — your effective monthly rent after vacancy, your total monthly expenses broken out by category, and your monthly mortgage payment separately.

Having the full expense breakdown on one screen lets you see exactly where the money goes and identify which line items have the most impact on your returns.

Using the Calculator to Make Better Offers

Work Backward from Your Return Target

One of the most powerful uses of the Cash Flow Calculator is reverse engineering your maximum purchase price. Enter your target cash-on-cash return (say, 10%), your rent estimate, and your expense assumptions, and work backward to find the highest price at which the deal still hits your return threshold.

That number is your maximum offer — not what the seller is asking, not what comparable sales justify, but what the property can support at your return requirements.

Run Stress Tests

Before you close, run three scenarios: base case (your most likely assumptions), conservative case (rent 10% lower, vacancy 5% higher), and pessimistic case (rent 15% lower, 10% vacancy, management fee added even if you plan to self-manage). If the deal still cash flows positively under the pessimistic case, you have found a resilient investment.

Deals that only pencil under optimistic assumptions are not investments. They are speculative positions dressed up as cash-flowing rentals.

The Difference Between Knowing and Guessing

The investors who build durable rental portfolios are not the ones who got lucky on a few properties. They are the ones who analyzed every deal with the same rigorous, complete framework — and made offers based on what properties could actually support, not what they hoped they would produce.

The Cash Flow Calculator is that framework, built into a tool you can use on every deal in under five minutes.

Available to all Core and Pro members. Start your 7-day free trial and run your first property analysis today.