Your Cap Rate Says Yes, Your Cash-on-Cash Says No: The Mortgage Constant Test for Rental Deals

Why a Good Cap Rate Can Still Be a Bad Rental

Most rental analysis starts with net operating income (NOI). NOI is your effective gross income (rent collected after vacancy and credit loss) minus operating expenses like taxes, insurance, repairs, management, and reserves. It deliberately excludes debt service, so it measures the property itself, not how you paid for it. That separation is useful, and it is also why NOI alone can mislead a buyer who plans to use a loan.

Cap rate is NOI divided by purchase price. A property producing $13,000 of NOI at a $200,000 price has a 6.5% cap rate. Cash-on-cash return is different: annual pre-tax cash flow (NOI minus your mortgage payments) divided by the total cash you put in, including down payment, closing costs, and any upfront repairs. Cap rate tells you what the building earns. Cash-on-cash tells you what your money earns.

The bridge between them is the mortgage constant, which is annual debt service divided by the loan amount. A $150,000 loan at 7% over 30 years costs about $998 a month, or roughly $11,975 a year. Divide that by $150,000 and you get a mortgage constant of about 7.98%. That number is the true annual cost of every borrowed dollar, because it includes principal paydown, not just interest.

The core rule is simple. When the cap rate is below the mortgage constant, leverage is negative. Each borrowed dollar costs more than it earns, and your cash-on-cash return drops below the cap rate. When the cap rate is above the mortgage constant, leverage is positive, the loan earns more than it costs, and cash-on-cash rises above the cap rate.

So that 6.5% cap rate property financed at a 7.98% constant is working against you. It might look fine on a listing sheet, and it might even be a solid all-cash buy. But with that loan, the more you borrow, the worse your return on cash gets. Before trusting any cap rate, compare it to your mortgage constant. That one comparison tells you which direction the loan pushes your return.

Worked Example: A $240,000 Duplex Bought All Cash vs. With a Loan

Take a duplex listed at $240,000, with each side renting for $1,200 a month. That's $28,800 in gross annual rent. Knock off 5% for vacancy ($1,440) and your effective gross income (EGI) lands at $27,360. Now subtract realistic operating expenses, not the optimistic ones on the listing sheet.

• Property taxes: $3,600
• Insurance: $1,800
• Management at 8% of EGI: $2,189
• Maintenance: $2,304
• Capex reserve: $1,440
• Water and trash: $1,200

Those total $12,533, leaving a net operating income (NOI) of $14,827. Divide by the $240,000 price and the cap rate is 6.18%. That's a respectable number in many markets, and plenty of investors would stop the analysis right here and make an offer.

Buy it all cash with $6,000 in closing costs and your cash-on-cash return is $14,827 / $246,000 = 6.03%. Closing costs drag it slightly below the cap rate, but the deal still behaves the way the cap rate suggests. Every dollar you put in earns about six cents a year.

Now finance it the way most investors actually buy. Put 25% down ($60,000), add the same $6,000 in closing costs, and your cash invested is $66,000. The $180,000 loan at 7.25% over 30 years costs $1,228 a month, or $14,735 a year. Subtract that from NOI and annual cash flow is just $92. Your cash-on-cash return is $92 / $66,000 = 0.14%, essentially zero on $66,000 of your money.

The reason is the mortgage constant: annual debt service divided by the loan amount, or $14,735 / $180,000 = 8.19%. Your borrowed money costs 8.19% a year while the property only earns 6.18% on its price. When the constant beats the cap rate, every borrowed dollar loses money, and leverage eats your return instead of amplifying it. That one comparison tells you the whole story before you ever open a spreadsheet.

Worked Example: Back-Solving the Price That Hits an 8% Cash-on-Cash

Instead of asking whether the duplex works at $240,000, flip the question around. Hold NOI at $14,827 and keep the loan terms identical: 25% down, 2.5% closing costs, 7.25% interest, 30 years, and an annual mortgage constant of 0.0818616. Then solve for the purchase price (P) that produces an 8% cash-on-cash return. This turns a vague "is it good?" into one number you can negotiate toward.

The setup is simple algebra. Annual debt service equals 0.75P x 0.0818616, or 0.061396P. Cash invested equals 0.275P (down payment plus closing), so 8% of that is 0.022P. Setting cash flow equal to that target gives $14,827 - 0.061396P = 0.022P, which rearranges to P = $14,827 / 0.083396. That works out to about $177,800.

Always verify a back-solved number before you trust it. At roughly $177,800:

• Debt service: about $10,916 per year
• Cash flow: $14,827 - $10,916 = $3,911
• Cash invested: about $48,892
• Cash-on-cash: $3,911 / $48,892 = 8.0%
• Cap rate: $14,827 / $177,800 = 8.34%

Notice the cap rate now sits above the 8.19% mortgage constant. That crossover is the whole story. Once the property yields more than the debt costs, every borrowed dollar adds to your return instead of eating it. At the $240,000 ask, the 6.18% cap rate sits well below the constant, which is why the loan drained nearly all the cash flow.

A "good negotiation" can still fall short. Talk the seller down to $205,000 and debt service drops to about $12,586, leaving $2,241 of cash flow on $56,375 invested. That is just under 4% cash-on-cash, half your target, because a 7.23% cap rate is still below the constant. Treat $177,800 like a flip MAO: it is your walk-away number for a rental, and anything above it means accepting negative leverage.

Edge Cases That Change the Verdict

The mortgage constant test only measures year-one cash flow, so it ignores the other ways a rental pays you. Each loan payment includes principal, and that paydown builds equity even when cash flow is thin. A $180,000 loan at 7% over 30 years has a 7.98% constant, but roughly $1,830 of the first year's $14,370 in payments goes to principal. Add appreciation and your total return can look healthy while your cash-on-cash looks weak. Decide which return you actually need before you call a deal dead.

Property taxes can move the verdict in either direction. Many counties reassess at the sale price, so the seller's tax bill is not your tax bill. If the seller paid $2,400 on an old assessment and your $240,000 purchase resets it to $4,200, your NOI drops $1,800 and the cap rate falls with it. Underwrite with the post-sale tax figure, never the listing's.

Interest-only and adjustable-rate loans lower the constant for a while. A 7% interest-only loan has a 7.0% constant, so a 7.5% cap rate flips to positive leverage on paper. That advantage ends when amortization starts or the rate adjusts, so test the deal at the fully amortizing payment too. A larger down payment works differently. It shrinks the dollars borrowed at a negative spread and pulls cash-on-cash closer to the cap rate, but it can never push cash-on-cash above the cap rate while the constant is higher.

Value-add deals are the biggest exception. If rehab raises rent, the cap rate on your all-in cost can clear the constant even when the purchase cap rate does not. That math depends on two numbers you cannot guess: the rehab cost and the post-repair value that sets your refinance loan. FlipSmrt returns a line-item rehab budget priced for the property's county and an ARV from adjusted sold comps within 0.5 miles. Plug those into your new NOI and refinance figures, and your first 3 analyses are free.

Mistakes That Make Negative Leverage Look Like a Deal

Negative leverage rarely looks negative on the listing sheet. It hides behind small errors that each nudge the numbers a little in your favor. Here is the $240,000 duplex again: $16,800 NOI (a 7% cap rate), a $180,000 loan at 7.5% over 30 years, and $15,103 in annual debt service. That gives a mortgage constant of 8.39%, which is higher than the cap rate, so the loan works against you.

1. Quoting cap rate on asking price, then buying with debt. A 7% cap feels solid until you compare it to the 8.39% constant. Skip that check and you never notice the $60,000 down payment earns about 2.8%.
2. Leaving out vacancy, capex, and management. Drop 5% vacancy ($1,800), capex ($2,400), and 8% management ($2,880), and NOI jumps to $23,880. That is a 9.95% "cap rate" that clears the constant on paper and fails in real life.
3. Putting debt service inside NOI. NOI is calculated before the mortgage. If you subtract the $15,103 first, you get a 0.7% cap rate, and then you subtract the mortgage again when you calculate cash flow.
4. Using the seller's tax bill. If the seller pays $2,600 on an old assessment and a reassessment at $240,000 and 1.8% brings it to $4,320, the extra $1,720 wipes out the entire $1,697 of cash flow.
5. Ignoring closing costs and reserves. Add $8,000 in closing costs and $6,000 in reserves and your cash invested becomes $74,000. Cash-on-cash falls from 2.8% to 2.3%.
6. Counting appreciation as cash flow. Three percent appreciation is $7,200 on paper. It does not cover a vacancy or a new roof, and you only collect it when you sell or refinance.

These errors stack. An investor who makes three of them can turn a 2.3% deal into what looks like a 10% deal. That investor finds out the difference only after closing, when the real expenses start coming in.

The fix is to follow a fixed order every time. Build NOI with every operating expense included. Re-estimate taxes at your purchase price. Divide by the price to get the cap rate, then compare it to the constant. Only after that, calculate cash-on-cash on your full cash invested.

FAQ: Cap Rate, Cash-on-Cash, and Leverage

What is a mortgage constant, and how do I calculate it quickly? The mortgage constant is your annual debt service divided by the loan amount. To get it fast, price a $100,000 loan at your quoted rate and term, multiply the monthly payment by 12, and divide by 100,000. A 30-year loan at 7% runs about $665.30 a month, or $7,984 a year, so the constant is roughly 7.98%. If the property's cap rate is below that number, every borrowed dollar costs more than it earns.

Is negative leverage ever acceptable? Sometimes, but only with a specific plan that closes the gap. A value-add deal where you can raise NOI within 12 months, or a purchase where you expect to refinance once rates drop, can justify a thin first year. Strong appreciation markets are the weakest reason, because appreciation is a forecast and your mortgage payment is a contract. If you accept negative leverage, write down the exact rent or NOI that flips it positive and the date you expect to hit it.

Should I compare cap rate or cash-on-cash when choosing between two rentals? Use cap rate to compare the properties themselves, since it ignores financing and tells you which asset produces more income per dollar of price. Use cash-on-cash to compare the deals you can actually execute, with your real down payment, rate, and closing costs. Two duplexes can share a 7.5% cap rate and land at very different cash-on-cash returns if one requires a higher-rate loan. When the two metrics disagree, cash-on-cash reflects what lands in your account.

How does a higher interest rate change the max price I can pay? It lowers it, because the cap rate you need rises with the constant. Say a property produces $18,000 of NOI. At 7%, staying above an 8% cap means paying no more than $225,000. At 8% interest, the constant climbs to about 8.81% ($733.76 monthly per $100,000), so your ceiling for positive leverage drops to about $204,300 ($18,000 / 0.0881).

The practical rule is simple. Compute the mortgage constant before you trust any cap rate on a financed deal, and treat that constant as the minimum cap rate the property must beat. It takes two minutes and a loan calculator, and it will save you from buying a property that looks great on paper and drains cash every month.