DSCR, what the lender actually computes, and why it is not the number you calculated

Debt service coverage ratio is one of the simpler numbers in real estate. Net operating income divided by debt service. If the property earns more than the loan costs, it is above 1.0.

It is also one of the most commonly miscalculated, and the miscalculation almost never comes from the division. It comes from the two numbers going into it. You and your lender can run the same ratio on the same property and disagree, because you are not using the same inputs.

The four places you and the lender diverge

1. Which rent. You are likely using the rent in place. An underwriter often uses the lesser of the rent in place and market rent from an appraiser's rent schedule. If your tenant is paying above market, the lender does not give you credit for it. If the unit is vacant, they use market rent with their own vacancy factor applied.

2. Which expenses. Your NOI probably reflects your own operating costs. The lender applies theirs, including a management fee whether or not you self-manage, a vacancy allowance whether or not you have a tenant, and a replacement reserve. Those three alone commonly take fifteen to twenty five percent off a self-managed owner's NOI.

3. Which payment. If the loan is interest-only, some lenders qualify it on the interest-only payment and some qualify on a fully amortising equivalent. The second is a larger number and a lower ratio on the same loan.

4. Whether taxes are reassessed. Underwriters generally use the post-sale tax figure. Sellers' pro formas generally do not. That single line moves the ratio more than anything else on this list.

What the cushion actually buys you

Lenders commonly want 1.20 or 1.25. It is easy to read that as bureaucracy. It is not. It is the answer to a specific question: how much can go wrong before the property stops paying its own loan?

At 1.00 the property covers the loan and nothing else. A single vacant month, a boiler, or an insurance renewal is paid out of your pocket, and there is no month in which the property earns it back.

At 1.25 the property produces a quarter more than the loan costs. That surplus is what absorbs the vacancy and the boiler. The ratio is not a hurdle the lender invented. It is roughly the amount of margin a small residential property needs in order to survive an ordinary year.

The trap in a DSCR loan specifically

A DSCR loan is underwritten on the property rather than on your income, which is why investors like it. The consequence is that the property has to carry the whole file. There is no salary behind it to reassure anyone.

So the ratio does not merely gate approval. It sizes the loan. When the ratio comes in under the threshold, the lender does not usually decline. They reduce the loan amount until the payment is small enough for the rent to cover, and you make up the difference in cash at closing.

That is the part that surprises people. The deal is not refused. It gets more expensive, quietly, a week before closing, and by then you have spent the inspection and appraisal money.

How to run it the way they will

1. Use market rent, or the rent in place if it is lower
2. Take out vacancy at the market rate, not zero
3. Take out management at the market rate, even if you manage it yourself
4. Take out a replacement reserve
5. Use the reassessed tax figure and a real insurance quote for this address
6. Divide by the annual payment, amortising, not interest-only

If the answer sits comfortably above the lender's threshold, you have a deal that survives a bad quarter. If it clears only when you assume no vacancy and no management, you do not have a financing problem. You have a property that only works while nothing goes wrong.

One number worth checking alongside it

DSCR asks whether the property covers the debt. It says nothing about whether the deal is worth doing.

A property can sit at 1.35 and still be a poor use of your money, because the ratio does not know what you paid, how much cash you put in, or what else you could have bought. Run cash-on-cash next to it. DSCR is a survival test. It is not a return.