Debt service coverage ratio is the first thing a rental-property lender looks at, and it is a single division: net operating income divided by annual debt service. There is no weighting and no adjustment. Everything interesting is in what goes into the two numbers.
Take a $300,000 loan at 7.5 percent over 30 years. The monthly payment is $2,097.64, so annual debt service is $25,171.72. Against NOI of $24,000 the ratio is 0.953, which is a decline at essentially every lender because the property does not cover its own debt. Against NOI of $33,500 the same loan gives 1.331.
The thresholds are conventional rather than mathematical, but they cluster tightly. Below 1.0 the property loses money before you are paid at all. Between 1.0 and about 1.15 it is marginal. Roughly 1.15 to 1.25 is tight, 1.25 to 1.35 is comfortable, and above about 1.35 is strong. Most DSCR programmes want to see 1.25, and on the loan above that means NOI of about $31,465.
Here is the part that catches people. Improving the ratio by raising rent is slow and capped by the market, but the denominator moves fast and is under your control at origination. Annual debt service is driven by the payment, and the payment is driven by term and rate far more than by principal. Extending the term lowers the payment and lifts DSCR immediately, at the cost of much more total interest. Nothing about the property changed; the ratio improved because the denominator shrank.
That is also why DSCR flatters long-amortisation loans and penalises short ones, and why comparing two properties on DSCR alone tells you as much about their financing as about their operations. A property at 1.30 on a 30-year schedule and one at 1.30 on a 15-year schedule are not equally strong.
The other half is NOI, which excludes debt service and income tax but is after operating expenses, vacancy and management. Understating vacancy is the most common way a DSCR calculation comes out optimistic, because it inflates the numerator directly.
Every figure above was recomputed from the amortization formula rather than quoted from a table, and each worked example was cross-checked against the threshold band it falls in. These are illustrative scenarios, not lender quotes, and thresholds vary by programme.
A calculator that works DSCR from your own NOI and loan terms is at dscrradar.com.