What Is the Debt Service Ratio?
Debt service ratio compares income available for debt payments against the total payments due over a period, including both interest and principal. It measures whether obligations can be met as they fall due, which is the test that actually determines default rather than any measure of total indebtedness.
Default is a timing event. A borrower with manageable total debt fails if a payment falls due and the cash is not there, and this ratio is the one that measures exactly that.
How it works
Divide income available for debt service by total debt service for the same period. For a company that is usually operating income; for a household, income after essential expenses.
Both halves of the payment count. Interest coverage ratios look only at interest; this includes principal repayment, which is usually the larger amount on an amortising loan.
Below 1.0 the arithmetic fails. Income does not cover the payments, and the shortfall must come from reserves, new borrowing or asset sales.
Total debt and serviceability are different
A large debt spread over thirty years at a low rate produces small payments and a comfortable ratio, despite the headline amount looking alarming.
A smaller debt amortising over three years produces much larger payments and can be unserviceable from the same income.
So the structure of the debt matters as much as its size, and any measure looking only at the total misses the variable that determines the outcome.
A worked example
A property generating 120 of net operating income. Its mortgage requires 100 a year in interest and principal.
The ratio is 1.2. Lenders commonly require something in that region on commercial property, because it leaves a margin for vacancy and repairs.
A tenant leaves and income falls to 90. The ratio drops to 0.9, the owner is paying 10 a year out of their own pocket, and the covenant has probably been breached.
Nothing about the property’s value changed in that. It may be worth exactly what it was the day before, and the position has become untenable purely because of timing.
Where the ratio appears in ordinary life
Mortgage affordability. Lenders assess what proportion of income the payments consume, and regulatory limits in many countries now cap that proportion explicitly.
And they stress-test it. A borrower must typically demonstrate they could service the loan at an interest rate several percentage points above the current one, which is this ratio calculated against a hypothetical.
Household debt service ratios are tracked nationally. Central banks publish aggregate figures because the share of income going to debt payments predicts consumer spending and financial stress better than total household debt does.
Which is the same insight at a different scale. A country with high household debt and low rates can have a comfortable aggregate ratio; the vulnerability appears when rates rise and the payments reprice, with the debt unchanged throughout.
The original data
This site’s thirty-year fee measurement: 5 basis points costs 1.5% of the final balance, 20 costs 5.8%, 75 costs 20.2%, 150 costs 36.5%.
Interest rates work on debt the way fees work on savings, and the effect on a payment is immediate rather than gradual. A floating-rate loan repricing upward changes the denominator of this ratio directly, with no change in the amount borrowed.
And the drawdown figures show what income must survive: 95% of bars sit below a prior peak, the longest stretch running 73 bars. A ratio of 1.05 leaves no room for a period like that, which is why lenders require a margin rather than mere sufficiency.
What breaks it without warning
Rate resets. A floating-rate or fixed-for-a-period loan reprices on a date, and the payment can rise substantially while income does not.
The end of an interest-only period. A loan switching to full amortisation sees its required payment jump sharply, and the date was known from the outset and is frequently forgotten.
Balloon maturities. A loan with small payments and a large final amount assumes refinancing will be available, which is an assumption rather than a term of the loan.
And income volatility. The ratio is calculated on a period’s income, and a borrower whose income varies needs a much larger margin than one whose income is contractual — which is the single most common reason a ratio that looked adequate turns out not to be.
How lenders build a margin into it
Required minimums vary by asset. Commercial property lending commonly requires a ratio comfortably above 1.0, and riskier income streams require more.
Stress testing is standard. The ratio is recalculated at a higher assumed interest rate, or with income reduced by an assumed vacancy or downturn, and the loan is sized so it still passes.
Cash reserves are often required alongside. A borrower may have to hold several months of payments in a controlled account, which is a buffer for exactly the timing problem this ratio measures.
And the covenant is tested periodically. A ratio that falls below the threshold triggers consequences before any payment is actually missed, which is the point - the covenant exists to create a conversation while there is still time for one.
When it fails
The characteristic failure is a ratio measured before a reset. A borrower comfortably services a loan at 1.4, the fixed period ends, the rate reprices upward, and the same income now produces a ratio below 1.0. Nothing about the borrower changed, the debt did not grow, and the loan they could afford became one they cannot — on a date that was printed in the original documents. The ratio was accurate about a payment schedule that had an expiry, and almost nobody recalculates it for the schedule that follows.
A second failure is using interest coverage instead, which ignores principal and can look twice as healthy as reality.
A third is calculating it on peak income, leaving no margin for an ordinary bad period.
A fourth is ignoring a balloon payment, which assumes refinancing that may not be available.
And a fifth is treating 1.0 as adequate. It means zero margin, and no income stream is certain enough to justify that.
Related
Cash-flow-to-debt ratio covers the broader capacity question. Debt ratio covers total indebtedness rather than serviceability. And paying off debt covers the practical side of reducing what is owed.
Nobody defaults because their total debt was too large. They default because a payment came due and the money was not there. This ratio is the only common measure that asks that question directly, and it is the one most people skip past.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.