The short answer

The debt with the largest total monthly interest charge is not necessarily the best place for the next extra dollar. Total interest reflects both the balance and the rate. To compare the marginal benefit of extra principal payments under similar terms, look at the rate applied to that extra amount.

What makes this decision different?

A large mortgage or installment balance may generate a substantial dollar charge while a smaller revolving balance has a higher rate. The question for an extra payment is how much future interest that payment avoids. Account restrictions and prepayment terms still matter, so the rate comparison is a starting point rather than a complete instruction.

How can you apply the idea?

Use these steps to connect the strategy with your actual account terms and available money. Keep any unresolved assumptions clearly labeled.

  1. List both the balance and rate for each candidate account.
  2. Compare the interest effect on the same extra-payment amount.
  3. Check account-specific terms before acting on the ranking.

What should the forecast not hide?

Do not use this simplified example as an exact statement calculation. Daily balances, compounding conventions, transaction dates, and account terms affect actual charges. The example isolates the difference between total interest and interest per dollar.

How should you explain the result to yourself?

Write the comparison in terms of the same extra dollar amount over the same period. That keeps the focus on marginal cost and avoids being distracted by the larger account’s total charge. Preserve the actual statement interest as a separate measure of what the account cost during the past cycle. The two measures can both be correct while pointing to different observations. If the account has more than one rate, identify which balance the extra payment would actually reduce. Without that application detail, even a correct rate comparison may describe a payment effect the creditor will not apply.

Worked example · illustrative numbers

Illustrative example: compare the payment effect

Assume a $10,000 balance at 6% and a $1,000 balance at 24%. Simple monthly interest is about $50 and $20 respectively. Yet an extra $100 avoids about $0.50 of next-month interest on the first and $2 on the second. The larger total charge comes from the larger balance.

Put this into practice with Debtless

Debtless is a completely free iPhone debt app with snowball, avalanche, and hybrid projections. Use its local ledger to compare plans with your own figures, then make and verify payments directly with your creditors.

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Common questions

Should I ignore the dollar interest charge entirely?

No. It helps explain account cost and progress. Use the rate and terms when deciding where an equal extra payment has the larger effect.

How should you explain the result to yourself?

Write the comparison in terms of the same extra dollar amount over the same period. That keeps the focus on marginal cost and avoids being distracted by the larger account’s total charge.

Sources & further reading

General education for U.S. readers, not individualized financial, legal or tax advice. Examples are hypothetical; lender terms and actual interest calculations can differ. Check your current statements and agreements.

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