Debt Avalanche Calculator

The avalanche method is the cost-minimizing solution to a constrained allocation problem. Given a fixed total monthly payment, every debt must receive at least its contractual minimum, and whatever remains is discretionary. Because interest accrues as balance multiplied by the periodic rate, a dollar directed at a 22 percent card avoids 22 cents of annual interest while the same dollar against a 5.5 percent student loan avoids only 5.5 cents. Ordering by annual percentage rate, highest first, and sending the entire surplus to the top-ranked debt therefore minimizes total interest across the whole schedule. Balance size is irrelevant to that ranking - only the rate matters, because the extra dollar earns the same avoided-interest return regardless of how large the balance behind it is. As each debt clears, its freed minimum payment rolls into the surplus, so the payment applied to the next target grows over time even though total household outlay stays flat. The size of the advantage over balance-ordered repayment depends entirely on rate dispersion. When one debt carries 24 percent and another 4 percent, avalanche can save thousands of dollars and several months; when every debt sits between 5 and 7 percent, the two orderings differ by a rounding error and the choice does not matter financially. Several modeling caveats apply. This simulation holds minimum payments fixed, whereas card issuers typically recalculate minimums as a percentage of the outstanding balance, which lowers the required payment as balances fall and slows real-world repayment for anyone paying only the minimum. It also assumes fixed rates, no new borrowing, no fees or penalty APRs, and payment applied at a consistent monthly interval. Promotional zero-percent balances that revert to a high rate on a known date are not handled by a static ranking and need separate treatment.

Ordering debts by interest rate is not an invention of consumer finance so much as an application of much older results. The mathematics of compound interest and amortized repayment were formalized in the seventeenth and eighteenth centuries, and by the twentieth century capital budgeting theory - notably Irving Fisher's rate-of-return framework and its successors - had made explicit the principle that scarce funds should flow to the use with the highest marginal return. Retiring a liability at rate r is exactly equivalent to a risk-free investment yielding r, so the optimal ordering falls out directly. What did not exist for most of that period was a household problem to apply it to. The consumer version emerged with unsecured revolving credit. BankAmericard launched in 1958 and the Interbank network that became Mastercard followed in 1966, and by the 1980s and 1990s American households routinely held several simultaneous obligations at sharply different rates - a mortgage near single digits, an auto loan somewhat higher, and revolving card balances often above 18 percent. That dispersion is what made ordering consequential. The Truth in Lending Act of 1968 mandated disclosure of the annual percentage rate in a comparable form, which is precisely the input the method needs, and the Credit CARD Act of 2009 later required issuers to apply payments above the minimum to the highest-rate balance within a single account, embedding the same logic in law. The term avalanche itself is retrospective, coined in personal finance writing during the 2000s as a contrast to the already-popular snowball label.