With several debts and a fixed amount to put toward them, you make every minimum payment and then choose where the surplus goes. There are two common answers. The avalanche sends it to the highest interest rate. The snowball sends it to the smallest balance.
A worked comparison
Take three debts: $8,000 at 22 per cent, $2,500 at 18 per cent and $12,000 at 7 per cent, with minimums of $160, $75 and $220, and $300 a month spare.
The avalanche clears everything in 42 months with $4,331.49 of interest. The snowball takes 43 months and $4,807.74. The avalanche saves $476.25, and the snowball clears its first debt one month sooner.
Why the avalanche always wins on interest
This is not a close mathematical question. Interest accrues per dollar per month at whatever rate applies, so a dollar aimed at the 22 per cent debt kills more future interest than the same dollar aimed at the 7 per cent one. The avalanche is optimal by construction.
What varies is the size of the win. When the rates are close together, the two methods nearly coincide. When they are far apart, or the high-rate debt is also large, the avalanche pulls further ahead.
Why the snowball still gets recommended
The snowball's argument is behavioural, not arithmetic. Clearing an account entirely removes a payment, a statement and a due date, and that first completion arrives sooner. A plan you keep to imperfectly beats an optimal plan you abandon in month seven.
That is a real effect, and it is not something a calculator can weigh for you. What the calculator can do is price it. If the avalanche saves $476 over three and a half years, you can decide whether that is worth paying for the earlier sense of progress. If it saved $5,000, most people would answer differently.
A middle path
Nothing forces a single method for the whole journey. Clearing one small balance first for the momentum, then switching to strict rate order, captures most of the interest saving and most of the early win. The order only matters for the surplus; the minimums are paid either way.
Put your actual balances, rates and minimums into the debt payoff strategy calculator and read the gap. If it is small, choose the method you will stick to. If it is large, that is information worth having before deciding.