Two orderings dominate discussion of multi-debt repayment. They agree on the total amount paid each month and disagree only on where the surplus goes, yet they produce different outcomes.
The two orderings
Both approaches assume minimum payments are made on every account, with a fixed surplus directed at one target account until it clears.
The avalanche directs the surplus at the highest interest rate first. The snowball directs it at the smallest balance first, regardless of rate.
When an account clears, its payment is folded into the surplus, so the amount attacking the next target grows each time a debt is retired.
Why the avalanche costs less arithmetically
Interest accrues on balances at their own rates. Money applied to the highest-rate balance removes the most future interest per unit paid.
Any other ordering leaves expensive balances outstanding for longer, so it necessarily costs more in total interest and usually takes marginally longer.
The arithmetic is not in dispute. If the plan is followed exactly to completion, the avalanche wins on cost every time.
Why the snowball survives longer in practice
Repayment plans fail through abandonment, not through arithmetic. A plan that is optimal but abandoned in month five returns nothing.
Clearing a small balance quickly removes an account, a statement and a payment from the picture. That visible reduction sustains the behaviour that the plan depends on.
The snowball also simplifies the situation faster. Fewer live accounts means fewer opportunities to miss a due date and trigger a fee.
Where the gap between them is small
The cost difference depends on how far apart the rates are and how far apart the balances are. Where those orderings roughly coincide, the two methods produce nearly identical schedules.
The gap widens when a large balance carries the highest rate while a small balance carries a low one, because the snowball then delays the expensive account considerably.
Even in unfavourable cases the difference is often modest relative to the total repaid, which is why the argument generates more heat than money.
What actually determines the outcome
The dominant variable is the size of the monthly surplus, not its destination. Increasing the amount directed at debt changes the timeline far more than reordering the queue.
The second variable is whether new borrowing continues alongside repayment. A plan competing with fresh spending on the same accounts makes little progress under either ordering.
Ordering is a genuine but second-order choice, and it is worth resolving quickly rather than treating as the central decision.