Highest-rate-first (avalanche) always costs least; smallest-balance-first (snowball) clears debts sooner and keeps people going. Both camps are right about different things, so this calculator simulates your actual debts month by month under each — freed minimum payments rolling forward, interest compounding monthly — and prices the difference, so you choose with the number in front of you. Your figures never leave your browser.
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Two mechanics do most of the work in any payoff plan, and both are method-agnostic. First, fix your total payment: card minimums shrink as balances fall, silently stretching the payoff — paying the same total every month (as this simulation does) is the single biggest accelerant. Second, roll freed minimums forward: when a debt clears, its payment joins the attack on the next. The avalanche-vs-snowball choice is real but usually second-order to these — the calculator shows exactly how second-order, on your debts.
Where this fits a life plan: debt payoff is the highest guaranteed return most households ever see (clearing a 22.9% card is a 22.9% tax-free return), which is why it sits ahead of investing in every sensible ordering. The full Hundred Summers plan carries your debts inside the whole projection — interest charged yearly, payoff dates, and what clearing them frees for pensions, ISAs and everything after — and the mortgage overpayment calculator handles the biggest debt separately, where the answer is genuinely closer.
Hundred Summers