Hundred SummersDebt payoff calculator: avalanche vs snowball, priced
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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Worked example
Computed when this page was built by the same engine that runs the calculator above, so the figures below and the live tool can never disagree. Change any input above to see your own.
| Inputs | |
|---|---|
| Credit card | £4,200 at 22.9%, £150 a month |
| Car finance | £9,000 at 9.9%, £260 a month |
| Extra a month | £200 |
| Results | |
|---|---|
| Avalanche (highest rate first): debt-free in | 25 months, £1,811 interest |
| Snowball (smallest balance first): debt-free in | 25 months, £1,811 interest |
| Interest difference | £0 |
The next question
Where does the money go once the debt is cleared? →Household budget calculator
Or answer it with everything else: the full Hundred Summers plan connects your pay, pensions, savings, property, tax and spending year by year, and stress-tests them across 500 simulated market histories.
Read the guide
- Inheriting money: the order of operations →A lump sum arrives: inheritance, redundancy, a house sale. The unhurried sequence: debt, buffer, wrappers, the tax on what the money earns next, and the £2,880 trick for a non-earning partner.
The mechanics that actually matter
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.