Debt snowball vs avalanche: the actual maths
There are two well-known ways to clear multiple debts, and the internet loves to argue about them. The honest answer is that they differ only in the order you attack — and understanding the maths tells you which suits you.
The two methods in one line each
- Debt snowball: pay the smallest balance first (while paying the minimum on everything else). When it clears, roll its payment onto the next-smallest.
- Debt avalanche: pay the highest interest rate first. When it clears, roll its payment onto the next-highest rate.
In both methods you put the same total toward debt each month. The only thing that changes is which debt gets the spare money after minimums. That single difference is the whole debate.
Why the avalanche costs less
Interest is charged on a balance at its rate, so the debt doing the most damage each month is the one with the highest rate, not the biggest balance. A simple way to see this is to estimate a month's interest as balance × APR ÷ 12. Take three debts:
| Debt | Balance | APR | Interest this month |
|---|---|---|---|
| Overdraft | £600 | 15% | £7.50 |
| Store card | £1,200 | 34% | £34.00 |
| Personal loan | £4,000 | 9% | £30.00 |
The store card is the smallest balance except the overdraft — but at 34% it is quietly costing £34 a month, more than the £4,000 loan. The avalanche attacks the store card first because it is the most expensive, so every spare pound kills the fastest-growing interest. Across the whole payoff, that means you pay less interest in total and finish sooner. Mathematically, the avalanche is always the cheapest route.
Why the snowball still wins for many people
The snowball would clear the £600 overdraft first — the smallest balance — even though the store card is more expensive. That gives you a whole debt gone quickly: one fewer payment, one fewer login, a visible early win. For a lot of people that momentum is the difference between sticking with a plan and giving up. The extra interest you pay for that motivation is often small in the scheme of things.
The power of either method is the roll-over: when one debt is cleared, its payment is added to the next target, so your monthly firepower grows as you go. A payoff plan that does not roll the freed-up payment onto the next debt is leaving its main advantage on the table.
So which should you pick?
- Want to pay the least interest and finish fastest, and you are disciplined about numbers? Choose the avalanche.
- Need motivation and quick wins to keep going? Choose the snowball — the small extra cost buys momentum.
- Not sure? Run both, compare the numbers side by side, and pick the one you will actually stick to. The best method is the one you finish.
Free, impartial debt advice is available from MoneyHelper (backed by the government's Money and Pensions Service), StepChange, Citizens Advice and National Debtline. gov.uk/debt-advice lists them. A spreadsheet is a planning tool, not a substitute for regulated advice.
Sources: gov.uk 'Get help with debt' and MoneyHelper for signposting; the interest illustration uses a standard balance × APR ÷ 12 estimate. General information, not financial advice.
General information, not tax or financial advice. Always confirm your own position with HMRC or a qualified adviser. This article was last checked against published gov.uk guidance on 13 July 2026. Rules and figures can change — always confirm your own position with HMRC or a qualified adviser.