Both methods are right about different things
Avalanche attacks the highest interest rate first. It is arithmetically optimal, by definition: no other order can possibly cost less, because you are always killing the most expensive money first.
Snowball attacks the smallest balance first. It clears individual debts sooner, which means the list gets shorter faster, and it costs more.
Most planners pick a side and argue for it. That hides the only thing that actually matters, which is the size of the gap. If highest-rate-first saves you £2,000, that is a serious argument and you should probably take it. If it saves £40 across four years, then the method you will genuinely stick to is the better method, and saying otherwise is pretending people are spreadsheets.
So this tool gives you both totals, both timelines, and the difference. Then you choose with the number in front of you rather than being told.
The rolling payment is the whole mechanism
Neither method does anything on its own. What makes both work is what happens when a debt clears.
Its payment does not go away. It rolls onto the next debt, on top of that debt's own minimum. Your total outgoing stays completely flat, but the amount attacking the target grows every time something falls, so the plan accelerates as it goes. The last debt in the queue often disappears startlingly fast, because by then it is absorbing every payment that came before it.
This is also the single point of failure. If the freed-up payment gets absorbed into normal spending each time a debt clears, neither method works at all and the timeline stretches out badly. The discipline is not in the ordering, it is in keeping the total constant.
The surplus is what does the work
The minimums keep every account in order. Everything above the minimums is what actually reduces the debt in any meaningful way, and it all goes to one target at a time.
If your budget only covers the minimums, the ordering question is academic: nothing is being targeted and no method applies. Even a modest surplus changes this out of all proportion, because the whole of it lands on a single debt rather than being spread across all of them.
When the answer is not a calculator
If your budget will not cover the minimum payments, no ordering strategy fixes that and this tool says so rather than producing a hopeful number.
That is the point to speak to StepChange, National Debtline or Citizens Advice. They are free, they are not selling anything, and they can do things arithmetic cannot: negotiate with creditors, get interest frozen, and arrange formal solutions. Paying a company for debt advice is almost never necessary in the UK, because the free services are genuinely good.
A word on consolidation
Consolidating into one lower-rate loan can be a real improvement, particularly where cards are in the high twenties. Two things decide whether it works.
First, whether the rate is genuinely lower once the term is accounted for. A lower monthly payment over a much longer term can cost more in total while feeling like progress.
Second, and more importantly, whether the cards stay clear afterwards. The well-documented failure mode is that consolidation clears the balances, the accounts stay open, and a year later there is a loan and the card balances are back. Consolidation is a refinancing tool, not a solution to the spending that created the balances.
Common questions
Avalanche or snowball, which is better?
Avalanche, meaning highest interest rate first, always costs less. It is optimal by definition: no other order can be cheaper. Snowball, meaning smallest balance first, clears individual debts sooner and costs more. The useful question is not which wins but by how much, because a £40 gap across four years is not an argument for anything while a £2,000 gap is.
Why do most tools pick a side?
Because each is right about something different. Avalanche is right about arithmetic. Snowball is right about people, in that a plan somebody actually finishes beats a cheaper one they abandon. Showing only one number hides the trade-off, so this tool gives both totals and the gap between them and lets you decide with the figure in front of you.
What actually makes either method work?
The rolling payment. When a debt clears, its payment does not disappear: it moves onto the next debt on top of that debt’s own minimum. Your total outgoing stays flat while the plan accelerates, because each cleared debt makes the next one fall faster. If you spend the freed-up payment instead, neither method works and the timeline stretches out badly.
What if I can only just cover the minimums?
Then the order makes no difference, because nothing is being targeted. Every debt receives exactly its minimum and none of them is being attacked. Even a small surplus changes the shape of the plan a lot, because all of it lands on one debt rather than being spread thinly across all of them.
What if I cannot cover the minimums?
No ordering strategy fixes that, and a calculator is the wrong tool. Speak to a free debt advice service: StepChange, National Debtline or Citizens Advice. They are free, they are not selling anything, and they can do things that arithmetic cannot, including negotiating with creditors and stopping interest.
Should I consolidate instead?
Possibly, if the consolidated rate is genuinely lower and the term is not much longer. The risk is well documented: consolidation clears the cards, the cards remain open, and a year later there is a loan and the card balances have returned. Consolidation only helps if the underlying spending has changed too.