The short answer
Both methods pay the minimum on every debt, then throw every spare pound at one target debt until it is gone and roll that payment onto the next. The only difference is how they choose the target. The avalanche picks the highest interest rate. The snowball picks the smallest balance. The avalanche always wins on total interest, and the snowball always clears its first account sooner. On the worked example below the avalanche saves £617.38 and finishes 1 month earlier, while the snowball retires its first card 15 months before the avalanche retires anything.
The worked example
Arguments about these two methods are usually made without numbers, so here is a concrete case. Three revolving balances totalling £7,900, with £400 a month available for all of them combined. The example is built so the two methods disagree from the first month: the smallest balance carries the lowest rate and the largest balance carries the highest, which is the situation where the choice actually matters.
| Debt | Balance | APR | Opening minimum | Avalanche rank | Snowball rank |
|---|---|---|---|---|---|
| Card A | £900 | 18.9% | £25.00 | 3 | 1 |
| Card B | £2,400 | 27.9% | £60.00 | 2 | 2 |
| Store card | £4,600 | 34.9% | £115.00 | 1 | 3 |
The minimum payment is modelled the way a UK card sets one, at 2.5 percent of the balance or £25, whichever is larger. Interest is charged monthly at the APR divided by twelve, which is the same convention the credit card payoff calculator on this site uses, so a single debt checked there will agree with the figures here.
What each method costs
Running both plans month by month on identical inputs gives the following. The row that decides the argument is not the interest row on its own, it is the interest row read next to the first cleared row.
| Outcome | Avalanche | Snowball | Difference |
|---|---|---|---|
| Order of attack | Store card, then Card B, then Card A | Card A, then Card B, then Store card | Rate order vs balance order |
| Months to debt free | 2 years 4 months | 2 years 5 months | 1 month sooner on avalanche |
| Total interest paid | £3,059.26 | £3,676.65 | £617.38 cheaper on avalanche |
| Total repaid | £10,959.26 | £11,576.65 | £617.38 |
| First debt cleared | Month 20 | Month 5 | 15 months sooner on snowball |
The interest gap is £617.38 on £7,900 of debt, which is real money but is not the landslide the usual framing implies. It is about 17 percent of the total interest bill, and it buys you exactly 1 extra month of freedom. Against that, the snowball hands you a cleared account in month 5, while an avalanche follower is still chipping away at the Store card with nothing finished until month 20.
When each debt disappears
The payoff dates show why the two plans feel so different to live through even though they land within a month of each other.
| Debt | Cleared on avalanche | Cleared on snowball |
|---|---|---|
| Card A (18.9%) | Month 28 | Month 5 |
| Card B (27.9%) | Month 26 | Month 14 |
| Store card (34.9%) | Month 20 | Month 29 |
The snowball follower is down to a single debt by month 14. The avalanche follower still has all three open at that point. Neither is wrong. One is optimising the interest bill and the other is optimising the odds that you are still following the plan a year from now.
The gap shrinks as your budget grows
The most useful thing to know about this decision is that it matters least when you are paying the most. Running the same three debts at different monthly budgets shows the avalanche advantage steadily collapsing.
| Monthly budget | Avalanche months | Snowball months | Avalanche interest | Snowball interest | Avalanche saving |
|---|---|---|---|---|---|
| £300 | 44 | 48 | £5,205 | £6,332 | £1,127 |
| £350 | 34 | 36 | £3,832 | £4,630 | £798 |
| £400 | 28 | 29 | £3,059 | £3,677 | £617 |
| £450 | 24 | 25 | £2,558 | £3,062 | £504 |
| £500 | 21 | 22 | £2,205 | £2,630 | £425 |
| £600 | 17 | 17 | £1,737 | £2,062 | £325 |
At £300 a month the choice of method is worth £1,127. At £600 a month it is worth £325. Over the same range, raising what you pay each month cuts the interest bill by roughly £3,467. Finding another £50 a month is worth more than any amount of agonising over the ordering rule, which is the opposite of how most of the advice on this subject is weighted.
The behavioural case for the snowball
The snowball is not simply the wrong answer with better marketing. A 2016 study by Kettle, Trudel, Blanchard and Haubl in the Journal of Consumer Research looked at what actually keeps people repaying, and found that concentrating repayment on one balance, and the discrete event of an account closing, raised motivation and the likelihood of clearing the total debt. Progress that you can see beats progress that is merely optimal, because a large balance falling from £4,600 to £4,200 does not feel like anything, whereas an account disappearing does.
So the honest framing is a price. On this example the snowball costs £617.38 and 1 month, and it buys a completed debt 15 months earlier. If you have started and abandoned a repayment plan before, that is a reasonable thing to pay for. If you have never had trouble sticking to one, there is no reason not to take the cheaper route.
Where both methods go wrong
Neither ordering rule protects you from the three things that actually derail repayment. The first is new spending on a card you are paying down, which resets the whole plan silently. The second is having no cash buffer, so the next unexpected bill goes straight back on the card you just cleared. It is usually worth holding a small emergency fund before throwing everything at the balances, even though that is technically suboptimal on interest. The third is a promotional rate expiring, which can move a debt from the bottom of the avalanche order to the top overnight, so re-sort your list whenever a rate changes.
It is also worth checking whether the payment you are planning is affordable at all rather than just ambitious. If your total debt payments are eating a large share of your income, the ordering question is a secondary one. The debt to income ratio calculator gives you that share in one number, and what counts as a good ratio explains where the usual thresholds sit.
How to run your own numbers
List every debt with its balance and its APR, taking the APR from the statement rather than from memory, since the headline rate advertised on a card is often not the rate you were given. Sort the list by rate for the avalanche or by balance for the snowball. Pay every minimum, then put everything left on the debt at the top. When it clears, add its whole payment to the next one and do not let that money quietly return to general spending, because the rolling payment is what makes either method accelerate.
To put a figure on any single debt in that list, the credit card payoff calculator takes a balance, an APR and a fixed monthly payment and returns the months to clear it and the total interest. If the rate on your statement is quoted in more than one way, the guide to APR versus interest rate covers which figure to use. The full set of finance tools is on the guides index and the calculator list.
One last caveat on the figures above. They assume fixed rates, no new spending, no missed payments and no fees, which is how every payoff model works and is never quite how a real year works. Treat the £617.38 gap as the right order of magnitude for the choice, not as a promise.