Same engine, different target
Which debt gets your spare money first? That is the entire disagreement between the two strategies this page compares. Everything else is identical: each month every debt accrues interest at its own rate (the APR divided by twelve), every minimum payment goes out, and whatever extra you can afford lands on a single target. Avalanche aims at the highest APR, because that balance is growing fastest. Snowball aims at the smallest balance, because closing an account entirely is a win you can feel.
Both run on the same engine underneath: the roll-over. A cleared debt's minimum does not drift back into your current account; it joins the extra payment attacking the next target. Payments accelerate as accounts fall, which is why the last debt on a list often vanishes far faster than its size suggests.
One assumption is doing more work here than any other: the model believes you keep paying. New spending on a card you are supposed to be clearing is the single most common reason a real payoff plan misses its date, and no ordering strategy survives it. The APRs are held fixed too, and real borrowing rarely obliges. Cards reprice, promotional 0% windows expire, and variable rates move, broadly tracking the Bank of England's Bank Rate; a rate change mid-plan shifts the answer.
Avalanche always wins on pure interest. It has to, since money is directed at the fastest-growing balance first. But the margin is usually smaller than people expect, and the case for snowball is not sentimental: a study of actual repayment behaviour found people who cleared small balances first were more likely to stay the course. So if the interest difference between your debts is large, take avalanche. If you have struggled to stay motivated before, take snowball. A plan you finish beats a mathematically perfect one you abandon in month four. If you want the steps rather than the argument, the walkthrough covers entering the balances and reading the date that comes back. And if the minimums themselves are the struggle, ordering is not the problem; free advice from a debt charity such as StepChange is the better first step.
Sam, £250 and one month's difference
Sam owes on old store finance (£1,300 at 9.9% APR, £30 minimum), a credit card (£6,400 at 26.9%, £130 minimum) and a car loan (£7,900 at 6.4%, £170 minimum), and can put £250 a month above the minimums.
Here the two strategies genuinely part company. Avalanche sends the £250 at the credit card first, as the dearest debt by far, and finishes everything in 32 months with £2,851 of interest paid. Snowball clears the small store balance first for the quick win, which leaves the 26.9% card compounding for longer, and finishes in 33 months with £3,256.
One month and about £405 apart. Real, but far smaller than the extra payment itself: drop Sam's £250 to £100 and avalanche needs 49 months while snowball needs 51, with interest north of £5,200 either way. How much you send beats where you send it. The gap between strategies widens as the rate spread grows; push the dearest card's balance and rate higher and avalanche's advantage runs into four figures, so run your own numbers above; the tool shows both strategies side by side and you never have to take a rule of thumb on faith.
What the projection leaves out, add mentally. Minimums are held constant even though most card minimums are a percentage of the balance and fall as you repay; holding them flat is the conservative choice, and it slightly understates how quickly the roll-over accelerates. Late fees, annual fees and over-limit charges are not modelled. Neither are balance transfers, though moving a balance to 0% can beat both strategies outright, at the cost of a transfer fee and the discipline to clear it before the window closes.
What else the spare £250 could do
Before attacking debt hard, most planners keep a small buffer, often around one month of essentials, so an unexpected bill does not go straight back onto the card; the emergency fund calculator sizes that buffer. As for clearing debt versus investing instead, compare the debt's APR against a realistic after-tax return. A 22% card is an unbeatable guaranteed return; a 2.1% student loan usually is not. The mortgage overpayment tool runs that same comparison for secured borrowing.
And the payoff date reaches further than the debt itself. Payments that end become money that can be invested, which is exactly what the FIRE calculator projects forward.