Math wins vs motivation wins.

Two strategies, one goal

You have multiple debts. You can only make minimum payments on some and extra on one. Which one do you overpay? Two schools of thoughtavalanche says pay the highest-interest debt first (mathematically optimal), snowball says pay the smallest balance first (behaviourally optimal). Both work. The interesting question isn't which is "right" — it's which one you'll actually stick to for 18 months.

The math: avalanche always wins on paper

Here's a realistic scenario. Three debts: 8,000 at 22% (credit card), 5,000 at 9% (personal loan), 2,500 at 18% (store card). Total 15,500. You can put 500 a month toward debts beyond the minimums. Avalanche (tackle the 22% card first) clears everything in 38 months with about 3,800 in interest paid. Snowball (tackle the 2,500 store card first) clears in 40 months with about 4,100 in interest. Avalanche saves 300 and finishes two months sooner. That's the gap the math sees.

The behavioural counterpoint

Research from Kellogg School of Management (Gal & McShane, 2012) found that people on the snowball plan were 15–20% more likely to actually complete debt payoff. The small early wins — clearing one debt entirely in four months, then another — kept motivation alive. Avalanche payers often stalled on the big, high-rate debt and lost momentum. The 300 of "extra" interest avalanche would have saved doesn't help if you quit the plan halfway through and never finish.

When to actually use which

Some honest heuristics:

Use snowball if this is your first serious attempt at paying down debt, you've felt overwhelmed by the total before, or your smallest debt is genuinely small (under 1,500). The psychological lift of clearing a balance entirely is worth the small extra interest.

Use avalanche if your smallest debt is large enough that clearing it would take 6+ months anyway (so there's no quick-win advantage), your highest-rate debt is meaningfully higher than the others (20%+ vs 8%), or you're the kind of person who's motivated by running the optimal math rather than seeing progress.

Use a hybrid if one of your debts is at a very high rate (store cards often run 30%+) — tackle that one first on avalanche grounds, then switch to snowball for the remainder. Nothing says you typically need to pick one method and stick with it.

What both methods require

Neither plan works if the debts keep growing. The calculator above assumes you stop adding to the balances you're paying down. If you're still spending on the same card you're overpaying, the payoff date is a moving target and the plan fails. Before running either strategy, the non-negotiable step is: stop the bleeding. Cut the cards up, switch to a debit card, remove saved payment details from retailer sites. You can't calculate your way out of active overspending.

A third option: balance transfer

Before picking snowball or avalanche, check whether a 0% balance transfer card is available to you. Moving 8,000 from a 22% card onto a 0%-for-24-months transfer card (with a typical 3% fee = 240) saves roughly 1,500 in interest over two years if you clear it within the promotional period. That's a bigger gain than choosing between snowball and avalanche produces. The catch: you need the discipline to clear it before the promotional rate ends, otherwise you've added a fee to a debt that's now at the post-promo rate (often 22%+).

Reading the calculator output honestly

The payoff date the tool shows assumes every payment lands on time at the stated amount. Real life includes months where the car needs new tyres or the washing machine dies. Build a 5–10% buffer into your timeline expectations, and celebrate finishing 2 months early if you do.

What comes after the last debt is cleared

The most common mistake at the end of a debt payoff is letting the monthly amount you were putting toward debt just quietly re-merge with general spending. Redirect it the day the last balance hits zero: half to an emergency fund until you have 3 months of expenses saved, then all of it into a pension or tax-advantaged savings account. The habit of paying 500 a month is more valuable than any individual pound of it. Losing the habit is how people end up back in debt two years later.

What this calculator doesn't capture

The math assumes fixed rates and steady payments. Real debt journeys include rate changes on variable cards, promotional rate resets, fee changes, and missed payments. The figure here is the best-case version of each plan. Use it to compare the two strategies, not to predict the exact month you'll be free.