Both strategies pay the minimum on every debt, then throw every spare pound at exactly one target debt until it's gone — then roll its old minimum payment into the pool and move to the next target. Avalanche always targets the highest interest rate first, which is mathematically guaranteed to minimise total interest paid. Snowball always targets the smallest balance first — it can cost more in interest, but clearing a whole debt fast is a real psychological win that keeps people going, which is exactly why it's the more popular method despite costing more on paper.
A £2,000 balance at 5% APR alongside a £15,000 balance at 24% APR, with £400 extra a month: avalanche clears both in 30 months for £4,953 in interest; snowball takes 33 months and £6,135 — avalanche wins here because the small balance also happens to carry the low rate, so snowball spends longer exposed to the expensive one.
It always saves equal or more total interest than snowball — that's mathematically guaranteed, since it always attacks the balance accruing the most interest first. The two strategies only give an identical result when the smallest-balance debt also happens to be the highest-rate one.
Behavioural research on real payoff attempts (notably by Northwestern's Kellogg School) found people using snowball were more likely to actually finish paying off all their debts — the quick wins from fully closing an account sooner keep motivation up in a way a slowly-shrinking largest balance doesn't.
No — minimums are paid on every debt every month regardless of strategy. Only the extra payment (and, once a debt is cleared, its freed-up minimum) gets redirected based on which strategy you're using.
With £0 extra, this tool just simulates minimum-only payoff — useful on its own, since some minimum payment schedules barely outpace interest and can take a genuinely long time to clear.