How this calculator works
Debt snowball and debt avalanche are the two most common structured payoff strategies. Both keep paying the minimum on every debt and add one fixed extra payment each month, but they disagree on where that extra payment goes first. Snowball targets whichever debt has the lowest balance, aiming for fast, motivating wins. Avalanche targets whichever debt has the highest interest rate, which is mathematically guaranteed to cost the least total interest for a given monthly budget.
The key mechanic in both strategies is the rollover: once a debt is paid off, its old minimum payment does not disappear — it gets added to the pile directed at the next targeted debt, so the total amount you pay every month stays the same from the first month to the last. This calculator simulates that rollover month by month for up to five debts at once, rather than approximating with a single blended payoff formula.
How each month is simulated
Every open debt accrues one month of interest: balance += balance × (APR ÷ 12)Every debt except the current target receives exactly its own minimum paymentThe target debt (lowest balance for snowball, highest APR for avalanche) receives its minimum payment plus everything left over from the fixed total monthly budget — including minimum payments freed up by debts already paid offThe 'target' is recalculated every month as balances change and debts get paid off. A payment is never allowed to push a balance below zero; any leftover from a debt that clears mid-month rolls into the following month rather than being redistributed within the same month.
Worked example: three debts, $150 extra per month
- Debt 1: $5,000 at 22% APR, $150 minimum. Debt 2: $8,000 at 6% APR, $200 minimum. Debt 3: $2,000 at 19% APR, $75 minimum. Extra payment: $150/month (total budget: $575/month).
- Snowball order (lowest balance first): Debt 3 ($2,000) is paid off in month 10, then Debt 1 ($5,000) in month 24, then Debt 2 ($8,000) in month 31. Total interest paid: about $2,439.
- Avalanche order (highest APR first): Debt 1 (22%) is paid off first, in month 21, then Debt 3 (19%) in month 24, then Debt 2 (6%) last, also in month 31. Total interest paid: about $2,364.
- Both strategies take the same 31 months to become debt-free here, but avalanche saves roughly $76 in interest by clearing the 22% balance before the 19% one.
Frequently asked questions
Does avalanche always pay less total interest than snowball?
Yes — for any fixed monthly budget split across the same debts, directing extra payments at the highest interest rate first is provably the interest-minimizing order. Snowball can only tie avalanche (never beat it), and it ties only in edge cases such as every debt sharing the same APR or there being only one debt.
Then why would anyone use snowball?
Behavioral research on debt payoff (including a widely cited 2012 study) found that people who clear their smallest balances first are more likely to stay motivated and pay off all their debt, even though it usually costs a bit more in interest than avalanche. Snowball trades a small amount of interest for earlier, more frequent 'wins.'
What if my minimum payments don't cover the interest?
If the fixed monthly budget (all minimums plus the extra payment) can't outpace the combined interest charged across your debts, no debt-clearing strategy will ever finish, and the calculator reports that directly instead of showing a misleading payoff date. Increasing the extra payment amount is the only fix.
Can I use this with fewer than five debts?
Yes. Leave any debt's balance at 0 and it is skipped entirely — the calculator simulates only the debts you actually enter, from one up to five.
Does the order debts are paid off change how much interest I pay if two debts have the same APR?
It usually makes only a small difference. When APRs are tied, there is no interest-minimizing reason to prefer one order, so snowball and avalanche land close together, though the exact figures can differ slightly depending on which debt happens to clear first.