Two popular ways to attack multiple debts disagree about the order. The snowball clears the smallest balance first for quick wins; the avalanche clears the highest interest rate first to minimise total interest. This planner runs both strategies month by month on your actual balances, rates and minimums, then shows which finishes first, what each costs in interest, and the month each individual debt disappears.
How the simulation works
Each simulated month does the same three things. First, interest accrues on every unpaid balance at its own monthly rate (APR divided by 12). Second, every debt except the current target receives its minimum payment. Third, the target debt receives its minimum plus the entire extra pool — your extra payment plus the minimums freed up by debts already cleared, which is what makes the payment snowball grow. If a target's payment cannot cover even its monthly interest, the plan stops and tells you so instead of pretending. Anything beyond a hundred years is likewise reported as unpayable rather than graphed.
A worked example
Take three debts: a 5,200 credit-card balance at 22.9% with a 156 minimum, a 12,400 car loan at 7.2% with a 285 minimum, and a 27,500 student loan at 5.8% with a 310 minimum. Paying only minimums clears everything in 77 months at a cost of about 11,880 in interest. Add an extra 350 a month and both strategies finish in 47 months at about 6,204 interest — roughly 30 months and 5,700 saved — clearing the card in month 12, the car in month 25 and the student loan in month 47. Here both strategies agree, because the highest rate also sits on the smallest balance; when a large balance carries the highest rate, the avalanche pulls ahead. To sanity-check any single loan's schedule first, run it through our loan calculator, and remember that interest working against you here is the same force that works for you in a compound interest investment.
What the plan leaves out
The simulation assumes fixed rates, fixed minimums and no new spending — real life is messier. It does not model annual fees, late penalties, penalty APRs, variable rates that move, balance-transfer promotions that expire, or tax effects such as student-loan interest deductibility. Keep a small emergency buffer before throwing every spare unit at debt: clearing a card and then borrowing it back at the same rate is motion without progress. Nothing here is financial advice; it is arithmetic on the figures you supply.
Snowball or avalanche — which to pick
On the same monthly budget the avalanche always ends with equal or lower total interest, and usually finishes at the same time or sooner, because less money is lost to interest along the way. The snowball's advantage is behavioural: clearing a whole debt early is visible proof of progress, and for some people that is what keeps the plan alive past month six. If the numbers say avalanche but you know yourself well enough to doubt you will stick with it, the strategy you actually follow beats the optimal one you abandon.