Add every debt you're carrying. This runs a real month-by-month simulation of both the snowball method (smallest balance first) and the avalanche method (highest interest rate first) side by side, so you can see the actual payoff date and total interest for each - not just the formula. Come back and update your balances any time - this remembers what you entered, right here in this browser.
The avalanche method almost always costs less in total interest, since it attacks your most expensive debt first - every month you delay paying down a high-rate balance is a month it keeps compounding against you. But a well-known finding on this topic, cited across financial research, is that people following the snowball method are more likely to actually finish, because eliminating a whole account early creates a real sense of progress the avalanche method doesn't give you until much later.
So the honest answer isn't "always use the mathematically optimal one" - it's that the method you'll actually stick with for the next year or two is usually worth more than the one that's a few percent cheaper on paper. This tool shows you both real numbers so that trade-off is a choice you're making on purpose, not a guess.
Both methods pay the minimum on every debt and roll the full freed-up payment into the next target the moment one debt is gone - that rolling mechanic is what makes payments accelerate over time under either approach. Treat the totals here as a close estimate rather than an exact figure: real card issuers vary slightly in how they time interest and minimum payments, which is true of every calculator on this topic, not just this one.
This remembers your debts the same way everything else on this site works - entirely in your own browser, never sent anywhere. Nothing needs an account or a login; if you clear your browser data or switch devices, it starts fresh, same as any other local setting. There's also a plain "start fresh" option if you'd rather not have it remembered at all.