If you have more than one debt, there are two well-known strategies for which one to attack first: the debt snowball and the debt avalanche. Both work the same basic way underneath: pay the minimum on everything, then throw every extra dollar at one target debt at a time. The difference is which debt you pick as the target. Snowball goes after your smallest balance first. Avalanche goes after your highest interest rate first. Here's what that difference actually costs, with real numbers.
A real example: two cards, $200 extra a month
Using the exact inputs from the debt snowball vs. avalanche calculator: a $6,000 balance at 24% APR with a $150 minimum payment, and a $2,000 balance at 10% APR with a $50 minimum payment. $200 extra a month to put toward whichever one you're targeting.
Snowball (smallest balance first, the $2,000 card): both debts are paid off in about 26 months, for $2,147 in total interest.
Avalanche (highest rate first, the $6,000 card): both debts are paid off in about 25 months, for $1,735 in total interest.
Avalanche wins here by about 1 month and $412 in interest. That's the general pattern: avalanche almost always costs less in total interest, because it's specifically targeting the debt that's charging you the most per dollar owed, while snowball can let a high-rate balance keep accruing interest longer if it happens to be the bigger one.
How the "roll forward" works
Both strategies share an important mechanic: when your target debt gets paid off, its minimum payment doesn't disappear, it rolls forward into the extra amount you're putting toward the next target. So your total monthly payment across all your debts never actually changes, only where the extra money is aimed shifts over time. That's what makes both strategies accelerate as you go: each payoff frees up more money to attack the next one.
So why would anyone pick snowball if it costs more?
Momentum. Clearing an entire balance, even a small one, is a visible, countable win. For some people that win is what keeps them actually sticking to a payoff plan instead of losing motivation halfway through. If knowing you'll see your first debt disappear in a few months (instead of over a year) is the difference between following through and giving up, that psychological benefit can be worth more than the extra interest in practice, even though the math says otherwise on paper.
This calculator exists so that trade-off is a choice you make with full information, not a guess. Run your own debts through it and see the real dollar gap between the two strategies for your specific numbers. Sometimes it's a few hundred dollars, like the example above. Sometimes, if your smallest balance and your highest rate are the same debt, there's no gap at all.
What this doesn't model
The calculator assumes a fixed payoff order decided at the start (by original balance for snowball, by APR for avalanche), which is the standard way both strategies are presented. It doesn't account for new charges, fees, or missed payments, and interest is calculated monthly on the remaining balance using each debt's own APR. If a required minimum payment on one of your debts is too small to even cover that month's interest, the calculator will flag that specific debt instead of showing a runaway number that never actually gets paid off.
What if you have more than two debts?
The calculator handles up to five debts at once, which covers the overwhelming majority of real situations, a couple of credit cards, maybe a personal loan, maybe a store card. The mechanic scales the same way regardless of how many debts you enter: pay every minimum, then stack the extra amount on whichever single debt the strategy says to target first, and move to the next one once that target hits zero. With more debts in the mix, the gap between snowball and avalanche tends to grow, since there are more opportunities for the two orderings to diverge from each other.
Picking a strategy that matches how you actually behave
It's worth being honest with yourself about which strategy you'll actually stick with for the full payoff period, which in the example above is over two years either way. If you've tried debt payoff plans before and lost momentum partway through, the visible wins from snowball might be worth more to you in practice than the $412 avalanche would save in this example. If you're confident you'll stay consistent regardless of which debt disappears first, avalanche is the mathematically better default. There's also a middle option some people use informally: start with avalanche, but if one debt is both small and annoying (not necessarily the highest rate), knock it out first for the mental win, then switch to strict avalanche ordering for the rest.
The takeaway
Avalanche usually saves more money because it directly targets your most expensive debt first. Snowball can feel more motivating because a balance disappears sooner. Neither is wrong, they're optimizing for different things (lowest cost versus fastest early win). What matters is knowing the real size of that trade-off for your own debts instead of guessing. Enter your balances, rates and minimums into the debt snowball vs. avalanche calculator and see which strategy, and how big a difference, applies to you.
This is an educational estimate, not financial advice. See the disclaimer for more.
More money math in my book. Check out Money Math for Your 20s.