Debt Snowball Calculator

How the debt snowball works

With the snowball method you pay the minimum on every debt, and throw every spare dollar at the smallest balance. When it’s gone, its freed-up minimum “snowballs” onto the next-smallest debt. Your total monthly outlay never changes — but the amount hitting the target debt grows with every payoff.

Note what that means: the snowball does something even when you have no extra money. With the three demo debts above and nothing extra, freed-up minimums alone still clear everything in 3 years 3 mo rather than letting each debt crawl to its own finish line.

Where the interest actually comes from

Every month each balance grows by its own monthly rate before your payment lands:

That single line explains the whole problem. The card’s $70 minimum looks like a payment, but $49.98 of it is rent on money you already spent — only $20.02 touches the principal in month one. At that pace the balance falls by about 0.8% a month while interest reloads, which is why minimum payments feel like running on a treadmill. They are.

Two things break the cycle, and the snowball uses both. First, a level payment: as the balance shrinks, the interest portion shrinks with it, so a fixed payment sends a bigger share to principal every single month — the effect compounds in your favour. Second, concentration: pointing the spare money at one balance instead of spreading it retires an account, and a retired account’s minimum is money you keep forever after.

Your statement is required by law to tell you how bad the first mechanism is on its own. Regulation Z obliges every card issuer to print, in bold: “Minimum Payment Warning: If you make only the minimum payment each period, you will pay more in interest and it will take you longer to pay off your balance” — alongside an estimate of how many years that would take. That estimate assumes minimum payments only, which is precisely the scenario this calculator is built to get you out of.

What each extra dollar actually buys

The demo debts total $12,100 with $380 of combined minimums. Here is what adding a fixed extra payment on top does — every row computed by the same engine as the calculator above:

The first dollars are worth the most. The initial $25 a month buys 3 months and $360.08 of interest. Going from $150 to $300 — twice as much money — buys only 6 further months and $408.21 more. If a big extra payment is not realistic, a small one is still worth starting: this curve is steepest at the beginning.

Why choose snowball over avalanche?

Mathematically the avalanche method (highest APR first) always costs the same or less interest — never more. So the honest question is not which is better in theory, but what the snowball’s version costs you in practice:

On these numbers, at $150 extra a month, choosing the snowball costs $120.68 more interest over the whole plan and not one extra month. Whether that is a good trade depends on you, not on arithmetic — and the row where the snowball costs nothing at all is worth noticing: when your smallest balance happens to also carry your highest rate, the two methods produce the identical plan and the choice is moot. Run your own numbers above before assuming you face a trade-off.

The snowball’s advantage is behavioural, and there is real evidence for it rather than folklore. Kettle, Trudel, Blanchard and Häubl, publishing in the Journal of Consumer Research (2016), found that “concentrated (vs. dispersed) repayment strategies tend to boost consumers’ motivation to become debt free, leading them to repay their debts more aggressively” — and that the effect was strongest when repayments were focused on the smallest accounts, because people judge their progress by the largest proportional reduction in any one balance. That is a description of the snowball. Note the careful limit of the finding: it is about motivation and how hard people repay, not a claim that the snowball is cheaper. A plan you finish beats a plan you abandon; a plan you would have finished either way should just take the cheaper order.

When the snowball is the wrong choice

• A 0% promotional balance is about to expire. The snowball ignores rates, so it can happily leave a promo balance untouched until the promo ends and the full APR lands retroactively on some cards. Check the payoff month for that debt against the promo end date, and jump it up the queue if the dates collide.
• The APR gap is extreme. Between a $500 debt at 5% and a $5,000 debt at 30%, the ordering matters far more than the table above suggests. When one rate is several times another, run the avalanche and compare before choosing.
• A debt is in collections, or wages are being garnished. Legal exposure outranks both methods. So does anything secured by something you need — a car you drive to work is not just a balance.
• Tax debt and federal student loans. These carry their own penalty, interest and hardship rules that no generic payoff order handles well; treat them separately.
• You have no buffer at all. A plan with zero slack turns the next flat tyre into new card debt at 25%, undoing months of progress. Most practitioners build a small starter fund first — see the emergency fund calculator.

Worked example

Getting the inputs right

• Balance: today’s payoff balance from each account portal, not last month’s statement balance.
• APR: the interest rate on your statement (the purchase APR for cards). If a card carries separate purchase and cash-advance rates, the cash-advance balance is usually the costlier one.
• Minimum payment: use the current dollar amount from your statement. Card minimums shrink as balances fall; keeping your payment fixed instead — which is what this calculator assumes — is exactly what makes payoff fast.
• 0% promo APR: enter the promo rate, then check the payoff month against your promo end date, per the warning above.
• If your minimums alone do not cover the monthly interest, the calculator will tell you the balance never shrinks rather than quietly returning a payoff date. That is a real answer: the budget, not the ordering, is the problem.