Snowball vs. Avalanche Calculator

How to use this calculator

1. 1List each debt with its balance, APR, and required minimum payment — statements show all three.
2. 2Set the extra amount you can pay beyond all minimums combined (start with $100 if unsure; even that reshuffles the finish line).
3. 3Read the two debt-free dates and interest totals: snowball pays smallest balance first, avalanche pays highest APR first.
4. 4Check the payoff table — the month each debt disappears is where the strategies visibly diverge.
5. 5Try a bigger extra payment: raising the cash almost always matters more than the ordering choice, and the chart shows both effects at once.

How the calculation works

each month, per debt:  interest = balance × APR ÷ 12  (rounded to cents)
cash pool = Σ minimums + extra  (constant while debts remain)
paymentᵢ = min(minimumᵢ + pool shareᵢ, balanceᵢ + interestᵢ)
surplus rolls to the next debt in line the same month

There is no closed-form formula — both methods are month-by-month simulations that differ only in the attack order, fixed before month one. Snowball orders debts by smallest balance (ties: higher APR first); avalanche orders by highest APR (ties: smaller balance first). Every debt is paid its minimum; the extra payment plus any freed minimums cascade onto the target debt until it hits exactly zero, then the next. A plan is impossible when, with balances remaining, the month’s total interest eats the entire cash pool — that is what the error message reports. Full conventions and the golden cross-check live in docs/FINANCIAL_FORMULAS.md.

Worked example

Assumptions and important notes

• Minimum payments stay at the entered amounts for the whole payoff. Real credit card minimums decline with the balance, which shortens issuers’ own projections — this model is deliberately constant so the two orders stay comparable.
• No new purchases, balance transfers, or fees (late fees, annual fees) are added along the way.
• Interest accrues monthly as balance × APR ÷ 12, rounded to cents; card issuers instead compound daily on your average daily balance, so real card costs run slightly higher.
• The attack order is fixed at month 0 and never re-sorted, even after balances change.
• Each debt’s final payment is trimmed to land its balance at exactly zero, and freed minimums roll onto the next target the same month.

Frequently asked questions