Debt Snowball vs Avalanche Calculator

What this calculator returns

The calculator runs two discrete monthly simulations on the same pair of debts, applying a constant total monthly budget across both strategies. Avalanche directs the surplus above minimum payments at the higher-rate debt first; snowball directs it at the smaller-balance debt first. The output is months to clear and total interest paid under each strategy, plus the interest difference between them. Both simulations roll up the freed-up payment when a debt clears — the defining feature of the snowball framing both methods share — so the total monthly outflow stays constant from month one until the last balance hits zero.

Why the strategies can produce different totals

The interest paid on a debt depends on how long the balance carries — the longer a high-rate balance sits, the more interest accrues. Avalanche directs the surplus payment at the higher-rate balance first, which clears that balance faster and reduces high-interest accrual earlier in the timeline. Snowball directs the surplus at the smaller-balance debt first, which clears it sooner but lets the larger debt accrue more interest in the meantime. When the higher-rate debt is also the larger debt, avalanche tends to win on both interest cost and total time. When the smaller debt is the higher-rate debt, both strategies converge to the same ordering and produce identical results.

How the minimum payments work

The simulation uses a currency-neutral minimum payment of monthly interest plus 1% of the original balance. This always covers interest and reduces the principal by at least 1% of the starting balance per month — close to how typical revolving credit minimums behave. The total monthly budget is the sum of both minimum payments plus the extra amount entered. That total stays constant across the simulation: when the targeted debt clears, the freed-up minimum is rolled onto the remaining debt rather than disappearing from the budget.

How payment size moves the timeline

Adding to the extra payment shortens both timelines non-linearly. Each extra unit of monthly payment reduces principal that future interest accrues on, which lowers each subsequent interest charge, which leaves more of every future payment to prioritising the principal. The compounding works in reverse here, which is why the difference between a small and a moderate extra payment is much larger than the difference between a moderate and a large extra payment.

What this comparison does not capture

The math assumes constant rates, on-time payments at the entered amount, and no new spending added to the balances during payoff. Real debt journeys often include rate changes (especially on credit-card balances), missed payments, fee charges, and continued spending on cleared accounts. The headline interest difference between strategies is the steady-state version — actual outcomes drift under those conditions.

Where the snowball-vs-avalanche debate matters

For two-debt situations where avalanche wins by a meaningful interest figure, the cost difference is real and quantified by this calculator. For situations where the difference is small or the strategies converge, the choice becomes a behavioural question rather than a mathematical one: clearing a small balance early can support follow-through on a long payoff plan, even when it costs slightly more in interest. The headline figure here lets the comparison happen on the actual numbers rather than on intuition.

Where to look next

The avalanche-vs-snowball-savings calculator extends the comparison to three debts with separate balances and rates per debt, useful when there are more than two balances to compare. The debt-payoff calculator runs the same single-debt math without the strategy comparison. The credit-card-min-payment-trap visualiser handles the percentage-of-balance minimum case typical for credit cards.