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 the freed-up payment onto the remaining debt once the first one clears, 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 instead of dropping out of the budget.

How payment size moves the timeline

Adding to the extra payment shortens both timelines, and not in a straight line. Money paid this month shrinks the balance next month's interest is charged on, so a little more of every later payment lands on principal instead of interest. The effect feeds on itself, which is why moving from no extra payment to a modest one changes the payoff date far more than moving from a modest one to a large one.

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 quantifiable. 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 puts the comparison on actual numbers, not intuition.

Where to look next

For a deeper look at one strategy on its own, the debt-avalanche calculator treats combined debt as a single line at the highest rate, while the debt-snowball calculator runs the same aggregate math at the average rate with an extra input that times the first clearance. Single-debt questions (months to clear and total interest at a fixed monthly payment) sit with the debt-payoff calculator, and the credit-card-payoff calculator applies that same math to a card at its APR.