Debt snowball vs avalanche calculator

What this calculator actually does

The calculator simulates your debts one month at a time. Each month it adds interest to every balance at the APR divided by twelve, subtracts the minimum payments you entered, and applies your extra payment to a single target debt until that balance reaches zero. When a debt is paid off, its minimum payment is rolled into the extra payment pool — this rollover is the entire engine of both methods.

Two things follow from that design. First, the comparison is fair: both methods get the same total monthly payment, so the only difference is the order in which your extra money lands. Second, the result is an estimate. Real card issuers recalculate minimums as balances fall, and some loans use daily rather than monthly interest accrual.

The difference between snowball and avalanche

Avalanche targets the highest APR first. Because interest is charged on the outstanding balance, attacking the most expensive dollar is the mathematically optimal move. For a set of debts with meaningfully different rates, avalanche almost always minimizes total interest.

Snowball targets the smallest balance first, regardless of rate. It usually costs more in interest, but it retires a whole account sooner, which some people find easier to sustain. The value of snowball is behavioral, not mathematical.

In practice the gap between the two is often smaller than people expect. If your debts carry similar APRs, the ordering barely matters and you should choose the one you will actually finish. If one debt is dramatically more expensive — a 29% store card next to a 5% student loan — avalanche's advantage is large.

Reading the output

• Debt-free month is the month the final balance reaches zero, projected forward from today.
• Total interest is the sum of every interest charge across the simulation. It is not the same as your current balance minus your original principal, because it starts from today.
• Total paid includes principal and interest from this point forward. Existing interest already baked into your balances is not double-counted.
• Interest saved compares your chosen method against the alternative, holding payments constant.

Assumptions you should check

1. Interest accrues monthly, not daily. Card issuers typically use average daily balance, which produces slightly different numbers.
2. Minimum payments stay fixed. In reality many issuers set the minimum as a percentage of the balance, so your minimum falls as you pay down.
3. No new charges. The simulation assumes you stop using the cards. Adding new spending resets the clock.
4. No fees. Late fees, annual fees, and balance-transfer fees are excluded.
5. Rates stay constant. A promotional 0% APR that expires in twelve months will not be modeled correctly — enter the post-promotion rate and treat the promo period as a separate scenario.

Where the rollover matters

The most common mistake people make is treating the "extra payment" as a one-off. The snowball method's power comes from the rollover: once the store card is gone, its $55 minimum joins the $150 extra, so the next target receives $205 per month. If you instead spend the freed payment, the snowball collapses into a slower version of paying minimums.

A worked example

Suppose you owe $4,200 at 24.99%, $1,800 at 26.99%, and $12,500 at 5.5%, with minimums of $105, $55, and $165. You add $150 per month.

• Avalanche attacks the 26.99% store card first, then the 24.99% card, then the student loan.
• Snowball attacks the $1,800 store card first anyway, then the $4,200 card, then the loan.

Because the smallest balance is also the highest rate here, the two methods produce nearly identical results. Change the store card to 9.9% APR and the ordering diverges: avalanche clears the 24.99% card first and saves meaningfully more interest.

When to use a different approach

If your total minimum payments exceed what you can afford, a payoff calculator is the wrong tool. Prioritize keeping housing and utilities current, contact creditors about hardship programs, and consider a nonprofit credit counselor. A consolidation loan can also make sense when it lowers your blended rate and you have addressed the spending pattern that created the balances.

What to do next

1. Pull the current balance, APR, and minimum from each statement.
2. Enter them above with an extra payment you can sustain for at least a year.
3. Run both methods. If avalanche saves less than a few hundred dollars, pick whichever keeps you motivated.
4. Automate the payments so the plan survives a busy month.
5. Re-run the calculator after any rate change or balance transfer.