Snowball vs. avalanche as the engine runs them
The Debt Payoff Calculator runs two plans over the same debts and the same monthly budget: a snowball, which sends the remainder to the smallest starting balance first, and an avalanche, which sends it to the highest APR first. This guide walks through what the engine does inside each month, where the two plans part ways, how to read the comparison it prints, and where its model stops. Every figure below is computed at build time by the same engine, on a fictional set of debts; it is the arithmetic made legible, not a recommendation. The other guides cover the site's remaining engines the same way.
One budget, paid every month
Before the two strategies differ on anything, they share one rule: the amount paid each month is constant. The engine adds every debt's minimum payment to the extra amount you enter, and that sum — the engine calls it the monthly budget — leaves your pocket every month until the last debt is cleared. The strategy never changes the budget. It only decides which open debt receives whatever is left once the minimums are paid.
Two conventions sit under that rule, and the calculator page discloses both. Interest accrues monthly at APR ÷ 12, so a $6,000 balance at 24% accrues $120.00 in its first month, and payments land at the end of each month, after that month's interest has been added. The sequence inside a month is fixed: interest is added to every open debt, every open debt receives its minimum, and only then does the remainder of the budget go to the target.
What "target" means in each strategy
The engine sorts the debts once, from their starting balances and APRs, before the first month runs. Under the snowball the order is ascending balance; under the avalanche it is descending APR. Ties keep the order the debts were entered in. The sort is never repeated.
After the minimums, the remainder cascades down that list. It goes to the first open debt in the order, capped at that debt's balance; if the debt clears with money left over, the leftover continues to the next open debt in the same month. A debt counts as cleared in the month its balance reaches zero (the engine tolerates a sub-cent remainder from floating-point arithmetic), and from the following month it accrues no interest and draws no minimum.
The tie rule is easy to see. Give the engine two fictional debts at the same APR — $3,000 and $1,500, both at 15% — with $100 extra. The avalanche has nothing to sort on, so it keeps the entry order and clears debt 1 first, in month 18. The snowball sorts on balance and clears debt 2 first, in month 12.
A fictional four-debt set
The rest of this guide uses four invented debts. The balances, APRs and minimums are chosen so that the two strategies sort them differently, and the extra amount is $200 a month. The last column is the engine's own first-month interest figure for each debt, which doubles as the floor its minimum must exceed.
| Debt | Balance | APR | Minimum | First month's interest |
|---|---|---|---|---|
| Debt 1 | $6,000 | 24% | $150 | $120.00 |
| Debt 2 | $2,500 | 18% | $60 | $37.50 |
| Debt 3 | $900 | 12% | $40 | $9.00 |
| Debt 4 | $11,000 | 7% | $230 | $64.17 |
The four minimums add up to $480; with the $200 extra, the engine's monthly budget is $680, and that is the figure paid every month under both strategies. The snowball's first target is Debt 3 ($900 at 12%), the smallest starting balance. The avalanche's first target is Debt 1 ($6,000 at 24%), the highest APR. In month one the two plans differ only in where the $200 remainder lands: on debt 3 under the snowball, on debt 1 under the avalanche.
The rollover, in the engine's terms
The payoff order the calculator prints is the list of months in which each debt reaches zero. Reading it alongside the standalone figure for each debt shows what the rollover does. The "alone" column below is a separate engine run of that one debt at its own minimum with no extra. The other two columns are the month the same debt clears inside each four-debt plan.
| Debt | Alone at its minimum: cleared in month | Snowball: cleared in month | Avalanche: cleared in month |
|---|---|---|---|
| Debt 1 | 82 | 27 | 22 |
| Debt 2 | 66 | 13 | 27 |
| Debt 3 | 26 | 4 | 26 |
| Debt 4 | 57 | 37 | 36 |
Under the snowball, debt 3 clears in month 4. From the next month its $40 minimum is no longer a minimum; it joins the remainder, so the second target now receives the extra plus that freed amount on top of its own minimum. Each elimination repeats the step, which is why the last debt in either order clears far sooner than it would alone, even though the budget never rose. The avalanche does the same thing in a different order: debt 1 clears in month 22 and its $150 minimum rolls forward.
The rollover does not depend on the extra. Run the same four debts with no extra at all and the budget is $480; the snowball finishes in 5 yr 1 mo and the avalanche in 5 yr 1 mo — a month difference of 0 — with $8,599 and $8,473 of interest respectively. The freed minimums alone carry the plan; the extra shortens it.
Reading the comparison
With the $200 extra, the engine reports the snowball debt-free in 3 yr 1 mo (37 months) with $4,249 of interest and $24,649 paid in total. The avalanche is debt-free in 3 yr (36 months) with $3,890 of interest and $24,290 paid in total. The comparison then prints two differences: snowball interest minus avalanche interest, here $360, and snowball months minus avalanche months, here 1.
The engine's own type comment documents the interest difference as never negative — in its words, the avalanche never loses on interest — because the remainder always goes to the balance accruing at the highest rate. Total paid is the starting balances plus the interest accrued, so the two totals differ by the same amount as the interest figures. None of this is a verdict: the engine computes both plans and the gap between them, and the reader decides which order to follow.
The size of the gap belongs to these particular debts. Where the balance order and the APR order coincide, the plans are the same plan. Hand the engine debt 1 on its own with the same extra and the comparison prints a difference of $0 and 0 months, because there is only one order to follow.
Where the engine stops
- Up to 10 debts. The plan needs at least one debt and accepts at most 10; the calculator's form has one row for each.
- A cap on plan length. A plan that is still running after 1200 months (100 years) is reported as out of range rather than simulated indefinitely.
- Minimums above the interest floor. Each minimum must be greater than zero and must exceed that debt's first month of interest. Set debt 1's minimum to exactly $120.00 and the engine refuses it with: "Debt 1: the minimum payment must exceed the first month's interest ($120.00) or the balance never falls."
- Fixed minimums and fixed APRs. The engine's inputs are a balance, an APR and a minimum per debt, plus one extra amount. A minimum stays at the amount entered for the whole plan, an APR stays at the rate entered, and nothing outside those inputs — new charges, fees, a rate that changes — is in the plan. A debt at 0% APR is accepted; its minimum only needs to be positive.
- Monthly accrual, end-of-month payments. As the calculator page words it, card issuers typically accrue daily and adjust minimums as balances fall. The engine models neither; it is a planning model, not a statement.
Common misreadings
- "Months to debt-free" is not a sum. It is the month in which the last debt clears — 36 for the avalanche above — not the months of the individual debts added together. The debts are paid in parallel; only the remainder is sequential.
- The payoff order is not the payment order. Every open debt receives its minimum every month in both strategies, and a cleared debt's minimum stays in the budget rather than leaving it. The order governs only where the remainder goes, and every total above assumes the constant outlay.
- The gap is not a constant. It is a property of the debts entered. Four debts whose balance order matches their APR order produce two identical plans; a single debt produces a difference of $0. Changing the extra changes both plans at once, as the no-extra run above shows.
Frequently asked questions
Does the engine recommend the snowball or the avalanche?
No. It runs both plans on the same debts and the same monthly budget and prints each one's months to debt-free, total interest and payoff order, plus the interest and month differences between them. The engine's own documentation notes that the interest difference is never negative — the avalanche never loses on interest — but it ranks nothing; the reader decides.
Why does the monthly budget never change?
By construction. The budget is every debt's minimum plus the extra amount, and the engine pays that constant total every month until the last debt clears. When a debt is eliminated its minimum does not disappear from the budget; it rolls into the remainder that goes to the next target. In the fictional four-debt set the budget is $680 a month under both strategies.
What happens when the remainder is larger than the target debt owes?
The remainder cascades. The engine pays the target up to its balance, and if money is left over in the same month it continues to the next open debt in the strategy's order. Two debts can therefore clear in the same month.
Why was a minimum payment rejected?
The engine requires each minimum to exceed that debt's first month of interest (balance × APR ÷ 12); otherwise the balance never falls and the plan never ends. The rejection names the row and the interest figure the minimum needs to beat — $120.00 for a $6,000 balance at 24%.
Is the target order recomputed as balances fall?
No. The order is sorted once from the starting balances (snowball, ascending) or the starting APRs (avalanche, descending) before the first month runs, and ties keep the order the debts were entered in. A debt that becomes the smallest balance partway through a snowball does not jump the queue.
How closely does this match a card statement?
It is a planning model: interest accrues monthly at APR ÷ 12 with payments at the end of each month, and the minimum is held fixed at the amount entered. As the calculator page words it, card issuers typically accrue daily and set minimums that shrink with the balance, so a statement will not reproduce the schedule line for line.
Related tools and guides
- Debt Payoff Calculator — run your own debts and extra amount through the two plans described here.
- Credit card payoff math — one balance under the same APR ÷ 12 convention, with the payment chosen instead of the order.
- All guides and the methodology page.
Not financial advice: this guide describes the arithmetic of the debt payoff engine on fictional inputs. The calculator models the arithmetic; it does not know your lender's terms. Your lender's figure may differ by a few cents depending on their rounding and day-count conventions — the difference is presentational, not an error. Values are processed locally in your browser and never transmitted. See the methodology page.