How This Debt Payoff Calculator Works

Updated October 27, 2026

The calculator is a loop, one month at a time, over the debts you typed. This page is that loop in words, plus a small example where extra money spills from one debt to the next. Guides that quote a total interest figure are using this same loop unless they say they stepped outside it for a fee or a rate change.

What is included

A row is included only when the balance is greater than zero and the minimum payment is greater than zero. A balance with a $0 minimum is left out entirely, even if the APR is high. You can list up to ten debts. The extra payment is one number for the whole plan, not a separate extra per card. The strategy is avalanche, snowball, or a side-by-side comparison of both.

Avalanche sorts by APR from high to low. If two APRs match, the smaller balance comes first. Snowball sorts by balance from small to large. If two balances match, the higher APR comes first. That sorted list is the attack order. It is the sequence extra money follows. It is not always the sequence of calendar months in which balances hit zero, because a large minimum on a low-rate loan can clear that loan while extra money is still aimed at a card.

One month

1. For every debt still open, interest is the balance at the start of the month times APR / 100 / 12. An 18% APR is a monthly rate of 0.015. The payment later in the same month does not reduce this interest charge.
2. The minimum you typed is applied next, but never more than balance plus interest. The part of the minimum above interest reduces principal. If the minimum is smaller than the interest, the balance grows.
3. Extra money for the month is the extra you typed, plus any minimums freed in earlier months. It is applied to the first debt in the attack order that still has a balance. If that payment finishes the debt, the remainder spills to the next debt in the same month.
4. A debt at or under one cent is removed. Its minimum is added to the freed pile starting the next month. It is not reused in the month it was paid off. The spill in step 3 is only unused extra, not the freed minimum.
5. The loop repeats until nothing is left, or until 600 months. The cap is there so a payment that never covers interest cannot run forever. In that case the page does not have a real payoff date.

The last month is usually a smaller payment. You are not charged a full minimum once balance plus interest is less than that minimum. Total paid is therefore not exactly "months times (minimums + extra)."

A spill, in dollars

Assumptions: a card with a $300 balance, 24% APR, and a $40 minimum; a loan with an $800 balance, 6% APR, and a $30 minimum; $80 extra; avalanche, so the card is first. These two rates divide cleanly, which makes the first month easy to check by hand.

Month 1. Card interest is 300 × 0.24 / 12 = $6.00. The $40 minimum leaves $266, and the $80 extra leaves $186. Loan interest is 800 × 0.06 / 12 = $4.00. The $30 minimum leaves $774. You pay $150, of which $10.00 is interest. No debt is finished, so month 2's extra pool is still $80.

Month 2 starts the card at $186. Interest is $3.72. After the $40 minimum and the $80 extra, the card balance is $69.72. The loan starts at $774, is charged $3.87, and ends at $747.87 after its minimum.

Month 3 finishes the card. Interest on $69.72 at 24% is $1.39 (1.3944 before rounding). After the $40 minimum, about $31.11 of the card remains, so $31.11 of the $80 extra pays it off and about $48.89 spills to the loan in the same month. The card's $40 minimum does not join the extra pool until month 4. Month 4's extra pool is $80 + $40 = $120, aimed entirely at the loan, which is the only debt left. The loan's balance at the end of month 3 is about $672.72.

The plan finishes in month 8. Total interest is $32.17 and total paid is $1,132.17. On screen those round to $32 and $1,132. Enter the two rows, set extra to 80, and choose Avalanche to confirm the month count, which is not rounded. Snowball lists this pair in the same order, because the $300 card is also the smaller balance, so both methods match. The example is here to show when a finished minimum starts being reused, not to show a strategy gap. A gap needs a debt that is small and low-rate at the same time. That is the prefilled personal loan on the calculator, worked in the avalanche guide.

Rounding

The loop does not round to the cent after each debt each month. It keeps the floating-point result and moves on. Totals in the guides are those running results rounded to the nearest cent at the end. The page you use rounds the big interest totals to the nearest dollar, so a guide can say $8,801.76 while the screen says $8,802. Months are whole numbers on both.

What the loop leaves out

• Average daily balance, a 365-day or 360-day divisor, and the length of a billing cycle. See the interest guide.
• Grace periods and residual interest.
• Annual fees, late fees, balance-transfer fees, and origination fees. Add a fee yourself by increasing the starting balance, then run the plan again. The transfer guide does that.
• An APR that changes on a future date. Run the months before the change by hand, then start a new calculator row with the leftover balance and the new APR.
• A minimum that is recomputed as a percentage of the new balance. The dollar you type stays the dollar.
• Biweekly payments and the day of the month you pay.
• Taxes, credit-score changes, and whether a lender will approve a new loan.