Formulas and sources
Each formula this calculator uses, where it comes from, and what it leaves out. "How it works" shows the same formula with your own numbers.
Interest each month
Fixed, Minimum, Compare and Multi-debt modes
- Interest
- that month's interest
- Balance
- the balance at the start of the month
- r
- monthly rate: the APR (annual percentage rate) ÷ 12
Each APR stays the same, nothing new is charged to the card, and interest is added once a month. Multi-debt does this for each debt.
Source: Based on the assumptions Regulation Z lets card issuers use for the repayment estimate on a statement, Appendix M1(b)(4) [1]. Your card charges interest by its own method, such as a daily rate on the average daily balance.
Payoff at a fixed payment
Fixed mode, and the fixed plan in Compare mode
- Balance
- the new balance on the left; the month's opening balance on the right
- P
- the fixed monthly payment
- Interest
- that month's interest, from the line above
This repeats every month until the balance is 0, for up to 600 months, and the last payment covers only what is left. A payment no larger than the first month's interest never clears the balance.
Source: Standard formula; no single publisher, on the assumptions in Appendix M1 [1]. It gives the same months as the CFPB's example of a $3,000 balance paid at $100 a month, and interest within $1 of its figures [3].
Minimum payment
Minimum mode, and the minimum plan in Compare mode
- Min payment
- that month's minimum payment
- Balance
- that month's opening balance
- m
- the minimum percentage, 1% to 10%
- Interest
- that month's interest, added under the % + interest rule
The floor is the dollar amount the minimum never goes below. The minimum is worked out again each month, and it is never more than the balance plus that month's interest. Unless you change them, the calculator uses 2% of the balance with a $25 floor. Your card agreement states its own rule and floor.
Source: Regulation Z sets no minimum-payment formula; a statement's repayment estimate uses the card's own, Appendix M1(b)(1) [1]. In 2025 the CFPB found that most issuers use 1% of the balance plus interest and fees, with a floor most often of $40 [2].
Payment that clears the balance in 3 or 5 years
Fixed, Minimum and Compare modes, when a payment never clears the balance
- P
- the same payment every month
- Balance
- the balance today
- r
- monthly rate: the APR ÷ 12
- n
- 36 or 60 months
At a 0% APR the payment is Balance ÷ n.
Source: Uses the formula in Regulation Z, Appendix M1(d), for 36 months [1]. The 60-month figure is the standard formula; no single publisher.
Snowball and avalanche
Multi-debt mode
- Budget
- the monthly budget, the same every month
- sum of original minimums
- every debt's minimum in the first month: the one you type, or the minimum rule above on its starting balance
- Extra
- the extra you add each month
Each month, interest is added to every debt and every minimum is paid. The rest of the budget goes to one target: the smallest balance (snowball) or the highest APR (avalanche). The plan stops if the budget does not cover a month's interest, or at 600 months.
Source: Based on the CFPB's snowball and highest-interest-rate methods [4]. The fixed budget is CalcTrail's own choice: a real card's minimum falls with its balance, while here that money stays in the plan. Within one card, Regulation Z sets the same highest-APR-first order for payments above the minimum [5].
Sources
- Consumer Financial Protection Bureau. 12 CFR part 1026 (Regulation Z), Appendix M1: Repayment Disclosures, paragraphs (b) and (d). eCFR, current as of October 1, 2026.
- Consumer Financial Protection Bureau. The Consumer Credit Card Market, section 4.1.1, page 70. Report to Congress, December 2025.
- Consumer Financial Protection Bureau. How to find the best credit card for you, page 1. January 2021.
- Consumer Financial Protection Bureau. How to reduce your debt. Blog post, July 16, 2019.
- Consumer Financial Protection Bureau. 12 CFR 1026.53 (Regulation Z): Allocation of payments, paragraph (a). eCFR, current as of October 1, 2026.
What this calculator does not cover
- It does not model variable APRs, balance-transfer or other fees, a grace period, or a daily periodic rate.
- A promotional rate that ends partway, or a penalty rate after a payment more than 60 days late, is not modeled.
- On a card with balances at different APRs, the issuer must apply payments above the minimum to the highest APR first [5]. Each card here has one APR.
Sources checked on .