These calculators use a monthly, fixed-rate repayment model. They are estimates for comparing scenarios, not lender payoff quotes. All website amounts are in US dollars. Calculations run in your browser.
1. Opening balance and interest
For each active debt, monthly interest equals the opening balance × APR ÷ 1,200. For example, $1,000 at 12% APR accrues $10 in the first modeled month. We add interest before applying that month’s payments. This is not a daily-average-balance or actual/365 calculation.
Money is converted to integer cents. APR accepts up to three decimal places. Interest is calculated using integer arithmetic and rounded half-up to the nearest cent separately for each debt each month. A 0% APR debt accrues no interest.
2. Minimums, then extra payments
The monthly plan budget is the sum of the original active debts’ entered minimums plus your extra payment. Each active debt receives its fixed minimum first, capped at the amount owed including the month’s interest. The rest goes to the priority debt.
If that debt is paid off, unused money flows to the next priority in the same month. Payments previously assigned to paid-off debts stay in the budget for future months. The final month uses only what is needed, so it can be smaller than the regular budget.
3. Priority and ties
Snowball sorts by the smallest opening balance each month. Equal balances use higher APR first, then input order. Avalanche sorts by highest APR, then smallest opening balance, then input order. The ordering is set before the month’s interest and payments. A debt paid through minimums can disappear before the priority debt, which is why payoff milestones may differ from the initial priority list.
4. What the baseline means
The fixed-minimum-only baseline pays each original fixed minimum and stops that payment when its debt is paid off. It does not recycle the unused portion of a final payment or the payments of completed debts. The plan keeps the original budget steady and includes extra payments.
Therefore, “interest saved” versus the baseline includes both extra payments and rollover. It does not isolate the effect of extra money alone. Actual card minimums may decrease as balances decline, so this fixed-payment baseline can understate the time and interest of a declining-minimum plan. Consult your statement for your issuer’s terms.
5. Dates and duration
You choose a starting month. Payment 1 is one calendar month later; payment 12 is twelve calendar months later. Results name a month and year, not a guaranteed day. Calendar calculations use UTC month boundaries so short months, leap years, and local daylight-saving changes do not shift the label.
An all-zero portfolio is already paid off and has a duration of zero months. A partial final payment still counts as one monthly period.
6. When payments cannot finish the plan
A payment below interest creates negative principal repayment: the balance grows. We show an early warning for a minimum below the first month’s interest, but keep simulating because extra money or later rollover may cover the gap.
If every remaining balance is unchanged or growing in a month, the plan stops as non-amortizing. Otherwise we simulate up to 600 months. A remaining aggregate balance over $10 billion also triggers a safety stop. A stopped plan has no complete payoff date, total lifetime cost, or savings comparison; amounts shown are only for the simulated period.
7. Limits you should check
The model excludes new borrowing, fees, penalty rates, changing APRs, grace periods, daily accrual, promotional expiries, lender-specific allocation rules, prepayment charges, and tax effects. It does not assess affordability or prioritize debts by legal consequences. Secured, delinquent, disputed, and specialized debts may need separate guidance.
Inputs support up to 30 debts, $1,000,000 per balance or payment, and APRs from 0% to 1,000%. These are numerical limits, not suggested borrowing terms. The native app supports additional planning features; this website does not claim exact parity with every app scenario.
Reproduce or report a result
Use the comparison calculator and expand the monthly schedules to inspect principal, interest, and remaining balances. Send a hypothetical reproduction if something does not add up.