| # | Payment ($) | Interest ($) | Principal ($) | Balance ($) |
|---|---|---|---|---|
| {{ row.month }} | {{ format(row.payment) }} | {{ format(row.interest) }} | {{ format(row.principal) }} | {{ format(row.balance) }} |
Revolving credit lets you carry a balance from month to month, but outstanding amounts accrue interest daily and compound each billing cycle. Even moderate annual percentage rates translate into substantial long-term costs when only minimum payments are made. Grasping these mechanics empowers you to weigh faster repayment against competing financial priorities.
This interactive tool simulates a single credit-card account under fixed, extra, or new monthly charges. You enter current balance, annual interest rate, and payment amounts; the engine instantly recalculates payoff time, total interest, and a detailed amortisation schedule. A charting layer visualises declining balances and the principal-to-interest split month by month.
Experiment by adding a modest $25 extra payment and observe the payoff date jump forward by years while the interest share shrinks dramatically, yet always verify the resulting repayment timetable with your monthly statement and issuer disclosures to avoid unpleasant surprises and maintain accuracy for your overall household finances planning.
The calculator applies simple-interest accrual to the running balance, compounds monthly, and deducts combined base + extra payments. It supports additional monthly charges, mirroring real-world card usage, and iterates until the balance reaches zero or a 50-year guard-rail.
| Symbol | Description | Typical Range |
|---|---|---|
| Bn | Balance at month n | $100 – $25 000 |
| r | Annual interest rate (APR) | 8 % – 29 % |
| P | Total monthly payment | $25 – $1 000 |
| C | Monthly new charges | $0 – $2 000 |
| Interest Share | Risk Band |
|---|---|
| < 20 % | Low |
| 20 – 49 % | Moderate |
| ≥ 50 % | High |
Assume a $5 000 balance (B0), 18 % APR, $150 base payment, no extras, and no new charges. Monthly rate r/12 equals 0.015. Substituting values:
The process repeats until B reaches zero after 46 months, during which \$6 900 is paid and \$1 900 represents interest.
Calculations are O(n) relative to payoff months and complete instantly in modern browsers. Memory usage is bounded by the amortisation array and typically remains below 20 kB.
Follow the flow below to model your repayment plan.
It projects months to zero balance, total paid, interest share, and provides a month-by-month amortisation schedule.
Yes. Enter the expected amount in Monthly new charges; the model adds it before computing each month’s interest.
No. Everything runs locally in your browser; nothing leaves your device.
The first-month payment must at least equal first-month interest. Increase the amount until the warning disappears.
The model rounds to the nearest cent and assumes interest compounds monthly; statement-level variations can shift totals slightly.