What this tool models
The simulation applies a simplified monthly rate, interest charge, and payment with cent-level rounding for up to 1,200 months. It stops when the first payment does not reduce the balance.
Interactive Learning Tool
Credit Card Interest Timeline
Show how a user-entered balance, APR, and payment amount affect a simplified repayment timeline.
This tool models user-entered assumptions for education. It does not recommend a financial decision.
Last reviewed:
What you enter
Enter a starting balance, APR, scheduled monthly payment, and an optional additional payment for a comparison. Results remain hidden until the form is valid.
Simplified timeline
Modeled repayment result
First 12 months
| Month | Starting balance | Interest | Payment | Principal | Ending balance |
|---|
View full schedule
| Month | Starting balance | Interest | Payment | Principal | Ending balance |
|---|
Formula explanation
Monthly rate = APR ÷ 12 ÷ 100.
Monthly interest = current balance × monthly rate.
Principal reduction = payment − monthly interest.
New balance = current balance + interest − payment. The final payment is capped at balance plus interest.
Assumptions
- The APR remains constant.
- One payment occurs every modeled month.
- No new charges or fees are added.
- Calculations round to cents each month.
Limitations
- Actual agreements may use daily balances, multiple rates, grace periods, fees, and additional rules.
- The simulation is capped at 1,200 months.
- This tool is educational modeling, not debt advice.
What this teaches
The result shows how interest and payment size interact, including why a payment can fail to reduce the starting balance under some assumptions.
Continue learning
Related lessons, tools, and practice
Sources
Official sources support the educational definitions. Calculations use the simplified formulas shown on this page.
Educational boundary
Financial Intelligence Lab provides simplified educational information, calculators, and games. This page does not provide individualized financial, investment, tax, legal, credit, student-loan, insurance, housing, retirement, career, business, or budgeting advice. Examples and formulas use simplified assumptions. Actual costs, taxes, rates, benefits, laws, eligibility rules, and personal circumstances vary.