What this calculator does
Takes current balance, APR, and either a fixed monthly payment or a target payoff date. Returns months to payoff, total interest paid, and a month-by-month amortisation schedule.
The formula
Monthly interest rate: r = APR / 12
Given fixed monthly payment P:
Each month:
interest = balance × r
principal = P − interest
balance = balance − principal
Repeat until balance ≤ 0
Total interest = sum of all monthly interest charges
Minimum payment mode (illustrative):
Minimum = max(£25, balance × min_rate)
where min_rate is typically 1–2% of balance
Each month minimum is recalculated on the new balance
(This mode shows how long minimum-only payments take)
Given target months n, solve for required payment P:
P = balance × r / (1 − (1 + r)^−n)
Assumptions
- No new purchases are added to the card during the repayment period.
- APR is constant throughout.
- Payment is made on time every month (no late fees).
- Minimum payment rate for illustrative mode is 1% of balance plus interest, which is common UK practice.
Data sources
No regulatory data is used directly in the formula. For context: UK Finance reports the average UK credit card APR was approximately 25% in 2024/25. FCA rules require the minimum payment to cover at least interest plus 1% of the outstanding balance.
Limitations
- Does not model new purchases on the card.
- Does not model minimum payment rules that may vary by lender.
- Does not model introductory 0% purchase periods.
- Late payment charges and the potential loss of a promotional rate are not included.
Worked example
Inputs: £2,500 balance, 24.9% APR, £100/month fixed payment.
r = 24.9% / 12 = 2.075% per month = 0.02075 Month 1: Interest = £2,500 × 0.02075 = £51.88 Principal = £100 − £51.88 = £48.12 New balance = £2,500 − £48.12 = £2,451.88 Continuing monthly amortisation: Payoff in ~32 months Total interest paid ~£690
Changelog
| Date | Change |
|---|---|
| May 2026 | Initial publication |