What this calculator does
Takes current balance, current APR, monthly payment, 0% deal length (months), and any transfer fee. Returns total interest saved vs staying on the current card, cost of the transfer fee, and net saving.
The formula
Without balance transfer — amortisation on existing card:
Each month: interest = balance × (APR / 12)
principal = payment − interest
balance = balance − principal
Sum all interest payments until balance = 0
With balance transfer:
Transfer fee = balance × fee_rate (e.g. 3%)
During 0% period (months 1..n): no interest accrues;
monthly payment reduces balance only
After 0% period: remaining balance amortises at revert APR
Total cost (with BT) = transfer fee + interest paid after 0% period ends
Net saving = total interest (without BT) − total cost (with BT)
Assumptions
- Minimum monthly payment kept constant throughout the comparison.
- No new purchases on either card.
- Transfer fee is charged upfront on the full transferred balance.
- Revert APR is applied to the remaining balance after the 0% period ends.
- Does not model an introductory period on the revert card.
Data sources
No external regulatory data is used — the user inputs their own APR and deal terms. Typical balance transfer fee range (1.5–3%) and typical revert APR (20–25%) are shown as defaults only and do not affect the calculation.
Limitations
- Does not model missed or late payment penalties, which may end the 0% deal early.
- Does not model minimum payment requirements (many lenders require a percentage of balance, not a fixed amount).
- Assumes the user makes the same fixed payment each month throughout the entire period.
Worked example
Inputs: £3,000 balance, 22% APR, £100/month payment, 24-month 0% deal, 3% transfer fee.
Without BT: Amortise £3,000 at 22% APR with £100/month → ~£730 total interest paid over ~37 months With BT: Transfer fee = £3,000 × 3% = £90 During 24 months at 0%: pay £2,400; remaining balance = £600 £600 at 22% APR, £100/month → ~£40 interest over 7 months Total cost with BT = £90 + £40 = £130 Net saving = £730 − £130 = £600
Changelog
| Date | Change |
|---|---|
| May 2026 | Initial publication |