Debt Repayment Calculator

How Debt Repayment Time Is Calculated

This calculator uses the standard reducing-balance amortisation formula that lenders use for credit cards, personal loans and overdrafts. Each month, interest is charged on the remaining balance at your annual rate divided by 12. Your payment first covers that month's interest, with whatever is left over reducing the principal. As the balance falls, less of each payment goes to interest and more goes to reducing what you owe, so payoff speeds up over time.

The number of months to clear the debt is calculated as:

n = -log(1 - (r × P / A)) / log(1 + r)

Where P is your starting balance, r is the monthly interest rate (annual rate divided by 12), and A is your monthly payment. If your monthly payment is less than or equal to the first month's interest charge, the balance will never reduce and the debt cannot be paid off at that payment level.

Why Extra Payments Make Such a Big Difference

Because interest is calculated on the reducing balance, extra payments compound in your favour. A higher monthly payment reduces the principal faster, which means less interest accrues the following month, which means even more of your next payment goes to principal. On a typical credit card balance at 20 percent or higher, increasing your payment by even $50 to $100 a month can cut years off the payoff time and save hundreds or thousands of dollars in interest.

Typical NZ Interest Rates by Debt Type

Rates vary by lender and depend on your credit history. Check your loan or credit card statement for your actual current rate, as this calculator only produces an estimate based on the rate you enter.

Tips to Pay Off Debt Faster

1. Pay more than the minimum whenever you can, even small extra amounts help
2. Apply any lump sums (tax refund, bonus, savings) directly to the balance
3. Consider a balance transfer or consolidation loan if it lowers your interest rate
4. Stop adding new spending to the debt while you are paying it off
5. If you have several debts, use the debt avalanche method (highest rate first) or the debt snowball method (smallest balance first) to structure your overall plan

Related Calculators

• Mortgage Calculators hub: all NZ mortgage and lending tools.
• Debt Avalanche Calculator: pay off the highest interest rate debt first.
• Debt Snowball Calculator: pay off the smallest balance first.
• Credit Card Interest Calculator: interest charges on card balances.
• Personal Loan Repayment Calculator: fixed-term loan repayments.
• Credit Card Minimum Payment Trap Calculator: why minimum payments cost so much.

Sources: Standard reducing-balance loan amortisation formula, as used by NZ banks and lenders for credit cards and personal loans. Commerce Commission NZ, Responsible Lending Code guidance on consumer credit (comcom.govt.nz). Sorted.org.nz debt repayment guidance.

This calculator provides indicative estimates only, assuming a fixed interest rate charged monthly on the reducing balance and no additional fees, penalty interest, or changes to the rate. Actual repayment terms depend on your lender's terms and conditions. This is not financial advice. Speak to your lender or a financial mentor if you are struggling with debt.