How credit card interest actually works (the maths might surprise you)
Published 21 April 2026
You know credit cards charge interest. But most people dramatically underestimate how fast it compounds — and how long it takes to pay off a balance if you only make the minimum payment each month.
Credit card interest is calculated and applied differently from most other types of debt. Understanding the mechanics helps you make better decisions about when to carry a balance and how to pay it off efficiently.
How daily compounding works
Most New Zealand credit cards use a daily interest calculation. The APR (annual percentage rate — e.g. 20%) is divided by 365 to get the daily rate (about 0.055%). That daily rate is applied to your outstanding balance each day and added to what you owe.
This means interest accrues on interest. A $1,000 balance at 20% APR doesn't just cost you $200 a year in interest — it compounds daily, and the total interest cost is slightly higher as each day's interest becomes part of the next day's balance.
The minimum payment trap
Most credit cards set minimum payments at around 2–3% of the balance. This sounds manageable, but it's designed to keep you in debt as long as possible while maximising interest paid.
A real example: $5,000 balance, 20% APR, minimum payment of 2% (or $25, whichever is higher). If you only ever pay the minimum, it takes approximately 30 years to clear the debt, and you'll pay over $7,000 in interest — more than the original balance. That same debt paid at $250 a month is cleared in 2 years with around $1,100 in interest.
The interest-free period and how to use it
New Zealand credit cards typically offer an interest-free period of 44–55 days on purchases. This applies only if you pay your full statement balance by the due date each month. If you carry any balance from the previous month, you lose the interest-free period entirely — interest starts accruing from the date of purchase.
This means there are really two ways to use a credit card: pay the full balance every month and pay zero interest, or carry a balance and pay significant interest. The hybrid approach — paying most but not all of the balance — gives you the worst of both worlds.
Practical rules for avoiding the trap
- Never spend on a credit card what you don't have in your bank account right now
- Set up automatic full payment on your due date if you always pay in full
- If you have an existing balance, stop using the card for new purchases while you pay it down
- Know your APR — it's on your statement and is the single most important number for understanding the cost of carrying a balance
Next: put it into practice
Step-by-step guides to do what this article describes.
Common questions
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