What the balance actually costs
An APR is a percentage, and percentages are easy to not feel. This turns yours into two numbers you can act on: how long the balance takes to clear, and what it costs you to get there.
Calculator
Paying this off
Fill in a balance, an APR and a payment.
Why the balance barely moves
Interest is charged on what you owe, every month, before any of your payment counts as repayment. At 23.99% on $4,800 that's about $95.96 in the first month — so a $150 payment reduces the balance by roughly $54, not $150.
That's the trap in a minimum payment. It's usually set just high enough to cover the interest and a sliver of principal, which is why a balance can sit there for years while you pay every month without fail.
Why a small increase does so much
The interest is fixed by the balance and the rate, so every extra dollar you pay goes entirely to the balance. That shrinks next month's interest, which sends slightly more of the following payment to the balance too. Small increases compound in your favour, in the same way the debt compounds against you.
If it says "never"
That isn't a bug or a rounding artefact. If your monthly payment is at or below the monthly interest, the balance genuinely does not fall — it holds or grows, no matter how reliably you pay. The payment has to clear the interest before any of it is repayment.
Knowing the number is the easy part
Isomoira shows the interest on a card as dollars a month rather than a rate, next to everything else you and your partner are spending — so the cost of carrying a balance sits where you'll actually see it, without either of you having to go looking.