What this calculator is estimating
Credit-card interest usually depends on the balance carried, the APR, and the issuer’s daily-balance method. This calculator simplifies that framework into a truthful planning estimate: it assumes the balance stays flat, converts APR into a 365-day daily periodic rate, and then shows the daily interest cost that balance would create.
That makes the page useful for quick questions such as how much a balance costs per day, what the interest looks like over a month, and how expensive it is to keep a balance revolving for a full year. It is a static-balance estimate, not a statement replica.