How Credit Card Interest Is Actually Calculated
Updated 16 February 2026
Credit card interest isn't charged like a simple loan — the calculation method is what makes carrying even a small balance surprisingly expensive.
Daily, not monthly
The advertised rate is monthly (commonly in the 3–3.75% range, translating to roughly 36–45% annualized), but interest actually accrues daily on the outstanding balance from the transaction date until it's paid off, then compounds into the next cycle if unpaid.
It applies retroactively
The part that surprises people: if you don't pay your full statement balance, interest isn't just charged on the unpaid amount going forward — it's typically charged from the original transaction date on the entire balance, including purchases you thought were still in their interest-free window. This is why paying 95% of a bill can still result in a real interest charge.