Enter your credit card balance, APR, and billing cycle length to calculate the interest you'll be charged this cycle and over a full year if the balance goes unpaid.
How to Use the Credit Card Interest Calculator
Enter the current balance, the card's APR, and the billing cycle length in days — 30 is a reasonable default, but check your actual statement if you want an exact match. All three outputs update immediately.
Credit card interest doesn't compound the way a savings account does; it's calculated daily against whatever the balance happens to be, then summed up over the billing cycle. That daily mechanic is what this calculator exposes.
Daily Periodic Rate
Daily rate = APR ÷ 100 ÷ 365
Cycle interest = balance × daily rate × billing cycle days
Annual interest if unpaid = balance × APR ÷ 100A 22% APR sounds like a single annual number, but the card issuer actually applies a tiny fraction of it — the daily periodic rate — to the balance every single day, then adds up 28 to 31 days' worth for the statement. On a $4,000 balance at 22% over a 30-day cycle, that's roughly $72 in interest for one month alone.
Stretch the same $4,000 balance across a 31-day cycle instead of a 30-day one and the interest charge rises by a couple of dollars — a small shift, but it's the reason statement periods that vary in length never charge quite the same interest on an otherwise unchanged balance.
The Full-Year Estimate Is Deliberately Simple
The "annual interest if unpaid" figure is a flat-rate estimate — balance times APR — not a compounding projection. Real cards compound daily, so a balance that truly sat untouched for a year would accrue slightly more than this number through interest on interest. Treat it as a floor, not a precise forecast: the point is showing roughly what a full year of carrying this balance costs, not modeling every day of compounding.
The gap between the two output cards is the whole argument for paying more than the minimum. One billing cycle's interest looks manageable in isolation; multiply it out to twelve cycles of a balance that isn't shrinking, and the number stops looking small.