What This Calculator Does
This tool estimates the interest a credit card charges in a single month on a given balance. Card issuers quote an APR (Annual Percentage Rate), but interest accrues every billing cycle. By dividing the APR by 12 you get the monthly periodic rate, which is then applied to your balance to find the interest cost for that month.
How to Use It
Enter your current statement balance and the APR printed on your card agreement (for example 19.99%). The calculator returns the interest charged that month, along with an approximate daily figure and the total interest over a full year if the balance stayed the same.
The Formula Explained
The core equation is simply $$\text{Monthly Interest} = \text{Balance} \times \frac{\text{APR \%}}{100 \times 12}$$ where APR is expressed as a decimal. For instance, a 24% APR becomes 0.24, and dividing by 12 gives a monthly rate of 0.02 (2%). Many issuers actually compound daily, so real charges can be slightly higher; this calculator uses the simple monthly approximation that is easy to verify against your statement.
Worked Example
Suppose you carry a $1,000 balance on a card with a 19.99% APR. The monthly rate is $$19.99 \div 100 \div 12 \approx 0.016658$$ or about 1.666% per month. Multiplying by $1,000 gives roughly $16.66 of interest for that month — around $0.56 per day, or about $199.90 across a year if nothing is paid down.
FAQ
Is APR the same as the monthly rate? No. APR is annual; divide it by 12 to approximate the monthly rate.
Why might my statement differ? Most issuers compound interest daily and base it on an average daily balance, so actual charges may be marginally higher than this simple estimate.
How do I avoid interest entirely? Pay the full statement balance by the due date each month — purchases in a grace period are not charged interest if the balance is cleared.
