How to Calculate Credit Card Interest
Credit card interest isn't a mystery fee - it's a straightforward formula applied to whatever balance you're carrying. Once you see the formula, the rest of the card's behavior - why interest is worst early on, why a small extra payment does so much - stops feeling like a black box.
The monthly interest formula
APR is your card's annual percentage rate; dividing by 12 converts it to a monthly periodic rate. Multiply that rate by whatever balance you're carrying at the start of the cycle, and you have the interest charge added before your payment is even applied.
Worked example, month by month
A $3,000 balance at 26.99% APR, paying $150/month. The monthly rate is 26.99% ÷ 12 ≈ 2.25%.
- Month 1: interest = $3,000 × 2.25% ≈ $67.42; principal paid = $150 − $67.42 = $82.58; new balance ≈ $2,917.42.
- Month 2: interest = $2,917.42 × 2.25% ≈ $65.60; principal paid ≈ $84.40; new balance ≈ $2,833.02.
- Month 3: interest ≈ $63.74; principal ≈ $86.26; new balance ≈ $2,746.76.
Notice the pattern: interest falls and principal rises every single month, even though the payment never changes. That's the mechanical reason early payments feel like they barely move the balance - a much larger share of the payment is interest at the start than near the end. Continuing this by hand for a multi-year balance is exactly what a calculator exists to automate, including a full month-by-month table.
Daily accrual vs. monthly accrual
The APR ÷ 12 model above is the standard simplified approximation, and it's what most payoff calculators use because it tracks actual issuer behavior closely enough to be reliable for planning. Some issuers instead accrue interest daily - APR ÷ 365 applied to your average daily balance - which matters more if your balance swings a lot mid-cycle from new purchases. If you want that more granular version, use our Daily Interest Calculator.
Frequently asked questions
What is the credit card interest formula?
Monthly interest = balance × (APR ÷ 12). Most US issuers compound monthly, applying one-twelfth of your annual percentage rate to whatever balance remains at the start of the billing cycle - though some compound daily using APR ÷ 365 against your average daily balance, which produces a very slightly higher effective cost.
Is credit card interest calculated daily or monthly?
It depends on the issuer, but most major US card issuers accrue interest daily against your average daily balance, then bill it once per statement cycle. The monthly APR ÷ 12 formula is the standard simplified model and typically lands within a rounding error of the daily-accrual result - see our Daily Interest Calculator if you want the more precise version.
How do I avoid paying credit card interest entirely?
Pay your statement balance in full by the due date every cycle. Most cards offer a grace period - typically 21-25 days after the statement closes - during which new purchases accrue no interest at all, provided the previous balance was paid in full. Carry any balance past that, and interest starts accruing immediately on new purchases too.
Why is my interest charge higher than balance × APR ÷ 12?
Usually because interest accrued daily against a balance that changed during the cycle - new purchases add to the average daily balance immediately, while payments only reduce it from the date they post. A simple end-of-month balance × APR ÷ 12 estimate can undershoot the real daily-accrual figure when spending happens mid-cycle.