Credit card interest is usually calculated using your card’s APR, a daily interest rate, and your average daily balance over the billing cycle. In practice, that means interest grows a little bit every day you carry a balance, not just once a month.

Key terms to know

  • APR (Annual Percentage Rate) : The yearly interest rate your card charges on balances you don’t pay in full.
  • Daily periodic rate: Your APR divided by 365; this is the rate used to calculate interest each day.
  • Average daily balance: The average of your balance on each day of the billing cycle, after payments and new purchases.

Basic formula (average daily balance)

Most major issuers use the average daily balance method.

  1. Convert APR to a daily rate:
    • Example: APR = 18% → daily rate = 0.18/365≈0.0004930.18/365\approx 0.0004930.18/365≈0.000493.
  1. Find your average daily balance:
    • Add up your balance for each day of the billing cycle and divide by the number of days.
  1. Calculate interest for the period:
    • Interest = daily rate × average daily balance × number of days in cycle.

So if your average daily balance is 1,000, APR is 18%, and the cycle is 30 days:

  • Daily rate ≈ 0.000493
  • Interest ≈ 0.000493 × 1,000 × 30 ≈ 14.79 in interest for that month.

Simpler “quick estimate” method

Some banks describe a rough monthly method for a quick estimate.

  • Divide APR by 12 to get a monthly rate.
  • Multiply that monthly rate by your current balance.

Example:

  • Balance = 1,000, APR = 16.99%
  • Monthly rate ≈ 16.99% / 12 ≈ 1.42%
  • Interest ≈ 1,000 × 0.0142 = 14.20 for the month (approximate).

This is less precise than the daily method but close enough for a quick mental calculation.

Why your interest changes

  • New purchases increase your daily balance, so later days in the cycle may add more interest.
  • Payments lower your balance on the day they’re applied; from that day on, interest is charged on the smaller amount.
  • Some cards compound daily, meaning each day’s interest is added to the balance and can itself earn interest the next day.

Always check your statement or your bank’s “How we calculate interest” section, because some cards use slightly different variations (like separate APRs for cash advances or balance transfers).

Quick Scoop (summary for your post)

  • Find your APR on your statement.
  • Turn APR into a daily rate by dividing by 365.
  • Work out your average daily balance over the billing cycle.
  • Multiply: daily rate × average daily balance × number of days in the cycle = interest for that period.
  • For a fast estimate, use APR ÷ 12 × current balance to get an approximate monthly interest charge.

Information gathered from public forums or data available on the internet and portrayed here.