The Basic Formula for Monthly Interest
To calculate your monthly interest charge, you need three pieces of information: your current balance, your card's annual percentage rate (APR), and the number of days in your billing cycle. The formula is: Daily Balance × Daily Rate × Number of Days in Cycle = Monthly Interest.
The daily rate is your APR divided by 365. For example, if your APR is 18%, your daily rate is 0.18 ÷ 365 = 0.000493. That daily rate is then multiplied by your balance for each day of the billing cycle, then summed and rounded to the nearest cent.
Most card issuers use the average daily balance method, which is the most common approach. This means they add up your balance for each day of the billing cycle, divide by the number of days, and apply the daily rate to that average. This is different from using only your statement balance on a single day.
Key Takeaways
- Monthly interest is calculated by multiplying your daily balance by your daily rate (APR ÷ 365) for each day of your billing cycle, then summing those amounts.
- Most issuers use the average daily balance method, which accounts for payments and new charges throughout the month rather than a single snapshot balance.
- Your billing cycle is typically 28 to 31 days, and the interest calculation runs from the first day to the last day of that cycle.
- If you carry a balance from the previous month, interest accrues on that carried balance immediately, even during any grace period for new purchases.
- You can find your APR, current balance, and billing cycle dates on your statement or in your online account.
Understanding the Average Daily Balance Method
The average daily balance method works like this: the issuer records your balance at the end of each day during your billing cycle. If you start the cycle with a $1,000 balance, make a $200 payment on day 10, and charge $150 on day 20, the issuer tracks all three balances across the full cycle.
They then add all daily balances together and divide by the number of days in the cycle. If your balances were $1,000 for 9 days, $800 for 10 days, and $950 for the remaining days, the average would be calculated across the full period. This average is then multiplied by the daily rate to produce your interest charge.
This method is fairer than using only your statement balance because it reflects the fact that you paid down part of the balance partway through the month. A method that used only your ending balance would charge you interest on money you no longer owed.
How Payments and New Charges Affect Your Calculation
Every payment you make reduces your daily balance starting the day after the payment posts to your account. If you pay $200 on the 10th, your balance drops on the 11th. The issuer's system records this lower balance for the remaining days of the cycle, which lowers your average daily balance and reduces the interest you owe.
New charges increase your daily balance the day they post. A $150 purchase on the 20th raises your balance that day and every day after. Both payments and charges flow into the average daily balance calculation, so the timing of both matters.
This is why paying early in your billing cycle reduces interest more than paying late: an early payment removes balance from more days of the cycle, lowering the average more significantly.
The Role of Your APR and Billing Cycle Length
Your APR is the annual rate, but interest accrues monthly. To convert it to a monthly rate, divide by 12. An 18% APR becomes 1.5% per month. However, the issuer does not simply apply 1.5% to your balance; they use the daily rate method instead, which accounts for the exact number of days in your cycle.
Billing cycles vary by issuer and card. Most run 28 to 31 days. A longer cycle (31 days) means more days for interest to accrue on the same balance, so you pay slightly more interest than on a 28-day cycle. You can find your exact cycle length on your statement or by logging into your online account.
The daily rate is always APR ÷ 365, regardless of cycle length. This standardizes the calculation across all issuers and ensures that a higher APR always produces a higher monthly charge, all else equal.
What Happens If You Carry a Balance From the Previous Month
If you do not pay your full statement balance by the due date, the unpaid amount carries to the next cycle. Interest begins accruing on that carried balance immediately, even if your card offers a grace period for new purchases.
A grace period typically applies only to new purchases made after the previous balance was paid in full. Once you carry a balance, no grace period applies to anything on the card—new purchases accrue interest from the day they post. This is why carrying a balance is expensive: you lose the grace period and pay interest on both old and new charges.
The carried balance is included in your average daily balance calculation for the new cycle, so it contributes to your interest charge from day one of the next billing period.
A Worked Example
Suppose your card has an 18% APR, your billing cycle is 30 days, and your statement shows these transactions:
| Date | Transaction | Balance |
|---|---|---|
| Day 1 | Starting balance | $2,000 |
| Day 10 | Payment | $1,500 |
| Day 20 | Purchase | $1,700 |
| Day 30 | Ending balance | $1,700 |
Your average daily balance is: ($2,000 × 9 days) + ($1,500 × 10 days) + ($1,700 × 11 days) = $18,000 + $15,000 + $18,700 = $51,700 ÷ 30 days = $1,723.33.
Your daily rate is 0.18 ÷ 365 = 0.000493. Your monthly interest is $1,723.33 × 0.000493 × 30 = $25.48 (rounded to the nearest cent).
Where to Find the Numbers You Need
Your credit card statement lists your APR near the top, usually in a box labeled "Interest Rates and Fees" or similar. It also shows your current balance, the dates of your billing cycle, and a line item for "Interest Charged" or "Finance Charge" that shows what you actually owed for that month.
Your online account portal typically displays your APR, current balance, and billing cycle dates in the account summary or settings section. Some issuers also show a breakdown of how interest was calculated, though not all do.
If you cannot find your APR or cycle dates, call the customer service number on the back of your card. They can confirm both in seconds and can also walk you through the calculation if you want to verify the interest charge on your statement.
Frequently Asked Questions
Why is my interest charge different from what I calculated?
The most common reason is using the wrong balance. You may have used your ending balance instead of your average daily balance, or you may not have accounted for the exact day a payment or charge posted. Issuers record transactions the day they post, not the day you made them. Check your statement for the exact posting dates.
Does interest accrue daily or monthly?
Interest accrues daily but is charged monthly. The issuer calculates a daily rate and applies it to your balance each day, then sums those daily amounts into a single monthly charge that appears on your statement.
Can I avoid interest by paying before my statement closes?
Only if you pay your full statement balance by the due date. Paying before the statement closes does not stop interest from accruing on the balance that will appear on that statement. You must pay the full amount owed by the due date to avoid interest.
What is the difference between APR and the monthly interest rate?
APR is the annual rate. To find the monthly rate, divide APR by 12. An 18% APR is roughly 1.5% per month. However, issuers use the daily rate method (APR ÷ 365) to calculate actual interest, which accounts for the exact number of days in your cycle.
If I pay my balance in full, do I still owe interest?
Only on any balance you carried from the previous month. If you pay your full statement balance by the due date and had no carried balance, you owe no interest. Interest only accrues on balances you do not pay in full.