The basic formula for monthly credit card interest

Credit card companies calculate your monthly interest charge by multiplying your average daily balance by your daily periodic rate, then multiplying by the number of days in your billing cycle. The formula is: (Average Daily Balance) × (Daily Periodic Rate) × (Number of Days in Billing Cycle) = Monthly Interest Charge.

Your daily periodic rate is your annual percentage rate (APR) divided by 365. For example, if your APR is 18%, your daily periodic rate is 0.18 ÷ 365 = 0.000493. This rate is applied to your balance each day, then added up across your entire billing cycle to produce the interest you owe at the end of the month.

Most credit card issuers use the "average daily balance" method, which is the most common way to calculate interest. This method accounts for the fact that your balance changes throughout the month as you make purchases and payments.

Key Takeaways

  • Your monthly interest charge depends on three things: your average daily balance, your daily periodic rate (APR ÷ 365), and the number of days in your billing cycle.
  • The average daily balance method adds up your balance for each day of the billing cycle, then divides by the number of days — this is what most issuers use.
  • A $5,000 balance at 18% APR costs roughly $75 in monthly interest, but the actual amount depends on when you made purchases and payments during the cycle.
  • Paying down your balance mid-cycle reduces your average daily balance and lowers the interest you owe that month.
  • You can request your issuer's calculation method in writing, and they must disclose it in your cardmember agreement.

How to find your average daily balance

To calculate your average daily balance, add up your balance for each day of your billing cycle, then divide by the number of days in that cycle. For example, if your balance was $1,000 for 10 days, $1,500 for 15 days, and $800 for 5 days in a 30-day cycle, your average daily balance is ((1,000 × 10) + (1,500 × 15) + (800 × 5)) ÷ 30 = $1,233.33.

Your credit card statement usually shows your average daily balance near the interest charge line. You do not have to calculate it yourself — the issuer does this and reports it to you. However, knowing how it works helps you understand why your interest charge is higher or lower than you expected.

If you made a large payment mid-cycle, your average daily balance will be lower than your ending balance, which means your interest charge will be lower too. This is why paying early in your billing cycle saves more on interest than paying at the end.

Finding your daily periodic rate and APR

Your daily periodic rate appears in your cardmember agreement and on your monthly statement. It is always your APR divided by 365 (or sometimes 360, depending on the issuer — check your agreement). If your APR is 20%, your daily periodic rate is either 0.0548% (20% ÷ 365) or 0.0556% (20% ÷ 360).

Your APR may not be a single number. Many cards have different APRs for purchases, balance transfers, and cash advances. Interest on purchases is what most people pay, but if you use your card for a cash advance or transfer a balance from another card, a different (usually higher) rate applies. Check your statement to see which rate is being used for your current balance.

Some cards have variable APRs that change when the prime rate changes. If your card is variable, your daily periodic rate will shift up or down a few times per year. Your statement will show your current rate and tell you whether it is fixed or variable.

Working through a real example

Suppose you have a card with an 18% APR and a 30-day billing cycle. Your balance was $2,000 for the first 15 days, then you made a $500 payment, leaving $1,500 for the remaining 15 days. Your average daily balance is ((2,000 × 15) + (1,500 × 15)) ÷ 30 = $1,750.

Your daily periodic rate is 18% ÷ 365 = 0.000493. Your monthly interest charge is $1,750 × 0.000493 × 30 = $25.84. If you had not made that $500 payment and carried $2,000 for the full 30 days, your interest would have been $2,000 × 0.000493 × 30 = $29.58 — a difference of $3.74 that month.

Over a year, making mid-cycle payments instead of waiting until the end of the cycle can save you $40 to $50 in interest, depending on how much you spend. The earlier you pay, the lower your average daily balance, and the less interest you owe.

Why different issuers may calculate interest differently

Most issuers use the average daily balance method, but a few use the "previous balance" method or the "adjusted balance" method. The previous balance method charges interest on your entire balance from the start of the cycle, ignoring payments you made during that cycle. The adjusted balance method subtracts your payments from your opening balance, then charges interest on that number.

The average daily balance method is the most common and usually results in the highest interest charge, which is why issuers prefer it. If your issuer uses a different method, it will be stated in your cardmember agreement under "How We Calculate Your Finance Charge" or a similar heading.

You can request this information in writing from your issuer, and they are required to send it to you within 30 days. If you are comparing cards, knowing the calculation method is less important than knowing the APR — the APR is what drives the total cost.

How grace periods affect your monthly interest

If you pay your full statement balance by the due date, you typically owe no interest on new purchases made during that cycle. This is called a grace period, and it usually lasts 21 to 25 days from the end of your billing cycle. However, the grace period does not explore to balance transfers or cash advances — interest on those starts accruing when ready.

If you carry a balance from one month to the next, you lose the grace period on new purchases. Interest begins accruing on new purchases the day they post to your account. This is why carrying a balance is expensive: you pay interest on old purchases and new ones simultaneously.

Some cards offer no grace period at all, or a shorter one. Check your cardmember agreement to see whether your card has a grace period and how long it lasts. Cards with rewards or low APRs often have standard 21-day grace periods, while some subprime cards have none.

Reducing your monthly interest charge

The most direct way to lower your monthly interest is to reduce your balance. Even a $100 payment mid-cycle lowers your average daily balance and saves you a few dollars in interest that month. Over time, small payments add up.

If you have multiple cards, paying down the highest-APR card first saves the most money. A $200 payment on a 24% APR card saves more in interest than a $200 payment on a 15% APR card. Prioritize high-rate cards while making minimum payments on the others.

If your APR is very high, you may be able to request a lower rate from your issuer, especially if you have a good payment history. A rate reduction from 22% to 18% cuts your monthly interest by roughly 18%, which compounds over months and years. It costs nothing to ask.

Frequently Asked Questions

Does my credit card company charge interest daily or monthly?

Interest accrues daily based on your daily periodic rate, but you are only charged once per month, at the end of your billing cycle. The daily charges are added together to produce your monthly interest charge. If you pay off your balance before the due date, you owe no interest.

What is the difference between APR and the interest charge on my statement?

APR is the annual rate; your monthly interest charge is what you actually owe that month based on your balance and the number of days in the cycle. A 24% APR does not mean you pay 24% of your balance each month — you pay roughly 2% per month (24% ÷ 12), but the exact amount depends on your average daily balance and the number of days in your cycle.

If I pay half my balance mid-cycle, does my interest charge go down?

Yes. Your interest charge is based on your average daily balance, not your ending balance. Paying half your balance mid-cycle lowers the average, which lowers the interest you owe that month. The earlier in the cycle you pay, the more interest you save.

Can I negotiate my APR to lower my interest charges?

You can request a lower APR from your issuer, especially if you have made on-time payments and have good credit. There is no may provide they will agree, but many issuers will reduce your rate by 1% to 3% if you ask. It takes a phone call and costs nothing.

Why is my interest charge higher than I calculated?

The most common reason is that you miscalculated your average daily balance or used the wrong daily periodic rate. Check your statement to see the average daily balance and daily periodic rate the issuer used, then recalculate. If the numbers still do not match, contact your issuer and ask them to walk you through their calculation.