Whether you’re trying to get out of debt or you’re applying for your first card and want to know how interest works, it’s important to understand how monthly interest is calculated.

Interest charges can quickly inflate your credit card debt and start eating into your disposable income. According to a 2026 report by The Century Foundation, about 111 million Americans carry credit card debt. But for a lot of people, that card debt is way more than a nuisance. The same report found that more than 27 million can’t afford to pay more than the minimum amount due, and very well could be slipping into a cycle of persistent debt. 

Figuring out your interest can help you understand the cost of carrying a certain balance from month to month. It can also be a huge motivator to figure out how to shift your budget or increase your income to start moving that balance in the right direction.

Key takeaways

• Collectively, Americans owe more credit card debt than ever — $1.27 trillion.
• An understanding of how debt works is step one for turning this problem around. 
• Calculating simple interest is a good way to estimate costs, but keep in mind, your card probably uses compound interest.
• Credit card interest calculators can help you estimate how long it will take you to pay off your balance and how much interest you’ll pay in total.

How monthly interest works on credit cards

Credit cards have an APR — annual percentage rate. To calculate monthly interest, banks turn this number into a daily periodic rate, and then apply it to your daily balance to get a daily interest charge. Add up all those daily interest charges, and you get your monthly interest. 

This calculation also includes daily accrued interest, because credit cards generally charge daily compound interest. With compound interest, you’re charged interest on interest. 

Before you get out a pencil and paper and try to do this yourself, you can simplify your life by finding the simple interest instead of the compound interest (a step-by-step guide on how to calculate it is below). Just know that the simple interest is always going to be a little less than the compound interest, which is what you’re actually paying.

Why you should know how to calculate monthly interest

Estimating your monthly interest and knowing how to calculate it can help you understand how much it costs to carry a balance on your card. You can also estimate what you’d pay if your balance were lower (or higher), so you have a clear grasp of what it costs to carry a certain amount of credit card debt month to month. 

Think of it this way: if you know that you’d pay about $150 a month if you don’t pay off your debt, you might be motivated to make some different financial decisions, like trying to reshuffle your budget around to free up more money to pay down your card. After all, that’s $150 less in disposable income. It’s $150 you can’t save or invest.

It helps you see the trade-off that comes with carrying debt or finding ways to get rid of it faster. 

Can you see how much interest you paid on your credit card statement?

Yes, you can look at your statement to see what you were charged in interest. Still, knowing how to calculate it lets you forecast upcoming interest costs and understand how your costs could change with a different rate or a different balance.

It also helps you wrap your mind around what factors impact interest, and what you can do to help reduce your interest costs. Let’s face it, it’s easy to bury your head in the sand when it comes to credit card interest and how much that debt is actually digging into your wealth potential. And a lot of Americans don’t pay attention.

A national consumer survey conducted by the Lending Club Corporation in 2024 found that 47.1% of Americans didn’t know what APR they were paying, and nearly half had no idea their interest rose by over 5 percentage points from March 2022 to July 2023 when the Federal Reserve issued a series of rate hikes. 

We’ve all been paying extremely high rates ever since, as you can see from the chart below.

Knowing how interest works and what factors go into the calculation banks use gives you greater control over your money.

Calculating monthly credit card interest manually in 4 steps

To calculate your monthly interest using simple interest, you need to know the following:

• APR: Your card’s annual percentage rate. You can find your current APR on your credit card statement. Look for a section outlining the interest charge calculation, usually toward the bottom of the statement.
• Daily periodic rate (DPR): APR divided by 365 (some cards divide by 360).
• Days in the billing cycle: Usually 30 or 31 days. You can find the days in each billing cycle toward the top of your credit card statement.
• Average daily balance: The sum of your daily balances for a billing period divided by the number of days in the billing period.

When calculating your monthly interest, you’re basically figuring out how much interest you pay per day, and then multiplying by the number of days in the billing cycle. So, for example, if you pay $0.80 a day in interest, in a 30-day billing cycle, your monthly interest cost would be $24.

Here’s how to do it in four steps using a fictional card balance as an example. Let’s say a card has the following: 

• APR: 23.99%
• Balance: $8,700
• Days in the billing period: 30

1. Calculate your daily periodic rate.

If your APR is 23.99%, you’d divide by 365 to find the daily periodic rate. Keep in mind, some banks divide by 360 instead of 365. The difference is negligible, so it’s okay to use either number if you’re not sure and just want to estimate your interest. 

23.99 ÷ 365 = .0657% 

To change the daily periodic rate to a decimal form, multiply by 100.

.0657% × 100 = .000657

The daily periodic rate is .000657.

2. Calculate your average daily balance.

To find your average daily balance, you’d look at your balance each day, add up the numbers, and divide by 30 to get the average. 

To keep this example simple, let’s assume the daily balance doesn’t change, so no charges are made, and no extra payments reduce the balance during the 30 days of the billing cycle. We’re also not calculating compound interest right now, so we’re not factoring in the additional interest charged on daily accrued interest. 

The average daily balance is $8,700.

3. Multiply the average daily balance by the daily periodic rate to find how much you pay per day in interest.

Now we’ll multiply the daily periodic rate by the daily balance. 

.000657 × $8,700 = $5.72

The daily interest charged is $5.72. 

4. Multiply the daily interest charged by the days in the billing cycle.

To calculate monthly credit card interest, multiply that daily charge by 30.

$5.72 × 30 = $171.60

Monthly interest is $171.60.

That’s how simple interest works. But most credit cards use daily compounding interest.

The compounding interest total is always going to be higher than the simple interest total. So, if you manually calculate simple interest, it’s likely slightly short of your actual interest. 

Here’s how to calculate interest that compounds daily, using the same numbers from our first example.

Credit card interest rate formula for interest that compounds daily

Monthly interest = Average daily balance × ((1 + Daily periodic rate)^30 − 1) 

Taking the same numbers we used for our simple interest example — $8,700 at a 23.99% APR, and a 30-day billing cycle — your monthly interest would be 1.000657 to the power of 30, then subtract 1, and multiply by $8,700.

$8,700 × (( 1.000657 ) ^30 – 1) = $173.12

Using the compound interest formula, your monthly interest is $173.12.

What can you do to lower your monthly interest?

So that average daily balance is your interest lever. Pull the lever to make it lower; pay less in interest. Let it get higher, pay more. 

So, how can you “pull the lever”?

That daily interest charge is based on the APR, the number of days in the billing cycle, and your average daily balance. You have zero control over two of those factors. You do, however, control the average daily balance.

• Make extra payments on your card during the month to pay down your balance when you can.
• If possible avoiding new charges on a card that already has a balance. When you spend money on your card and don’t pay it off each month, that new charge goes to the back of the line and keeps accruing interest.
• Know the APR on your cards. If you can, retire your card with the highest rate and just focus on paying it off.
• If you absolutely have to use credit to make a purchase, make sure it’s the one in your wallet with the lowest rate. 

How to check your APR

You can check your rate by looking at your monthly statement to see what you’re being charged. This number might be different than the APR that was advertised when you signed up for your card. Here’s what can cause your rate to change:

• Your creditworthiness: If your credit score drops or other factors make you less creditworthy, for example, if your credit utilization is high, your card might quietly bump up your rate.
• The prime rate goes up: When the Federal Reserve increases the federal funds rate, the prime rate goes up as well. As credit card banks use the prime rate to determine your rate, you can count on a higher APR as well. 

Using credit card interest calculators

If you don’t want to do the math manually, you can use credit card interest calculators to help you understand costs. Debt repayment calculators will show you a general estimate of what it will cost to pay off your debt in a certain period, and interest calculators will show you a monthly interest estimate

• Credit Karma has a debt repayment calculator and a card balance transfer calculator.
• Discover has a debt repayment calculator that shows the same overall interest costs as Credit Karma’s. It also lets you see what your costs would be if you pay off a little more or less each month.
• Credit Ratings will show you an estimate of your monthly interest, daily interest, and what you’ll pay over 12 months if nothing changes. 

What’s your rate?

Do you know your current APR? If you’re not sure, take a look at your last statement. 

Also, look at how much you paid in interest last month, and then picture what you could do with that money if you weren’t in credit card debt. Instead of paying a card company that money each month, you could be putting it away for retirement, saving for a home, or investing. 

You could use that money to earn interest, rather than accruing interest charges, so you get paid instead of the other way around. 

Sounds nice, right? Okay, now take that positive feeling and take one step toward making that vision a reality. Cancel a streaming subscription you don’t use. Commit to staying under a certain budget for groceries this month. Put off purchases you don’t actually need right now. 

Every little bit frees up a little more cash, lets you pay down your balance a little more, and moves you in the right direction. 

Frequently asked questions

What’s the interest on a credit card with a $10k balance?

With a $10k balance and a 22% APR, estimated monthly interest would be about $182, which means you’re paying about $6 a day in interest. 

How does compounding interest on a credit card work?

With compounding interest, your daily interest charge (which is the daily periodic rate multiplied by your average daily balance) gets added to each day’s balance. Then, the next day’s balance is slightly higher, and so on, through each day of the billing period, which makes your average daily balance higher.

Will I be charged interest if I pay my balance each month?

No, you won’t be charged interest. If you can pay off your balance each month, do it. You won’t just save on interest; paying off your balance or most of it will help keep your credit utilization low, which can lead to a higher credit score.