How credit card interest is worked out
Each month the card adds interest to what you owe, and your payment then comes off the total. Interest = balance × monthly rate. A US card quotes an APR, and under Regulation Z the APR is the periodic rate multiplied by the number of periods in a year (12 CFR 1026.14(b)), so a monthly rate is the APR divided by 12. That is the default here. Many issuers charge interest on a daily periodic rate of APR ÷ 365 instead.
The second setting follows that daily rate: it compounds APR ÷ 365 across an average month of 365 ÷ 12 days, which gives a monthly rate of 1.7649% at 21% against 1.7500% when the APR is divided by 12. Check your cardholder agreement for how your issuer works out your balance.
Worked example: $5,000 at 21% with $200 a month
On a $5,000 balance at 21% APR, paying $200 a month clears the card in 34 months (2 years 10 months) with the APR divided by 12. The first month's interest is $87.50, the total interest is $1,633.20 and you repay $6,633.20 in all.
With daily compounding the same card takes 34 months and costs $1,653.73 in interest. Discover's online calculator, with these inputs, showed 34 months and total interest of $1,654 when we checked it on 26 October 2026.
A second check: $8,000 at 18% with $200 a month
Calculator.net's credit card calculator defaults to $8,000 at 18% with $200 a month. Dividing the APR by 12 gives 62 months (5 years 2 months) and $4,308.98 of interest. The page showed 5 years and 2 months and $4,308.61, a few cents different because its schedule rounds each month's payment.
Clearing the balance in a set time
To clear $2,000 at 15% in 36 months, choose the set number of months: the payment is $69.33 a month and the total interest is $495.90. A longer term lowers the payment but raises the interest.
What paying only the minimum costs
Your card issuer sets the minimum payment, usually a share of the balance plus the month's interest and fees, with a floor. The minimum fields take your statement's rule. With the example rule of 1% plus interest and a $25 floor, the first minimum payment on this balance is $137.50, and paying only the minimum would take 228 months (19 years) and cost $7,707.72 in interest. The $200 plan saves $6,074.52. The $25, 1% and interest defaults are examples, not any issuer's terms.
When the payment is too low
If your payment does not cover the month's interest, the balance grows. At 21% on $5,000 the first month's interest is $87.50, so a payment of $87.50 or less is refused, with a message that shows the figure you need to beat.
What this calculator leaves out
It models one card with no new spending, a payment every month and a fixed rate. It does not include promotional 0% periods, balance transfer fees, annual fees, late fees, cash advance charges, or the grace period that stops interest on new purchases when you pay the statement balance in full. Treat it as a planning tool and check figures against your statement.
To see the effect of extra payments, or a loan to pay the card off, use the credit card loan payoff calculator.
To model a percentage-of-balance payment, use the credit card repayment calculator.
Use the right country page
This page uses dollars and US card terms. The country choices above the inputs open separate local pages. Choosing United Kingdom opens the UK credit card interest calculator, which works in pounds and turns the APR into a monthly rate with the effective-rate convention. Inputs reset and amounts are not converted.
Inputs for the credit card interest calculator
How the yearly rate becomes a monthly rate
Most tools divide the APR by 12. Daily compounding charges interest on interest within the month.
How you want to pay
Enter a fixed monthly payment, or the number of months you want to take to clear the balance.
Card balance
How much you owe on the card now.
Card APR
The yearly rate on your statement. Enter the APR, not the monthly rate.
Monthly payment
Used for a fixed payment. The amount you pay each month.
Months to clear
Used when clearing in a set number of months. A whole number up to 600.
Minimum payment: share of the balance
Used for the minimum comparison. Check your statement for the issuer's rule.
Minimum payment also covers the month's interest
Some issuers include the month's interest in the minimum payment. Check your statement.
Minimum payment: lowest amount
The lowest amount the card asks for in a month, from your statement.
CALCULATOR QUESTIONS
US credit card interest calculator questions
How do you calculate credit card interest?
Multiply the balance by the monthly rate, add it to what you owe, then take off your payment, and repeat each month. On $5,000 at 21% APR with $200 a month the total interest is $1,633.20 over 34 months.
Is the monthly rate the APR divided by 12?
On this page it is by default, which is how Regulation Z relates the periodic rate to the APR. Issuers that use a daily rate of APR ÷ 365 charge slightly more: $1,653.73 instead of $1,633.20 in this example.
How much do I need to pay to clear a card in 36 months?
On $2,000 at 15% you need $69.33 a month. The total interest is $495.90.
What happens if I pay less than the interest?
The balance grows, so the calculator shows a message instead of a result. At 21% the first month's interest on $5,000 is $87.50, so you need to pay more than that.
Does a grace period change this?
Yes. If you pay your statement balance in full each month, new purchases usually carry no interest, so there is nothing to calculate. This page is for a balance you carry from month to month.