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Credit Card Interest Calculator

Work out the interest a card charges in one billing cycle, or how long and how much it costs to pay off a balance with minimum payments only.

Level: Grade 7

  • Common Core standard 7.RP.A.3Use proportional relationships to solve multistep ratio and percent problems. Examples: simple interest, tax, markups and markdowns, gratuities and commissions, fees, percent increase and decrease, percent error.
Printable worksheet

Credit Card Interest Calculator

Know the answer you need? Pick an input to work backward.

Inputs
What do you want to find?

For one cycle, use the average daily balance if your statement shows it.

Result

Interest this billing cycle

$71.89

Carrying $3,500.00 at 24.99% APR for a 30-day cycle adds about $71.89 of interest.

Daily periodic rate
0.06847%
Interest per day
$2.40
Interest over a year at this balance
$874.65
Monthly estimate (APR ÷ 12)
$72.89

See the step-by-step solution ↓

Step-by-step solution

  1. Cards charge interest daily, so the daily periodic rate is the APR as a decimal divided by 365

    DPR=0.2499365=0.00068466

  2. Interest for one day is the balance times the daily rate

    $3,500.00×0.00068466=$2.40

  3. Multiply by the 30 days in the cycle

    $3,500.00×0.00068466×30=$71.89

  4. As a cross-check, a year of interest at this balance is the balance times the APR

    $3,500.00×24.99%=$874.65

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

Please note: Results are estimates for planning and comparison. Lenders, banks, and tax authorities may round differently or include fees not modeled here, so confirm figures with your provider before deciding.

What this calculator does

See what carrying a credit card balance really costs you in dollars. A card's APR (annual percentage rate) is a yearly figure, but interest is actually charged one day at a time. Card companies divide the APR by 365 to get a daily rate, called the daily periodic rate. They charge it on your balance each day of the billing cycle and add the total to your statement. Seeing that total in dollars makes the cost much easier to judge.

This calculator has two modes. The billing-cycle mode shows the interest for one statement period. It uses your balance, your APR and the number of days in the cycle. The minimum-payment mode shows what happens if you pay only the minimum each month. You'll see how many months it takes to clear the balance and how much interest you pay along the way. You can set the minimum rule to match your card's terms.

Where it's used

Why this math matters outside the classroom:

  • Monthly interest charge. Carrying a $2,000 balance at a 24% APR for 30 days adds about $39.45 of interest using the daily rate.
  • Reading a statement. Cardholders check the interest line on a statement by multiplying the average daily balance by the daily rate and the days in the cycle.
  • Minimum payments. Running the balance month by month shows how slowly a balance falls when only the minimum is paid.
  • Comparing cards. Shoppers compare the interest one month's balance would cost on cards with different APRs.

Grade level and standards: Grade 7; Common Core State Standards for Mathematics 7.RP.A.3. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose whether you want one cycle's interest or the cost of paying only the minimum.
  2. Enter the balance and the card's purchase APR.
  3. For one cycle, enter the number of days in the billing period. For minimums, enter the percent, whether interest is added on top, and the lowest minimum the card allows.
  4. Read the interest for the cycle. Or read the months to pay off, the total interest and the year-by-year table.

What the inputs mean

Balance
For one cycle, use the average daily balance from your statement for the closest match. For the minimum-payment mode, use the balance you start paying down from, with no new purchases.
Days in the billing cycle
Billing cycles usually run 28 to 31 days. Your statement shows the exact number.
Minimum payment percent
The share of the balance your card asks for each month. Check the card agreement, or the minimum shown on a recent statement.
Minimum adds that month's interest to the percent
Many cards set the minimum as a percent of the balance plus that month's interest. Others use a flat percent of the statement balance. Untick this box for the second kind.
Lowest minimum payment
The smallest minimum the card will ask for, used once the balance gets small. If you owe less than this, you just pay off the balance.

Formula and method

daily periodic rate = APR ÷ 100 ÷ 365
cycle interest = balance × daily rate × days in cycle
minimum mode, each month: interest = balance × APR ÷ 100 ÷ 12
minimum = max(floor, p × balance + interest)   or   max(floor, p × (balance + interest))
new balance = balance + interest − minimum

In the formulas, p is the minimum payment percent as a decimal (1% is 0.01). The floor is the lowest minimum your card allows, and max( ) means "whichever is larger".

In the billing-cycle mode, interest is the balance times the daily rate, for each day of the cycle. Using the average daily balance gives the same result as adding up each day's interest one by one. That's the idea behind the average daily balance method. Interest charged on interest within one cycle (compounding) is left out. At typical balances, it changes the answer by only a few cents.

The minimum-payment mode moves forward one month at a time. It adds a month of interest, then pays that month's minimum. The minimum shrinks as the balance shrinks, so progress slows down each year. Finally the floor takes over and clears the debt.

Worked example

You carry $3,500 on a card at 24.99% APR through a 30-day billing cycle.

  1. Daily periodic rate: 0.2499 ÷ 365 ≈ 0.00068466, or about 0.0685% a day.
  2. One day's interest: $3,500 × 0.00068466 ≈ $2.40.
  3. Thirty days: $3,500 × 0.00068466 × 30 ≈ $71.89.
  4. Now say you pay only the minimum: 1% of the balance plus interest, with a $25 floor. The first minimum is $35.00 + $72.89 ≈ $107.89.
  5. Repeating that month after month takes 201 months, and total payments come to about $9,654.16.

Result: The cycle adds about $71.89 of interest. Paying only the minimum would take 201 months (16 years 9 months). It would cost about $6,154.16 in interest, far more than the original $3,500 balance.

Understanding the result

The cycle interest is the price of carrying your balance for one month. Multiply it by 12 for a rough yearly cost. Or compare it with the yearly figure shown, which is just the balance times the APR.

In the minimum-payment mode, the total interest is the number to watch. If it's close to the balance itself, or larger, paying a fixed amount above the minimum will save you a lot. The credit card payoff calculator shows how much.

Assumptions and limitations

  • The APR stays the same. There are no new purchases, cash advances, fees or late charges.
  • Billing-cycle interest uses a 365-day year. The minimum-payment mode uses APR ÷ 12 each month.
  • Every minimum is paid on time, and amounts aren't rounded to the cent.
  • Some cards use a 360-day year or add interest on interest daily. Some charge different rates for purchases, balance transfers and cash advances. Those details aren't modeled.
  • It doesn't account for a grace period, when no interest is charged because you paid the last balance in full.
  • Card minimum rules vary, and some include fees in the minimum. Check your card agreement for the exact formula.

Frequently asked questions

Why does my statement show slightly different interest?

Your balance probably changed during the cycle as you made purchases and payments. Card companies charge the daily rate on each day's balance. So entering the average daily balance printed on your statement gives the closest match.

Why does paying the minimum take so long?

Most of each minimum payment goes to interest, and the minimum gets smaller as the balance does. That keeps payments small. But it also means the balance shrinks only a little each month, so paying it off stretches over many years.

What happens if the minimum doesn't cover the interest?

The balance would grow instead of shrinking, so the calculator shows an error. Cards that add the month's interest on top of a percent of the balance avoid this. That's why that rule is common.

Is the daily rate really APR divided by 365?

Often, but not always. Some card companies divide by 360 instead. Your card agreement states the daily periodic rate, so you can compare it with the one shown here.

How this calculator is checked

Method: Daily balance method: interest = balance × (APR ÷ 365) × days. Minimum-payment mode: monthly simulation at APR ÷ 12 with a percent-of-balance (plus interest) minimum and a dollar floor. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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