Credit card interest calculator
Check the finance charge printed on your statement: 1000 carried for a 30-day cycle at 18% APR costs 14.79 in interest. You also get the daily rate and the new balance if nothing is paid. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Balance
- 1,000
- Annual rate percent
- 18
- Days
- 30
- Daily periodic rate
- 0.0004931507
- Daily rate percent
- 0.04931507
- Interest amount
- 14.79
- New balance
- 1,014.79
Credit card interest is the finance charge a revolving account adds when you carry a statement balance through a billing cycle. Issuers convert the annual percentage rate into a daily periodic rate, then multiply that daily rate by the balance and by the number of days in the cycle. This calculator accepts those three inputs—balance, APR as a percent, and days—and returns the interest charged, the daily periodic rate, and the new balance after the charge. Arithmetic is deterministic, with no network, no sampling, and no clock.
How issuers turn APR and days into a finance charge
The finance charge on a revolving credit card is almost always an average-daily-balance identity, not a simple annual percentage applied to the whole year in one lump. The issuer converts the annual percentage rate into a daily periodic rate by dividing the APR, expressed as a percent, by one hundred and then by three hundred sixty-five, the day-count this calculator uses. That daily rate is the cost of carrying one currency unit for one calendar day.
The interest charged for a billing cycle is then the product of three quantities: the balance you are carrying, or a constant average daily balance when purchases and payments do not change the running total during the cycle; the daily periodic rate; and the number of days in the statement period. A thirty-day cycle at eighteen percent on one thousand currency units therefore costs one thousand times 0.18 times thirty over three hundred sixty-five, which is about fourteen dollars and seventy-nine cents after ordinary half-away-from-zero rounding to two decimal places.
There is no grace-period clock, no purchase-versus-cash split, and no compounding of unpaid interest inside the same cycle: those rules live on the cardholder agreement. This page answers only the transparent balance-APR-days identity that every statement already prints next to interest charged.
Input contract, rounding, and error behaviour
A negative balance is rejected because a credit—the issuer owes you—is not a finance charge on an amount you owe; that is the documented error this calculator exists to enforce. A negative APR is rejected for the same honesty. Zero days is rejected because a cycle of length zero does not define a finance charge.
Zero balance with a valid APR and days is allowed and yields interest of zero. The path uses no Date, no randomness, and no network, so goldens compare byte-for-byte and automated tests can pin fourteen dollars and seventy-nine cents without re-implementing the identity by hand.
Where a billing-cycle interest figure earns its keep
People reach for a credit card interest calculator when a statement lands with a finance charge they want to reproduce from the printed APR and cycle length, when a zero-percent introductory period is about to expire and they want the first full-APR cycle in cents, or when a payoff plan needs next month's interest before they decide whether to pay in full. Budget coaches use the same three-number identity to show why carrying a revolving balance is expensive even for a short cycle of thirty days.
Spreadsheet models that already project purchases still need a single-cycle oracle that matches the statement method rather than a generic simple-interest helper that takes time in years. Software tests and interview drills need a deterministic interest field they can assert without opaque macros. This identity is not a minimum-payment projector, not a debt-payoff timeline, and not a compound-interest account that capitalizes over years: those searches belong to other tools.
It is the number every revolving statement already prints next to the APR and the cycle length.
What you can do with it
Reproduce a statement finance charge
Enter the statement balance, the disclosed APR, and the number of days in the cycle to check that the interest line matches the average-daily-balance identity before you dispute or pay.
Price the first cycle after a 0% intro
When a promotional APR expires, run the new rate against the remaining balance and the next cycle length so the first full finance charge is a known number of cents.
FAQ
How is credit card interest calculated from balance, APR, and billing-cycle days?
The daily periodic rate is APR/100/365. Interest charged equals balance times that daily rate times the number of days in the cycle, rounded to two decimal places.
Does this credit card interest calculator use a 365-day or a 360-day year?
It uses 365 days. The daily periodic rate is the APR divided by 100 divided by 365, which is the day-count most U.S. revolving disclosures use for a daily rate.
Is this the same as a minimum-payment or simple-interest calculator?
No. This identity uses one cycle's balance, APR, and days only. A minimum-payment tool projects months to pay off; a simple-interest tool takes time in years.
What does the new balance in the result show?
It is the balance you entered plus the interest for the cycle, so you can see the next opening figure if nothing is paid. For 1000 at 18% over 30 days, that is 1014.79.
What does this calculator leave out?
It applies only the balance, APR and days on your statement. There is no grace period, no split between purchases and cash advances, and no interest charged on unpaid interest inside the same cycle.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/fin/credit-card-interest \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"balance":1000,"annual_rate_percent":18,"days":30}'const res = await fetch("https://api.kit.forhosting.com/fin/credit-card-interest", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"balance": 1000,
"annual_rate_percent": 18,
"days": 30
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/fin/credit-card-interest",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"balance": 1000,
"annual_rate_percent": 18,
"days": 30
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/fin/credit-card-interest", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"balance":1000,"annual_rate_percent":18,"days":30}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"balance":1000,"annual_rate_percent":18,"days":30}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/fin/credit-card-interest", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"balance": 1000,
"annual_rate_percent": 18,
"days": 30
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "fin.credit_card_interest",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_abs_value | 1000000000000000 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
A negative balance is rejected with invalid_input because a credit on the account is not an amount you owe. Use the tool free in the browser, or call the API at $0.002 per successful item for worksheets, payoff sketches, and statement checks that must see the same cents every time.
How issuers turn APR and days into a finance charge
Written out, interest equals balance times annual_rate_percent divided by one hundred times days divided by three hundred sixty-five. The response echoes the three inputs, reports the daily periodic rate as a decimal and as a percent, returns interest in currency units, and adds new_balance equal to the original balance plus that interest so you can see the next opening figure if nothing is paid.
Input contract, rounding, and error behaviour
POST a JSON body with three required fields. balance is the amount you owe in one currency unit: a statement balance, or a constant average daily balance when the running total did not move. annual_rate_percent is the APR as a percent, for example 21.99 for 21.99 percent, not a decimal 0.2199. days is the length of the billing cycle as a positive integer, typically twenty-eight to thirty-one, and at most three hundred sixty-six so a leap-year or long first statement still fits. Aliases amount and principal for balance; apr, annual_percentage_rate, and rate for the APR; and billing_days or billing_cycle_days for days are accepted so spreadsheets and typed forms can reuse the same contract. Missing fields, empty strings, non-finite values, and magnitudes above the documented ceiling fail with invalid_input naming the field. Currency results, interest and new_balance, round to two decimal places with half-away-from-zero rounding; the daily periodic rate keeps more decimal places so clients can show the disclosure figure without re-deriving APR over 365.
Where a billing-cycle interest figure earns its keep
On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split—free interactive exploration and a priced, identical API—matches the rest of the KIT tier-A catalog and lets you move from a kitchen-table check to an automated pipeline without changing the mathematical result.
Integration use cases
Feed a payoff sketch or unit test
Call the same contract from a worksheet or CI fixture to pin next-cycle interest without re-implementing APR over 365 or rounding to cents by hand.
Technical questions
What happens if the credit card balance is negative?
The task fails with invalid_input. A negative balance is a credit the issuer owes you, not an amount that accrues a finance charge, so the request is not billed.
What does a credit card interest calculator request cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.