Credit Card Interest Calculator: Monthly Charge & Payoff

How credit card interest is charged

On the defaults — a $3,500.00 balance at 21.99% APR over a 31-day cycle, paying $125.00 a month — the daily periodic rate is 0.0602% a day (exact 0.0602466%), the interest this cycle is $65.37, the interest in a 30-day month is $63.26, the interest in a year is $769.65, the interest share of the $125.00 payment is 52.30%, and the payoff takes 40 months for $1,454.37 of interest and $4,954.37 total. The whole calculation is three moves: divide the APR by 365 for the daily rate, multiply the balance by the daily rate and the days for the cycle charge, and sim the payoff month by month at APR ÷ 12.

Five things about the model are worth stating plainly. First, the daily periodic rate is 21.99% ÷ 365 = 0.0602% a day. Second, the cycle interest is $3,500.00 × 21.99% × 31 ÷ 365 = $65.37. Third, the 30-day interest is $3,500.00 × 21.99% × 30 ÷ 365 = $63.26 and the annual interest is $3,500.00 × 21.99% = $769.65. Fourth, the interest share is $65.37 ÷ $125.00 = 52.30% — more than half of the payment is interest. And fifth, the payoff sim’s first-month interest is $64.14 (APR ÷ 12), a little lower than the cycle interest $65.37 (31 days ÷ 365) — the two figures differ by design, not by mistake; all listed in Accuracy & limitations.

The worked example, line by line

Setup: $3,500.00 balance, 21.99% APR, 31-day cycle, $125.00 a month.

Line 1 — the daily periodic rate. The daily periodic rate is 21.99% ÷ 365 = 0.0602% a day (exact 0.0602466%).

Line 2 — the cycle interest. The interest this cycle is $3,500.00 × 21.99% × 31 ÷ 365 = $65.37.

Line 3 — the month and the year. The interest in a 30-day month is $3,500.00 × 21.99% × 30 ÷ 365 = $63.26, and the interest in a year if the balance stays is $3,500.00 × 21.99% = $769.65.

Line 4 — the share of the payment. The interest share is $65.37 ÷ $125.00 = 52.30% — more than half of the payment is interest.

Line 5 — the payoff. Paying $125.00 a month, month by month at APR ÷ 12 with the monthly interest rounded to cents, clears the balance in 40 months for $1,454.37 of interest and $4,954.37 total.

Paying only the interest

Paying exactly one cycle’s interest — $65.37 — leaves the balance at $3,500.00; next cycle’s interest is $65.37 again. The balance never falls. There is no payoff month for an interest-only payment — the calculator does not run the payoff sim for it, because there is nothing to pay off. This is the trap of paying the smallest amount that feels like progress: the statement shows a payment made, and the balance does not move.

Every branch, checked

$5,000 at 24.99% APR, 30 days, pay $200: Balance $5,000.00, APR 24.99%, 30 days, payment $200.00/month: daily periodic rate 0.0685%, cycle interest $102.70, 30-day interest $102.70, annual interest $1,249.50, interest share of payment 51.35%, payoff 36 months, total interest $2,135.18, total paid $7,135.18 — A bigger balance at a higher rate costs $102.70 in one cycle — and $200 a month still takes 36 months to clear it.

$1,000 at 18.99% APR, 30 days, pay $100: Balance $1,000.00, APR 18.99%, 30 days, payment $100.00/month: daily periodic rate 0.0520%, cycle interest $15.61, 30-day interest $15.61, annual interest $189.90, interest share of payment 15.61%, payoff 11 months, total interest $97.23, total paid $1,097.23 — A small balance at a lower rate costs $15.61 a cycle, and $100 a month clears it in 11 months.

$10,000 at 29.99% APR, 31 days, pay $500: Balance $10,000.00, APR 29.99%, 31 days, payment $500.00/month: daily periodic rate 0.0822%, cycle interest $254.71, 30-day interest $246.49, annual interest $2,999.00, interest share of payment 50.94%, payoff 29 months, total interest $4,033.78, total paid $14,033.78 — A $10,000 balance near 30% APR costs $254.71 in one cycle — half the $500 payment is interest.

$3,500 at 0% APR, 30 days, pay $125: Balance $3,500.00, APR 0.00%, 30 days, payment $125.00/month: daily periodic rate 0.0000%, cycle interest $0.00, 30-day interest $0.00, annual interest $0.00, interest share of payment 0.00%, payoff 28 months, total interest $0.00, total paid $3,500.00 — At 0% APR there is no interest at all — every dollar of the $125 payment cuts the balance, and 28 months clears it.

$3,500 at 21.99% APR, 28 days (February), pay $125: Balance $3,500.00, APR 21.99%, 28 days, payment $125.00/month: daily periodic rate 0.0602%, cycle interest $59.04, 30-day interest $63.26, annual interest $769.65, interest share of payment 47.23%, payoff 40 months, total interest $1,454.37, total paid $4,954.37 — A shorter February cycle costs $59.04 instead of $65.37 — three fewer days saves $6.33 — and the payoff is unchanged at 40 months.

$3,500 at 21.99% APR, 31 days, pay $200: Balance $3,500.00, APR 21.99%, 31 days, payment $200.00/month: daily periodic rate 0.0602%, cycle interest $65.37, 30-day interest $63.26, annual interest $769.65, interest share of payment 32.69%, payoff 22 months, total interest $759.09, total paid $4,259.09 — Paying $200 instead of $125 nearly halves the payoff — 22 months instead of 40 — and saves $695.28 of interest.

$3,500 at 21.99% APR, 31 days, pay $250: Balance $3,500.00, APR 21.99%, 31 days, payment $250.00/month: daily periodic rate 0.0602%, cycle interest $65.37, 30-day interest $63.26, annual interest $769.65, interest share of payment 26.15%, payoff 17 months, total interest $581.81, total paid $4,081.81 — Paying $250 a month clears the balance in 17 months for $581.81 of interest — less than half the $1,454.37 at $125 a month.

The reverses, worked backwards

Balance for $100.00 of interest in a 30-day month at 21.99% APR: $5,532.82 — the exact balance is 5532.8179. At the displayed $5,532.82 the interest is $100.00. $5,532.81 gives $100.00 as well at cent rounding — both round to $100.00 — and $5,532.82 is the smallest cent value whose unrounded interest reaches $100.00. The star is DOLLARS.

APR for $50.00 of interest in a 30-day cycle on a $3,500 balance: 17.38% — the exact APR is 17.3810%. At the displayed 17.38% the interest is $50.00. 17.39% gives $50.03; 17.37% gives $49.97. The star is RATE.

Days for interest to reach $75.00 on a $3,500 balance at 21.99% APR: 36 days — the exact days are 35.5681. At 36 days the interest is $75.91; at 35 days it is $73.80. 36 days is the smallest whole number of days reaching at least $75.00. The star is DAYS.

Monthly payment for the interest to be exactly 50% of the payment: $130.74 — twice the cycle interest of $65.37. At $130.74 the share is 50.00%. $130.73 and $130.75 both round to a 50.00% share as well, but $130.74 is exactly twice $65.37. The star is DOLLARS.

APR for $1,000.00 of annual interest on a $3,500 balance: 28.57% — the exact APR is 28.5714%. At the displayed 28.57% the annual interest is $999.95, $0.05 under the $1,000.00 target — rounding. 28.56% gives $999.60; 28.58% gives $1,000.30. The star is RATE.

Balance for $200.00 of interest in a 31-day cycle at 24.99% APR: $9,423.12 — the exact balance is 9423.1241. At the displayed $9,423.12 the interest is $200.00. $9,423.11 and $9,423.13 both round to $200.00 as well. The star is DOLLARS.

What the defaults assume

The defaults are a $3,500.00 balance at 21.99% APR over a 31-day cycle, paying $125.00 a month — cycle interest $65.37, annual interest $769.65, payoff 40 months. With a $5,000.00 balance at 24.99%, Balance $5,000.00, APR 24.99%, 30 days, payment $200.00/month: daily periodic rate 0.0685%, cycle interest $102.70, 30-day interest $102.70, annual interest $1,249.50, interest share of payment 51.35%, payoff 36 months, total interest $2,135.18, total paid $7,135.18 — the cycle interest is $102.70. With a $1,000.00 balance at 18.99%, Balance $1,000.00, APR 18.99%, 30 days, payment $100.00/month: daily periodic rate 0.0520%, cycle interest $15.61, 30-day interest $15.61, annual interest $189.90, interest share of payment 15.61%, payoff 11 months, total interest $97.23, total paid $1,097.23 — the cycle interest is $15.61. With a $10,000.00 balance at 29.99%, Balance $10,000.00, APR 29.99%, 31 days, payment $500.00/month: daily periodic rate 0.0822%, cycle interest $254.71, 30-day interest $246.49, annual interest $2,999.00, interest share of payment 50.94%, payoff 29 months, total interest $4,033.78, total paid $14,033.78 — the cycle interest is $254.71. With 0% APR, Balance $3,500.00, APR 0.00%, 30 days, payment $125.00/month: daily periodic rate 0.0000%, cycle interest $0.00, 30-day interest $0.00, annual interest $0.00, interest share of payment 0.00%, payoff 28 months, total interest $0.00, total paid $3,500.00 — no interest at all. With a 28-day February cycle, Balance $3,500.00, APR 21.99%, 28 days, payment $125.00/month: daily periodic rate 0.0602%, cycle interest $59.04, 30-day interest $63.26, annual interest $769.65, interest share of payment 47.23%, payoff 40 months, total interest $1,454.37, total paid $4,954.37 — the cycle interest is $59.04. With a $200.00 payment, Balance $3,500.00, APR 21.99%, 31 days, payment $200.00/month: daily periodic rate 0.0602%, cycle interest $65.37, 30-day interest $63.26, annual interest $769.65, interest share of payment 32.69%, payoff 22 months, total interest $759.09, total paid $4,259.09 — the payoff is 22 months. With a $250.00 payment, Balance $3,500.00, APR 21.99%, 31 days, payment $250.00/month: daily periodic rate 0.0602%, cycle interest $65.37, 30-day interest $63.26, annual interest $769.65, interest share of payment 26.15%, payoff 17 months, total interest $581.81, total paid $4,081.81 — the payoff is 17 months. With a balance of $0, Balance $0.00, APR 21.99%, 31 days, payment $125.00/month: daily periodic rate 0.0602%, cycle interest $0.00, 30-day interest $0.00, annual interest $0.00, interest share of payment 0.00%, payoff 0 months, total interest $0.00, total paid $0.00 — no interest, though the daily rate of 0.0602% still exists. With a $10.00 payment, Balance $3,500.00, APR 21.99%, 31 days, payment $10.00/month: daily periodic rate 0.0602%, cycle interest $65.37, 30-day interest $63.26, annual interest $769.65, interest share of payment 653.70%, payoff never — the balance grows because the $10.00 payment is below the first month’s interest of $64.14 — the balance grows. With a $100,000.00 balance, Balance $100,000.00, APR 21.99%, 31 days, payment $125.00/month: daily periodic rate 0.0602%, cycle interest $1,867.64, 30-day interest $1,807.40, annual interest $21,990.00, interest share of payment 1,494.11%, payoff never — the balance grows because the $125.00 payment is below the first month’s interest of $1,832.50 — the cycle interest is $1,867.64. Over a 365-day year, Balance $3,500.00, APR 21.99%, 365 days, payment $125.00/month: daily periodic rate 0.0602%, cycle interest $769.65, 30-day interest $63.26, annual interest $769.65, interest share of payment 615.72%, payoff 40 months, total interest $1,454.37, total paid $4,954.37 — the cycle interest is $769.65, the same as the annual figure. Over a 366-day leap year, Balance $3,500.00, APR 21.99%, 366 days, payment $125.00/month: daily periodic rate 0.0602%, cycle interest $771.76, 30-day interest $63.26, annual interest $769.65, interest share of payment 617.41%, payoff 40 months, total interest $1,454.37, total paid $4,954.37 — the cycle interest is $771.76, $2.11 more than the 365-day year.

Output · formula · meaning

Who uses this

• The cardholder who just got a statement: they learn this cycle’s interest is $65.37 on a $3,500.00 balance at 21.99% APR — and they check the $5,532.82 reverse to see the balance that would cost $100.00 of interest.
• The borrower paying $125.00 a month: they learn 52.30% of the payment is interest and the payoff takes 40 months — and they check the $130.74 reverse to see the payment that makes interest exactly half the payment.
• The shopper on a 0% APR offer: they learn the interest is $0.00 and $125.00 a month clears the balance in 28 months — and they check the 28.57% reverse to see the APR that would cost $1,000.00 a year.
• The February budgeter: they learn a 28-day cycle costs $59.04 instead of $65.37 — three fewer days saves $6.33 — and they check the 36-day reverse to see how many days it takes the interest to reach $75.00.

Accuracy & limitations

The math is first-principles: the daily periodic rate is the APR divided by 365, the cycle interest is the balance times the APR times the days over 365, and each figure is rounded to cents half-up. What the model cannot do is reproduce your issuer’s exact charge: it assumes the balance stays constant through the cycle (average daily balance — real issuers use your actual daily balances, so a payment mid-cycle lowers the charge); it ignores new purchases, fees, penalty APR, and grace periods (a balance paid in full by the due date accrues $0); it assumes a fixed payment every month; and paying only the interest leaves the balance unchanged forever. The payoff sim rounds the monthly interest to cents each month and never rounds the balance. Use it to see what one cycle costs and how fast a fixed payment clears the balance — then check your statement for the exact charge.

Frequently asked questions