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Credit Card Installment Calculator

Enter the installment amount, number of periods and per-period fee rate, choose whether fees are charged each period or upfront, and get the nominal rate, real APR (IRR), monthly payment and total fees.

"Only 0.6% a month" sounds cheap until you notice that the fee is charged on the full amount while the balance falls every month — by the end you are paying for money you have already repaid. Enter the amount, the number of instalments and the fee, choose how the fee is charged, and the headline rate and the true annualised cost appear side by side. They commonly differ by a factor of nearly two.

The IRR is the number that matters: repaying 12,000 over 12 instalments plus 60 a month in fees leaves an average balance of only about half the face value, so the true annualised rate lands at roughly 1.8 to 1.9 times the quoted fee rate. The practical test is to compare that true rate with what the same money earns elsewhere — if the plan costs more than your savings yield, paying in full is the better deal.

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How to use

  1. Enter the instalment amount, the number of periods and the fee per period.
  2. Choose whether the fee is charged each period or once up front.
  3. Compare the nominal annual rate with the true annualised rate (IRR).
  4. Compare the IRR against your own cost of funds before choosing to spread the payment.

How it works

Why the real APR far exceeds the nominal

The key is the interest basis: a credit-card installment fee is charged on the **full original principal**, but the principal declines monthly. For 12,000 over 12 periods at a 0.75% per-period rate, each period's fee stays 12,000 x 0.75% = 90, totaling 1,080; but by the 12th period you actually owe only 1,000 and still pay a 90 fee — the real APR works out to about 17.48%, far above "0.75% x 12 = 9%."

How IRR is reverse-engineered

The real APR uses cash-flow IRR: record the up-front principal received as a positive flow and each repayment as a negative flow, solve for the period rate that zeroes the NPV, then annualize with (1 + period rate)^12 - 1. This is the common convention for comparing mortgages and consumer loans, so it's directly comparable with other borrowing products' APRs.

Impact of the fee-charging method

If the fee is charged once up front (with later periods repaying only principal), the funds are committed earlier and larger, so the real APR is higher than when charged per period: the same 12,000 over 12 periods at 0.75% gives an IRR APR of about 18.98% charged up front. Most banks charge per period; a few consumer-finance products charge up front — which method applies significantly affects the cost.

Calculation basis: the true annual rate is the internal rate of return of the payment cash flows (solved by bisection) annualised as (1+i)^12 − 1; the nominal annual rate is simply the per-period fee × 12. Fees and early-settlement rules differ between issuers, so the instalment agreement is what counts.

Code example

JavaScript The real annual rate of an instalment plan (IRR) 

function installmentRealRate(amount, months, feeRatePerMonth) {
  const fee = amount * feeRatePerMonth;
  const payment = amount / months + fee;      // repayment per period
  // Cashflow: amount received (positive), payment per period (negative)
  let lo = 0, hi = 0.1;                       // bisect on the monthly rate
  for (let i = 0; i < 100; i++) {
    const mid = (lo + hi) / 2;
    const pv = Array.from({ length: months }, (_, t) =>
      payment / Math.pow(1 + mid, t + 1)).reduce((a, b) => a + b, 0);
    pv > amount ? lo = mid : hi = mid;
  }
  return { monthly: (lo + hi) / 2,
           annual: Math.pow(1 + (lo + hi) / 2, 12) - 1 };
}
// 10,000 over 12 periods at 0.6% each: 7.2% nominal, about 13% real annual rate

Python Verifying with numpy_financial 

import numpy_financial as npf

amount, months, fee = 10000, 12, 60
payment = amount / months + fee

irr_m = npf.irr([amount] + [-payment] * months)
annual = (1 + irr_m) ** 12 - 1

round(annual * 100, 2)   # ≈ 13.03 (7.2% nominal)

FAQ

Why is 0.75% x 12 = 9% but the real APR 17%?

Because the fee is charged on the full original principal while your balance declines monthly. On average you actually use only about half the principal, so the same 1,080 fee spread over "average funds used" nearly doubles the annualized cost. This is where installment products are most often misunderstood.

Should I look at nominal or real APR?

For decisions, the real APR. The nominal is just "per-period rate x 12," ignoring the declining principal; the real APR (IRR) is what can be compared on one dimension against mortgage rates, credit-loan rates and money-market returns.

Why is an up-front fee more expensive?

Because the money is paid earlier. Paying the entire fee in the first period means those funds are committed longest and discounted least, so the IRR is higher. In the 12,000-over-12-periods-at-0.75% example, per-period charging gives about 17.48% and up-front about 18.98%.

Does prepayment save the fee?

Most banks state that fees already charged are not refunded on early settlement, and a few add part of the remaining periods' fees. So under per-period charging, prepayment usually saves only the not-yet-charged portion — check the installment agreement, and always ask "are fees refunded on prepayment" before signing.

Which is cheaper, installments or minimum repayment?

Installments are almost always cheaper. With minimum repayment, the unpaid portion accrues daily interest of 0.05% (about 18.25% annual), often with full-amount penalty interest (some banks charge on the unpaid portion), so the real cost usually exceeds installments. Both are far above ordinary consumer loans — owing nothing is best.

How does the real APR compare with consumer loans?

Compare the annualized figures directly. Installment-plus-fee IRRs generally run 13-18%, while bank consumer loans commonly run 3-8% and housing-fund loans lower. So "interest-free but fee-charging" installments often cost far more than they sound.

Why does a longer term raise the real APR?

The longer the term, the more periods charge the fee on the full principal while the principal is spread thinner, lowering the average funds used relative to principal, so the annualized rate rises. At the same fee rate, 24 periods yields a higher real APR than 12.

Are the installment amount and term recorded?

They aren't uploaded. IRR solving and fee calculation run entirely in the browser, so the server never receives these numbers; the on-page history stays local and is clearable in one click.