Installment Calculator
Enter the amount, number of installments and fee rate to see monthly payments, total fees and estimated annual cost. Compare multiple plans side by side. For reference only.
Updated July 11, 2026
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How to use
- 1
Enter the total amount and installment count.
- 2
Enter the fee or interest rate.
- 3
See monthly payment, total cost and compare plans.
How it works
The amortization formula, derived
The formula balances two competing forces: the principal shrinks as you pay it down, but interest accrues on whatever balance remains. Setting the present value of an annuity equal to the principal and solving for the payment gives:
M = P · r · (1+r)ⁿ ÷ ((1+r)ⁿ − 1)
Here r is the periodic rate — for monthly payments, divide the nominal APR by 12. Do not compound it. n is the total number of payments: a 5-year monthly loan has n = 60.
APR vs. interest rate vs. real cost
These three numbers are commonly confused, and lenders use the ambiguity to their advantage.
Interest rate is the cost of borrowing the principal, full stop.
APR (Annual Percentage Rate) wraps certain upfront costs — origination fees, points, some insurance — into a single annualized figure. In the U.S. it is defined by Regulation Z (Truth in Lending). But APR does not capture every cost: late fees, some add-on products, and the time value of compounding are often excluded.
Real cost (IRR) — to know what a loan truly costs, compute the internal rate of return on the full cash-flow stream: every disbursement (negative) and every payment including all fees (positive). When a quote looks too good, the difference is almost always in the fees the APR omits.
- APR is mandatory disclosure in many jurisdictions — but its scope varies by country
- Longer terms lower the monthly payment but raise total interest paid
- Even a small rate difference compounds: 4% vs. 5% over 30 years is tens of thousands on a mortgage
How this is calculated
- Formula:
- Monthly payment M = P · r · (1+r)ⁿ ÷ ((1+r)ⁿ − 1), where P = principal, r = monthly rate (nominal APR ÷ 12), n = number of months.
- Applies to:
- Standard fixed-rate amortization; does not model variable rates, fees, or APR-vs-interest differences.
Advertised APR includes some fees but not always all of them, so it can differ from the true interest rate. The real total cost of credit is best measured by the internal rate of return (IRR) on the full payment schedule including all fees. Results are estimates for comparison only — not financial, credit, or lending advice. Confirm exact figures with your lender and the loan agreement.