Formula and calculation method
The monthly periodic rate equals the entered annual percentage rate divided by 12. Each modeled month, the calculator rounds the balance's finance charge to the nearest cent, adds it to the balance, and then applies the entered payment. The final payment is capped at the amount then due.
The calculation repeats until the modeled balance reaches zero. Total interest is the sum of modeled finance charges, and total paid is the sum of modeled payments. All money state uses whole cents; only the bounded rate multiplication uses JavaScript floating-point arithmetic before cent rounding.
Inputs, outputs, and boundaries
- Inputs: one positive current balance, one fixed annual rate from 0% through 100%, and one positive constant monthly payment, all in U.S. dollars.
- Outputs: payoff months, years-and-months duration, total interest, total paid, final payment, first-month breakdown, and an annual summary.
- Bounds: balance and payment accept $0.01–$1,000,000,000.00 with at most two decimal places; rate accepts at most six decimal places; payoff must occur within 1,200 modeled months.
- Invalid input: blank, signed, exponent-form, excessive-precision, zero, and out-of-range currency are rejected. The payment must exceed the first modeled month's interest so the balance decreases.
Aletheia conventions
This is a single-balance, fixed-rate, constant-payment model. It applies finance charge before one end-of-month payment and uses monthly cent rounding. The annual rate is treated as the rate you enter; the tool does not calculate or verify an APR.
Limitations and financial disclaimer
This estimate is not an account statement, required minimum payment, lender or servicer payoff quote, credit offer, or recommendation. Actual creditors may use daily balances, different day counts, multiple or variable rates, promotions, fees, penalties, payment-allocation rules, and different rounding. New charges are not modeled.
The calculator does not assess affordability, hardship, delinquency, collections, settlement, consolidation, refinancing, forgiveness, bankruptcy, taxes, credit scores, or legal rights. Compare the estimate with your agreement and current statement. Ask the creditor or servicer for an exact payoff amount when one is needed.
Worked example
For a $1,000.00 balance at a fixed 12% annual rate and a $100.00 monthly payment, the modeled monthly rate is 1%. The first finance charge is $10.00, so $90.00 of the first payment reduces principal. Under monthly cent rounding, the estimate reaches zero in 11 months, with $58.98 total interest and a $58.98 final payment.
Common questions
Why can an actual payoff amount differ?
A creditor may include interest through a specific date, fees, penalties, or account-specific terms. This calculator has none of that account data.
What if my rate or payment changes?
This release assumes both remain constant. Calculate separate scenarios for comparison, but treat each as an estimate rather than a prediction.
Does this choose which debt to pay first?
No. It models one balance and does not recommend a snowball, highest-rate, consolidation, settlement, or other strategy.
Related Aletheia tools
Use the Loan Calculator to estimate a fixed payment from a term, or the Compound Interest Calculator for a transparent compound-growth scenario.
Version and verification
Calculation version 1.0.0; formulas, boundaries, and deterministic fixtures last verified 2026-08-11. Authority pages were retrieved Aug. 11, 2026. Automated coverage includes formula fixtures, bounds, cent rounding, privacy, keyboard use, responsive layout, contrast, and Axe. Manual VoiceOver remains waived and is not claimed as passed.