Fidamen

Auto Loan APR Calculator with Extra Payments

This calculator estimates scheduled payments, total interest, and the impact of extra payments on auto loans. Use it to compare a standard schedule to scenarios with one‑time or recurring extra contributions and to see approximate time and interest savings.

The tool supports monthly schedules and a dedicated bi‑weekly accelerated mode. Results are estimates for planning and do not replace lender payoff quotes; always verify final payoff amounts with your lender.

Updated Nov 13, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record dca8ef495f85 • Reviewed by Fidamen Standards Committee

Compute scheduled periodic payment, total amount paid and total interest for a fixed-rate auto loan using the provided APR, term, and payment frequency.

Inputs

Results

Updates as you type

Scheduled payment

$471.78

Number of payments

60

Total interest

$3,306.85

OutputValueUnit
Scheduled payment$471.78USD
Number of payments60—
Total interest$3,306.85USD
Primary result$471.78

Visualization

Methodology

Calculations use standard amortization formulas for fixed-rate installment loans. Periodic rate = APR / payments_per_year. Scheduled payment is the annuity payment that amortizes principal over the specified number of periods.

Recurring extras are converted to a per‑payment basis when the user provides a monthly extra and the payment frequency differs. One‑time extras are applied in the period specified by the UI (default: immediately). Payoff estimates with extras use the closed‑form inverse of the annuity formula to estimate the new number of periods; this is an analytical approximation that assumes extras are applied consistently each period.

For secure handling and engineering controls, implementation should follow NIST guidelines for secure development and IEEE best practices for numerical stability. Quality management and process controls should align with ISO 9001 principles; workplace safety for any staff involved follows OSHA guidance.

F.A.Q.

Does this calculator give an exact lender payoff?

No. This calculator provides estimates based on standard amortization math. Actual lender payoff amounts can include daily interest accrual methods, fees, rounding rules, or different compounding conventions. Always request a payoff statement from your lender for an exact figure.

Can I use monthly extras with bi‑weekly payments?

Yes. If you enter a monthly extra and choose a bi‑weekly schedule, the calculator converts the monthly amount into an equivalent per‑payment amount for the chosen payment frequency before estimating payoff effects.

Are the results compliant with any standards?

The numeric methods are standard financial formulas. For implementation and data handling we recommend following NIST guidance for secure software development, IEEE principles for numerical stability, ISO 9001 for quality management processes, and OSHA for workplace safety. This calculator is for planning and informational use and is not a regulated disclosure.

How accurate are the payoff and interest‑saved estimates?

Estimates are accurate for loans using consistent periodic interest accrual and fixed payments. Differences may arise from daily interest accrual, lender rounding, applied fees, payment timing, or if the lender applies extras differently. Expect small variances; treat results as a planning guide.

Sources & citations

Further resources

Versioning & Change Control

Audit record (versions, QA runs, reviewer sign-off, and evidence).

Record ID: dca8ef495f85

What changed (latest)

v1.0.0 • 2025-11-13 • MINOR

Initial publication and governance baseline.

Why: Published with reviewed formulas, unit definitions, and UX controls.

Public QA status

PASS — golden 25 + edge 120

Last run: 2026-01-23 • Run: golden-edge-2026-01-23

Engine

v1.0.0

Data

Baseline (no external datasets)

Content

v1.0.0

UI

v1.0.0

Governance

Last updated: Nov 13, 2025

Reviewed by: Fidamen Standards Committee (Review board)

Credentials: Internal QA

Risk level: low

Reviewer profile (entity)

Fidamen Standards Committee

Review board

Internal QA

Entity ID: https://fidamen.com/reviewers/fidamen-standards-committee#person

Semantic versioning

  • MAJOR: Calculation outputs can change for the same inputs (formula, rounding policy, assumptions).
  • MINOR: New features or fields that do not change existing outputs for the same inputs.
  • PATCH: Bug fixes, copy edits, or accessibility changes that do not change intended outputs except for previously incorrect cases.

Review protocol

  • Verify formulas and unit definitions against primary standards or datasets.
  • Run golden-case regression suite and edge-case suite.
  • Record reviewer sign-off with credentials and scope.
  • Document assumptions, limitations, and jurisdiction applicability.

Assumptions & limitations

  • Uses exact unit definitions from the Fidamen conversion library.
  • Internal calculations use double precision; display rounding follows the unit's configured decimal places.
  • Not a substitute for calibrated instruments in regulated contexts.
  • Jurisdiction-specific rules may require official guidance.

Change log

v1.0.0 • 2025-11-13 • MINOR

Initial publication and governance baseline.

Why: Published with reviewed formulas, unit definitions, and UX controls.

Areas: engine, content, ui • Reviewer: Fidamen Standards Committee • Entry ID: 1b45363969a5