Fidamen

Auto Loan Interest Calculator

This calculator estimates scheduled payments, accelerated payoff time, total interest, and total amount paid for auto loans when using monthly, bi‑weekly, weekly, or a custom payment frequency. You can include fees financed into the loan balance, separate upfront fees, and optional extra payments per period to model faster payoff.

Results are intended for planning and comparison only and do not represent a loan offer. Use the inputs that match your lender disclosure (APR, financed fees, and term) for the most accurate estimate.

Updated Nov 6, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 4bb3c9025844 • Reviewed by Fidamen Standards Committee

Standard bi‑weekly schedule: 26 payments per year.

Inputs

Advanced inputs

Custom frequency options

Results

Updates as you type

Scheduled payment per period

-$0.37

Payment per period (with extra)

-$0.37

Estimated payoff time (years)

5

Total interest estimated

-$25,048.08

Total paid (includes upfront fees)

-$48.08

OutputValueUnit
Scheduled payment per period-$0.37USD
Payment per period (with extra)-$0.37USD
Estimated payoff time (years)5years
Total interest estimated-$25,048.08USD
Total paid (includes upfront fees)-$48.08USD
Primary result-$0.37

Visualization

Methodology

Calculations use standard amortization formulas to compute periodic payments and total interest. When extra payments are included, the calculator estimates the reduced number of payments using the closed‑form solution for a fixed periodic payment; this assumes each extra payment fully reduces principal at each period.

The tool supports multiple payment frequency models by converting the annual percentage rate to a periodic rate for the chosen frequency and computing the number of scheduled periods from the loan term.

For custom frequencies, the number of payments per year is taken from the custom input. Upfront fees are added to total paid but are not included in financed principal unless marked as financed.

Key takeaways

Use the bi‑weekly mode to model 26 payments per year and compare total interest and payoff time against monthly or weekly schedules.

Provide APR, financed fees, and term from your lender disclosure for the most reliable estimate. Consider lender posting rules and possible prepayment penalties before acting.

F.A.Q.

Why does a bi‑weekly schedule often shorten the loan faster than monthly?

A bi‑weekly schedule makes 26 payments per year (equivalent to 13 monthly payments), so more principal is paid sooner compared with 12 monthly payments. That reduces outstanding principal and the total interest paid over the life of the loan.

Are these numbers exact for every lender?

No. Lenders may apply different day‑count conventions, rounding rules, or compounding approaches. Use your loan disclosure (truth in lending statement) as the authoritative source. This calculator follows standard amortization math and provides consistent, comparable estimates.

How accurate are the accelerated payoff estimates when I add extra payments?

The calculator uses the closed‑form amortization solution to estimate payoff with fixed extra payments each period. This is accurate for fixed‑rate loans with payments applied immediately to principal. If your lender posts payments differently or charges prepayment penalties, results will differ.

What security and quality standards apply to the calculator?

Numeric algorithms aim to follow best practices for stability and reproducibility consistent with IEEE floating point guidance. Software security and data handling should follow national guidance such as NIST recommendations. This tool provides estimates and does not store sensitive loan data by default.

Sources & citations

Further resources

External guidance

Versioning & Change Control

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

Record ID: 4bb3c9025844

What changed (latest)

v1.0.0 • 2025-11-06 • 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 6, 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 • 2025-11-06 • 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: ac4b3ce313f0