Fidamen

Auto Loan Interest Calculator with Extra Payments

This calculator estimates payments, payoff time, and interest for auto loans and shows how one-time or recurring extra payments change the outcome. Use it to compare a baseline monthly amortization against accelerated options.

Enter your loan amount, APR and term. Add a one-time extra or recurring extra payment to see updated payoff months and estimated interest savings. For biweekly schedules the tool uses 26 payments per year (two payments per month in most months) to illustrate typical acceleration effects.

Updated Nov 16, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record b4a3f1ee78ab • Reviewed by Fidamen Standards Committee

Baseline monthly amortization schedule without extra payments used as the comparison point for savings.

Inputs

Advanced inputs

One-time extra payment

Recurring extra payment

Results

Updates as you type

Monthly payment

-$1.53

Term (months)

60

Total interest (no extras)

-$20,091.67

OutputValueUnit
Monthly payment-$1.53USD
Term (months)60months
Total interest (no extras)-$20,091.67USD
Primary result-$1.53

Visualization

Methodology

Monthly amortization uses the standard fixed-rate loan formula: payment = P * r / (1 - (1+r)^-N) where r is the periodic rate and N is the number of periods.

One-time and recurring extra payment effects are computed using closed-form formulas for remaining balance and the analytical solution for number of periods remaining using logarithms when the periodic rate is non-zero.

Biweekly schedules use a 26-payment-per-year convention; where exact day-level accrual matters (e.g., leap years or differing billing cycles), results are approximate. All calculations assume payments are applied at scheduled intervals and interest compounds at the stated periodic rate.

Key takeaways

Use the standard amortization results as the baseline. Adding extra payments (one-time or recurring) reduces remaining balance and total interest; recurring small amounts often compound into meaningful savings.

Treat all outputs as estimates. Confirm exact payoff and savings with your lender because billing practices, daily accrual, rounding and fees can change results.

F.A.Q.

Does the calculator account for fees, taxes or insurance?

No. This tool models interest-only amortization on the principal and does not include separate fees, title taxes, GAP insurance, or other charges. Add those to the principal if you want them included in the schedule.

How accurate are the savings estimates for extra payments?

Estimates use analytical formulas and assume scheduled application and compounding at the stated periodic rate. If your lender applies payments differently (daily interest accrual, minimum payment rules, or prepayment penalties) your actual savings may differ.

What happens if APR is zero?

Formulas that divide by the periodic rate require special handling when APR = 0. The calculator falls back to equal principal division (principal / number of periods) in that case. If you rely on a zero-APR promotion, verify with lender statements.

Are biweekly results exact?

Biweekly estimates assume 26 equal payments per year and that each payment is applied immediately. Exact amortization can differ if your lender reindexes payment dates, charges fees, or applies daily interest. Use the biweekly mode as an illustrative comparison.

Sources & citations

Further resources

Versioning & Change Control

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

Record ID: b4a3f1ee78ab

What changed (latest)

v1.0.0 • 2025-11-16 • 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 16, 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 • 2025-11-16 • 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: fdb414b05344