Fidamen

Auto Loan Interest-Only Calculator

This calculator compares an interest‑only auto loan structure (pay only interest for a fixed initial period, then amortize) with a fully amortizing loan that repays principal and interest over the full term.

Use realistic inputs for principal, APR, payment frequency, interest‑only length, and total term. Results show periodic payments, total interest, and total cash paid for clear decision support.

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

Governance

Record 98f7fca34541 • Reviewed by Fidamen Standards Committee

Calculates the periodic interest‑only payment during the interest‑only period, then the amortizing payment that repays principal over the remaining term.

Inputs

Results

Updates as you type

Interest‑Only Payment (per period)

$137.50

Amortizing Payment (per period after IO)

$143.45

Total Interest Paid

$73,140.70

Total Cash Paid

$103,140.70

OutputValueUnit
Interest‑Only Payment (per period)$137.50—
Amortizing Payment (per period after IO)$143.45—
Total Interest Paid$73,140.70—
Total Cash Paid$103,140.70—
Primary result$137.50

Visualization

Methodology

Interest‑only periodic payment equals the principal multiplied by the periodic interest rate: principal × (APR/100 ÷ payments per year). During the interest‑only period, principal remains unchanged.

After the interest‑only period the calculator computes the amortizing payment required to repay the principal over the remaining number of periods using the standard annuity formula.

All rate calculations use the nominal APR divided by the payment frequency to obtain a periodic rate; compounding between payment periods is not modeled beyond that periodic rate.

Worked examples

Example 1: $30,000 principal, 5.5% APR, monthly payments, 2‑year IO, 60‑year term. The calculator shows the monthly interest‑only payment during year 1–2, the monthly amortizing payment after year 2, and total interest over the loan life.

Example 2: Shorter term reduces total interest in a fully amortizing loan; interest‑only can lower early payments but often increases total interest unless followed by refinancing or principal prepayments.

F.A.Q.

Does this calculator include fees, taxes, or insurance?

No. This tool models interest and principal only. Add fees, taxes, and insurance to your budget separately; they are not included in payment or total cost outputs.

Is APR the same as the periodic rate used here?

This calculator uses the nominal APR divided by payments per year to compute a periodic rate. It does not perform additional APR to effective annual rate conversions or include fees in APR.

What happens if the interest‑only period equals the full term?

If the interest‑only period equals the total term, the tool shows zero amortization periods and an amortizing payment of zero; total interest equals the interest paid during the IO period.

How accurate are the results?

Results follow standard finance formulas. Rounding and floating‑point arithmetic can introduce small differences; see the accuracy and standards notes in citations.

Sources & citations

Further resources

Versioning & Change Control

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

Record ID: 98f7fca34541

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: 2bd15fc9200b