Fidamen

Boat Loan Amortization Calculator with Extra Payments

This calculator models boat loan amortization and the effect of adding recurring or one-time extra payments. It estimates periodic payment amounts, the number of payments to payoff, total interest paid, and interest savings from extra payments.

Use the standard method to see a direct comparison of scheduled payments versus making recurring extras. Use the target-payoff method to find the periodic payment needed to retire the loan in a specified time.

Updated Nov 9, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record ea80aa667093 • Reviewed by Fidamen Standards Committee

Calculate the regular periodic payment for the loan and then estimate the reduced payoff time and interest when making recurring extra payments and a single one-time principal reduction.

Inputs

Results

Updates as you type

Scheduled periodic payment (no extras)

-$1.25

Total interest without extras

-$30,150.00

Payments until paid off (with recurring extras)

—

Total interest with extras

—

Interest saved

—

OutputValueUnit
Scheduled periodic payment (no extras)-$1.25USD
Total interest without extras-$30,150.00USD
Payments until paid off (with recurring extras)—payments
Total interest with extras—USD
Interest saved—USD
Primary result-$1.25

Visualization

Methodology

We compute periodic interest as the nominal annual rate divided by the number of payments per year and use the standard fixed-payment amortization formula to derive the base periodic payment.

To estimate the payoff time with recurring extras we apply the closed-form inverse of the capital recovery formula to approximate the reduced number of payments. One-time extras are treated as an immediate principal reduction in the estimate; detailed schedules with time-specific one-time reductions require an amortization table and iterative application.

Worked examples

Example 1: $30,000 principal, 6% APR, 10 years, monthly. Recurring extra $100 reduces the number of payments and saves interest shown in the outputs.

Example 2: To pay off a $20,000 loan in 5 years at 5% APR with biweekly payments, use the target-payoff method to determine the required periodic payment before/after recurring extras.

F.A.Q.

How accurate are the payoff and interest savings estimates?

Estimates use closed-form amortization formulas and simple adjustments for recurring extras. They are accurate for recurring extras applied each payment period. Because one-time extras are applied as an immediate principal reduction for summary outputs, exact schedules may differ slightly; for transaction-level accuracy, generate a full amortization schedule and apply extras at the exact payment index.

Do you calculate APR, fees, or balloon payments?

This tool models standard fixed-rate amortization and extra principal payments. It does not compute lender fees rolled into APR, balloon structures, interest-only periods, or changing interest rates. For APR compliance or disclosures, consult your lender's statements.

What payment frequencies are supported?

Monthly (12), biweekly (26), and weekly (52) frequencies are supported. The periodic rate and number of periods are adjusted accordingly.

How should I treat one-time extra payments?

For a reliable summary, enter one-time extras as an immediate principal reduction. For precise timing (for example a one-time payment applied mid-loan), use a detailed amortization schedule that applies the one-time extra at the specific payment number.

What standards and practices guide the calculator's development?

This tool follows secure software and numeric handling guidance and documentation practices referenced from standards organizations for numerical reproducibility, testing, and user safety.

Sources & citations

Further resources

Versioning & Change Control

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

Record ID: ea80aa667093

What changed (latest)

v1.0.0 • 2025-11-09 • 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 9, 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 • 2025-11-09 • 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: ceee1e87c26d