Fidamen

Boat Loan Interest Calculator with Extra Payments

This calculator helps you estimate monthly or biweekly payments for a fixed-rate boat loan and shows how recurring extras or one-time lump-sum payments affect payoff time and total interest. Results are estimates intended for planning and comparison, not guaranteed loan offers.

The tool supports standard amortization, recurring monthly extras, a single lump-sum payment applied at a chosen month, and a 26-payments-per-year biweekly mode. Use conservative inputs and confirm exact terms with your lender.

Updated Nov 21, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 0ee89f2675ed • Reviewed by Fidamen Standards Committee

Calculate baseline monthly payment and total interest for a fixed-rate loan with monthly payments and no extra payments.

Inputs

Advanced inputs

Recurring extra payment details

One-time lump-sum details

Results

Updates as you type

Monthly payment

$340.64

Total interest paid (no extras)

$10,877.27

Loan term (months)

120

OutputValueUnit
Monthly payment$340.64USD
Total interest paid (no extras)$10,877.27USD
Loan term (months)120months
Primary result$340.64

Visualization

Methodology

Payment and amortization formulas use standard fixed-rate loan mathematics: periodic rate = APR/periods-per-year, and fixed payment is derived from the annuity formula. When extras are applied, the calculator recomputes the number of periods required to amortize the remaining principal under the same scheduled payment (unless the payment frequency/mode changes).

This implementation follows software quality and testing practices consistent with ISO 9001 (quality management) and IEEE standards for software verification. For data handling and security alignment, NIST guidelines are referenced. Results are rounded conservatively to avoid overstating savings.

F.A.Q.

Are results exact to my loan offer?

No. This calculator provides estimates based on the inputs you supply and standard amortization formulas. Actual lender offers may use different compounding, fees, insurance requirements, or daily interest accrual. Confirm final numbers with your lender.

Should I enter APR or nominal rate?

Enter the APR or the annual nominal interest rate that the lender states for the loan's interest calculation. If your lender quotes an APR including fees, that will affect comparisons. Clarify with the lender which rate they use to compute payments.

How accurate are the savings estimates for extra payments?

Estimates assume extras are applied as specified and that scheduled payments remain unchanged unless you change payment frequency. Rounding and differences in day count conventions can change final cents. Treat savings as indicative.

Is my data stored or shared?

This configuration documents calculation logic and content only. Any live implementation should follow NIST security recommendations for sensitive data handling. Avoid entering personally identifiable information unless you trust the host.

Sources & citations

Further resources

Versioning & Change Control

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

Record ID: 0ee89f2675ed

What changed (latest)

v1.0.0 • 2025-11-21 • 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 21, 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 • 2025-11-21 • 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: 2f82856bbe4e