Fidamen

Personal Loan APR Calculator with Extra Payments

This tool models personal loan amortization and the effect of extra payments. Use it to compare scheduled payments to scenarios where you add a recurring extra amount or make a one-time lump-sum principal reduction.

Results estimate payoff time, total interest paid, interest saved, and payments saved. Calculations use standard amortization formulas; lender fees or prepayment penalties are not included unless entered as adjustments to loan amount or APR.

Updated Nov 22, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 60fb708d8c50 • Reviewed by Fidamen Standards Committee

Standard amortization using periodic interest rate with a fixed additional payment applied each period until payoff.

Inputs

Results

Updates as you type

Scheduled periodic payment (no extra)

-$0.83

Periodic payment including recurring extra

-$0.83

Estimated payments until payoff (with extra)

—

Total interest paid (with extra)

—

Interest saved

—

Payments saved

—

OutputValueUnit
Scheduled periodic payment (no extra)-$0.83USD
Periodic payment including recurring extra-$0.83USD
Estimated payments until payoff (with extra)—payments
Total interest paid (with extra)—USD
Interest saved—USD
Payments saved—payments
Primary result-$0.83

Visualization

Methodology

Calculations use periodic compounding: periodic_rate = APR / payments_per_year and standard amortization algebra to compute payment amounts and remaining balances.

When estimating reduced term from higher payments or lump sums, the tool solves for the number of payments using logarithmic inversion of the amortization equation. Edge cases (zero or negative remaining balance) are handled conservatively; see accuracy notes below.

For traceability and testability we follow best practices for numerical stability and input validation inspired by standards for measurement and verification (NIST), quality management principles (ISO), and engineering numerics (IEEE). This calculator is informational only and not a regulatory disclosure.

Key takeaways

Use the recurring-extra method to see the impact of adding the same extra amount to every payment. Use the lump-sum method to model a one-time principal reduction at a specific payment number.

This calculator is a model based on standard amortization mathematics and references measurement and numerical best practices. It is not a substitute for official loan disclosures.

F.A.Q.

Does this calculator compute the legally defined APR used in loan disclosures?

No. This tool computes interest accrual and amortization using APR as an annual nominal rate for periodic compounding. It does not include lender fees, points, insurance, or finance charge disclosures required by law. For legally binding APR disclosures consult your loan documents or your regulator.

Can I model biweekly payments or irregular extra payments?

Yes. Use Payments per year = 26 for biweekly schedules. For irregular or ad-hoc extras, model each event as a lump-sum at the appropriate payment number or use the recurring extra field for constant additional amounts.

What if my lender applies the lump sum before interest accrues for the period?

This calculator assumes extras are applied at the same schedule timing described in the scenario: recurring extras are applied each payment period and lump sums are applied immediately at the specified payment number. If your lender applies payments differently, results will vary.

How accurate are the 'payments saved' and 'interest saved' values?

Values are computed from closed-form amortization formulas and are accurate within rounding tolerance for the mathematical model. They do not account for rounding rules, day-count conventions, compounding differences, or lender-specific processing. See accuracy & standards below.

Sources & citations

Further resources

Versioning & Change Control

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

Record ID: 60fb708d8c50

What changed (latest)

v1.0.0 • 2025-11-22 • 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 22, 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 • 2025-11-22 • 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: 0eabd5656d02