Fidamen

Personal Loan Interest Calculator with Extra Payments

This calculator models personal loan amortization and shows how recurring extra payments or a one-time lump-sum payment change your payoff date and total interest paid. It supports standard schedules and an alternate biweekly schedule for comparison.

Use the inputs to reflect your actual loan terms (principal, APR, term, scheduled payments per year). Enter recurring extras or a lump-sum amount to see estimated savings and shortened payoff time. Results are estimates and intended for planning, not as a legal payoff statement.

Updated Nov 2, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 64e6beaf2009 • Reviewed by Fidamen Standards Committee

Compute periodic payment, accelerated payoff and interest savings when you add a recurring extra payment applied each scheduled payment.

Inputs

Results

Updates as you type

Scheduled payment (no extra)

$495.03

Total payment per period (with extra)

$495.03

Estimated payoff (years) with extra

5

Estimated total interest (with extra)

$4,701.80

Estimated interest saved

$0.00

Estimated payments saved

0

OutputValueUnit
Scheduled payment (no extra)$495.03—
Total payment per period (with extra)$495.03—
Estimated payoff (years) with extra5years
Estimated total interest (with extra)$4,701.80—
Estimated interest saved$0.00—
Estimated payments saved0payments
Primary result$495.03

Visualization

Methodology

Calculations use standard amortization formulas. The periodic rate is annual APR divided by scheduled payments per year. The scheduled payment (without extras) is computed using the fixed-payment amortization formula; if the periodic rate is zero, payments are principal divided by number of payments.

Recurring extras are added to each scheduled payment and the new payoff is estimated by solving the amortization balance equation for the number of payments. A single lump-sum reduces principal at the chosen payment number and remaining payments are recalculated.

Biweekly schedule uses 26 payments per year and recalculates periodic rate and scheduled payment accordingly. All outputs are mathematical estimates and assume extras are applied immediately and reduce principal on the same day payments are processed.

Worked examples

Example: $20,000 principal, 6% APR, 5 years (monthly). Scheduled payment ≈ formula result. Adding $100 extra per month reduces payoff years and cuts total interest—calculator shows estimated savings.

Example: A $5,000 lump-sum at payment 12 on a 7-year loan reduces principal and the calculator estimates the new payoff date and interest saved.

F.A.Q.

Are results exact or guaranteed?

Results are estimates for planning. Actual payoff and interest depend on lender posting rules, compounding specifics, payments timing, fees, and whether extras are applied to principal. Use this tool for comparison, not as an official payoff figure.

How does the calculator treat extra payments?

Recurring extras are assumed to be applied to principal at each scheduled payment. A lump-sum is applied at the chosen payment number immediately before calculating remaining payments.

What if APR is zero?

When APR is zero the calculator uses linear amortization: payment = principal / total payments and extra payments reduce the number of payments proportionally.

Why might my lender's payoff differ?

Lenders may have minimum posting periods, interest accrual methods, prepayment penalties, or apply payments differently. Confirm with your lender for exact payoff balance and procedures.

How accurate is the math and implementation?

Formulas are standard financial mathematics. Implementation accuracy follows best practices for numerical stability, but very small rounding differences can occur. See citations for recommended numerical methods and validation standards.

Is this tool compliant with any standards?

This tool follows general engineering and QA best practices and references authoritative organizations for validation and numerical reliability; it is not a certified financial product and users should verify with their lender.

Sources & citations

Further resources

Versioning & Change Control

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

Record ID: 64e6beaf2009

What changed (latest)

v1.0.0 • 2025-11-02 • 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 2, 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 • 2025-11-02 • 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: e8285b0b488f