Fidamen

Personal Loan Amortization Calculator with Extra Payments

This advanced amortization tool models how extra principal payments affect a fixed-rate personal loan. It supports standard amortization, recurring extra principal per payment, and a single lump-sum paydown applied at a chosen payment number.

Use the calculator to estimate new payoff timing, approximate months saved and total interest under each scenario. Results use closed-form amortization formulas; the app includes guidance for edge cases and accuracy limits.

Updated Nov 5, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 09fb0ea05aaa • Reviewed by Fidamen Standards Committee

Models a fixed recurring extra amount applied each payment period to principal. Calculates new payoff schedule using closed-form approximation for number of payments and total interest.

Inputs

Results

Updates as you type

Payment with extra principal

$188.71

Estimated payments to payoff

60

Total interest (with extra)

$1,322.74

Approximate months saved

-0

OutputValueUnit
Payment with extra principal$188.71USD
Estimated payments to payoff60payments
Total interest (with extra)$1,322.74USD
Approximate months saved-0months
Primary result$188.71

Visualization

Methodology

Periodic rate is derived from the nominal annual rate divided by the number of payments per year. Regular periodic payment uses the standard fixed-rate annuity formula.

Recurring extra payments are modeled by adding the extra amount to the regular payment and using the logarithmic closed-form solution to estimate how many periods are required to amortize the balance. A single lump-sum paydown uses the closed-form balance formula to compute the outstanding principal immediately before the lump and then recalculates remaining payoff using the annuity solution.

These closed-form formulas are widely used in financial calculations. Where the periodic rate is zero, the tool falls back to simple division (principal divided by number of payments).

Key takeaways

This advanced calculator provides fast, closed-form estimates for how extra payments affect payoff timing and interest. Use recurring-extra mode for steady additional principal and lump mode for one-time paydowns.

For full legal, tax, or loan-servicer-specific calculations, obtain official payoff quotes from your lender. This tool is an estimator and includes accuracy caveats and guidance consistent with best practices for financial calculators.

Worked examples

Example: $10,000 loan, 5% APR, 5 years, monthly. Regular monthly payment is computed by the annuity formula. Adding $100 recurring reduces number of payments and total interest; the tool estimates months saved and revised total interest.

Example: Same loan with a $2,000 lump payment on payment 12. The tool computes the outstanding balance at month 12, subtracts the lump, and estimates the new remaining term and interest.

F.A.Q.

Does the calculator consider fees, prepayment penalties or changing rates?

No. This tool models only fixed-rate, fixed-term loans without additional fees or prepayment penalties. Results do not include third-party fees, escrow, insurance, taxes, or variable-rate adjustments.

Is the APR the same as the periodic interest rate?

No. APR is an annual percentage rate; the calculator converts APR to a periodic rate by dividing by the number of payments per year. For loans with financed fees or compounding definitions differing from simple periodic compounding, consult loan documents or a licensed professional.

What happens if I enter zero interest rate?

If the periodic interest rate is zero, the calculator uses straightforward division: remaining principal divided by payments remaining. Some closed-form logarithmic formulas are undefined at zero; the tool handles that as a special case.

How accurate are the months saved and remaining-payments estimates?

Estimates use closed-form amortization formulas and assume every payment is applied on schedule and exactly as specified. They do not simulate irregular timing, partial periods, or mid-period interest accrual; for precise schedules or lender-specific rules, request an amortization schedule from your servicer.

Sources & citations

Further resources

External guidance

Versioning & Change Control

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

Record ID: 09fb0ea05aaa

What changed (latest)

v1.0.0 • 2025-11-05 • 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 5, 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 • 2025-11-05 • 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: 047912fdc854