Fidamen

Student Loan Amortization Calculator with Extra Payments

This tool estimates how extra payments — either recurring contributions or a one-time lump sum — affect the amortization of a fixed-rate student loan. Use it to gauge payoff time, interest savings, and how different payment strategies change your total cost.

Results are estimates based on fixed-rate math and typical amortization formulas. The tool does not model loan-specific rules such as capitalization timing, variable interest rates, borrower benefits, institutional fees, or income-driven repayment enrollments.

Updated Nov 11, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record adaa7ff0eb60 • Reviewed by Fidamen Standards Committee

Models the effect of adding a fixed extra payment each period on payoff time and interest. Assumes extra payment is applied to principal in each period and borrower keeps scheduled payment amount plus extra.

Inputs

Advanced inputs

Recurring extra payment

One-time lump sum

Results

Updates as you type

Scheduled payment

$325.58

Payment with extra

$375.58

Months to payoff (estimated)

99.6895

Total interest (estimated)

$7,441.28

Interest saved (estimated)

$1,628.18

OutputValueUnit
Scheduled payment$325.58—
Payment with extra$375.58—
Months to payoff (estimated)99.6895months
Total interest (estimated)$7,441.28—
Interest saved (estimated)$1,628.18—
Primary result$325.58

Visualization

Methodology

Scheduled payment is computed using the standard fixed-rate annuity formula: payment = r * principal / (1 - (1 + r)^-n), where r is the periodic rate and n is the scheduled number of payments.

Recurring extra payments are applied each period in addition to the scheduled payment and reduce principal faster. Estimated payoff months with extra payments are calculated by solving the geometric progression for remaining balance using logarithms.

A one-time lump-sum reduces principal at the chosen payment index. Remaining payoff is estimated by computing the balance immediately before the lump payment, subtracting the lump amount, and re-solving the amortization equation for remaining periods under the assumption the scheduled payment amount is maintained.

Key takeaways

This advanced amortization calculator supports three scenarios: standard amortization, recurring extra payments, and a single lump-sum payment. It provides estimated payoff timing, total interest, and interest savings using closed-form amortization formulas.

Outputs are estimates. For exact payoff figures and instructions on applying extra payments, contact your loan servicer. Use this tool to compare strategies and approximate savings before making payment decisions.

F.A.Q.

How accurate are these estimates?

Estimates use standard amortization math and continuous formulas for payoff time. They are accurate for fixed-rate loans with payments applied each period and no fees or balance capitalization events. Edge cases such as payment totals that are less than or equal to the interest due per period can produce invalid estimates; the tool warns when inputs make payoff impossible without increasing payment.

Does this model income-driven repayment, loan forgiveness, or variable rates?

No. This calculator assumes a fixed interest rate and scheduled payments. Income-driven plans, forgiveness programs, variable rates, deferment, forbearance, payment exemptions, or capitalization rules are not modeled. Consult your loan servicer or plan documents for those scenarios.

Are rounding and payment timing included?

Results are shown as decimal estimates for payoff months and currency values rounded for display. The model assumes payments occur each period on schedule. Real-world rounding, payment timing differences, and servicer policies can change exact amounts; treat outputs as close estimates, not billing statements.

What if an extra payment would pay off the loan early?

If an extra payment would fully repay the balance before the next scheduled payment, the model may compute a fractional number of periods. Many lenders will accept a final smaller payment; check with your servicer for exact payoff handling.

Sources & citations

Further resources

Versioning & Change Control

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

Record ID: adaa7ff0eb60

What changed (latest)

v1.0.0 • 2025-11-11 • 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 11, 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 • 2025-11-11 • 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: 128a400086c0