Fidamen

Mortgage Amortization Calculator with Extra Payments

This calculator models mortgage amortization and shows how recurring extra payments or a single lump-sum principal payment change payoff timing and total interest paid. It produces conservative numeric estimates, highlights assumptions, and surfaces how results were calculated.

Use the recurring extra payments option to see the effect of adding a fixed amount to each scheduled payment. Use the lump sum option to estimate the immediate effect of applying a one-time principal payment.

Updated Nov 7, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 387446606d6b • Reviewed by Fidamen Standards Committee

Estimates the effect of recurring extra payments or a single lump-sum payment on payoff timing and interest paid. Provides conservative numeric estimates and identifies assumptions and rounding limits.

Inputs

Advanced inputs

Recurring extra payment

Lump sum payment

Results

Updates as you type

Payment including extra

$97.57

Estimated years to payoff

—

Estimated interest with extra payments

—

Estimated interest saved

—

OutputValueUnit
Payment including extra$97.57USD
Estimated years to payoff—years
Estimated interest with extra payments—USD
Estimated interest saved—USD
Primary result$97.57

Visualization

Methodology

Calculations use standard amortization formulas: the periodic rate is APR divided by the number of payments per year. The scheduled payment is computed from the loan principal, periodic rate, and number of payments using the standard annuity formula.

For recurring extras, we model an increased periodic payment (scheduled payment plus the extra). The estimated payoff number of payments is computed by solving the annuity equation for the remaining term; natural logarithms are used to invert the geometric series. For a lump sum applied immediately, we subtract the lump from principal and recompute the remaining term using the same approach.

Numerical results are approximate. Rounding, payment timing conventions, escrow, fees, variable-rate resets, prepayment penalties, and lender-specific posting rules are not modeled. Always verify results with your lender and account statements.

Key takeaways

This advanced calculator supports three scenarios: scheduled payments only, recurring extra payments, and a one-time lump-sum payment. It reports estimated payoff duration and estimated interest saved.

Results are estimates. Confirm with your lender before making financial decisions.

F.A.Q.

Are results exact for every lender?

No. This tool provides mathematical estimates. Actual payoff timing and interest depend on lender posting rules, daily interest accrual conventions, potential prepayment penalties, escrow adjustments, and whether payments are applied to principal immediately. Use results for planning and verify with your lender.

Does the calculator model adjustable-rate mortgages or interest-only periods?

Not in this version. This tool assumes a fixed APR for the modeled term and that payments are applied according to the standard amortization schedule. For adjustable-rate loans or interest-only periods, consult a lender-specific schedule or an advisor.

How accurate are the logarithm/inversion formulas?

Formulas use standard algebraic inversion of the annuity equation. Results are subject to floating-point rounding; see the accuracy note. For very small extra payments close to zero, the estimated payoff may be numerically unstable.

Is this financial advice?

No. This is an educational calculator. It does not replace professional financial, tax, or legal advice.

Sources & citations

Further resources

External guidance

Versioning & Change Control

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

Record ID: 387446606d6b

What changed (latest)

v1.0.0 • 2025-11-07 • 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 7, 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 • 2025-11-07 • 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: 0f39307b1768