Fidamen

Home Loan Interest Calculator with Extra Payments

This calculator estimates how recurring extra payments (monthly or annual) and single one-time payments can reduce total interest and shorten the life of a home loan. It produces approximate payoff timing and interest figures based on standard amortization mathematics.

Results are intended for planning and comparison. They are approximations; for an exact amortization schedule and legally binding APR disclosures, consult your lender or a licensed financial professional.

Updated Nov 1, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record d8670e6c7508 • Reviewed by Fidamen Standards Committee

Inputs

Results

Updates as you type

Scheduled periodic payment (no extras)

-$2.43

Approximate periodic payment including recurring extras

-$2.43

Estimated payoff time (months, approx.)

—

Total interest paid without extras (approx.)

-$300,875.00

Estimated total interest paid with extras (approx.)

—

Estimated interest saved (approx.)

—

Estimated months saved (approx.)

—

OutputValueUnit
Scheduled periodic payment (no extras)-$2.43USD
Approximate periodic payment including recurring extras-$2.43USD
Estimated payoff time (months, approx.)—months
Total interest paid without extras (approx.)-$300,875.00USD
Estimated total interest paid with extras (approx.)—USD
Estimated interest saved (approx.)—USD
Estimated months saved (approx.)—months
Primary result-$2.43

Visualization

Methodology

Scheduled periodic payment is computed using the standard fixed-rate amortization formula with periodic rate r = annual_rate/100 / payments_per_year and N = term_years * payments_per_year.

Recurring extras are treated as additions to the periodic payment (extra monthly and an annual sum divided across the payment periods). One-time extra payments are distributed across the original number of payments for approximation. The payoff time with extras is estimated by solving the closed-form equation for number of periods: N' = -ln(1 - P*r / M_total) / ln(1 + r), where M_total includes recurring extras.

This tool follows engineering and information-quality best practices for calculators and provides accuracy caveats. It is not a replacement for a full-period amortization schedule that tracks balance changes month-by-month when extras start later or vary over time.

F.A.Q.

Are the results exact?

No. Results are approximations. Recurring extras and one-time payments are folded into the periodic payment for a closed-form estimate. Exact schedules require stepwise amortization (period-by-period balance updates) which this quick calculator does not perform.

Does this calculator use APR or nominal rate?

This tool uses the nominal annual interest rate you enter. APR may include fees and other costs; use lender-provided APR disclosures for legally required comparisons.

How should I enter an annual extra paid once per year?

Enter the total extra amount you plan to pay each year in the 'Extra annual lump sum' field. The calculator approximates that amount spread evenly across payment periods for the estimate.

What if extra payments start later in the loan?

This calculator approximates extras as if applied throughout. If extras begin later, results will overstate interest saved and months saved. For precision, generate a full amortization schedule that applies extras starting at the specific month.

Sources & citations

Further resources

External guidance

Versioning & Change Control

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

Record ID: d8670e6c7508

What changed (latest)

v1.0.0 • 2025-11-01 • 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 1, 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 • 2025-11-01 • 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: 1745727ed78f