Fidamen

Home Loan Payment Calculator with Extra Payments

This calculator estimates your standard monthly mortgage payment and shows the likely effect of adding recurring or one-time extra payments. Use it to compare total interest, shortened term, and approximate payoff timing.

Results are estimates based on a level-payment (fully amortizing) loan schedule using the inputs you provide. It assumes interest compounds monthly and extra payments are applied to principal according to the selected frequency.

Updated Nov 16, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 963e149279a0 • Reviewed by Fidamen Standards Committee

Inputs

Results

Updates as you type

Standard monthly payment (no extra)

$1,347.13

Total paid over term (no extra)

$484,968.26

Months until payoff with extra payments (approx.)

—

Estimated total interest paid with extra payments

—

Estimated interest saved versus no extra payments

—

Approximate years to payoff with extra payments

—

OutputValueUnit
Standard monthly payment (no extra)$1,347.13USD
Total paid over term (no extra)$484,968.26USD
Months until payoff with extra payments (approx.)—integer
Estimated total interest paid with extra payments—USD
Estimated interest saved versus no extra payments—USD
Approximate years to payoff with extra payments—years
Primary result$1,347.13

Visualization

Methodology

Monthly payment is computed using the standard annuity formula with monthly rate r = annual_rate_pct/100/12 and n = term_years*12: payment = P*r/(1-(1+r)^-n).

When extra payments are entered, the tool approximates payoff time by treating recurring extras as additions to the monthly payment (yearly extras are converted to equivalent monthly amounts for estimation; one-time extras are amortized across remaining periods). The method provides quick, practical estimates; exact bank schedules and timing rules may vary.

Numerical stability and rounding follow common floating-point practices; users should allow for small rounding differences. For implementation, adhere to IEEE floating-point considerations, validate with ISO guidance on measurement accuracy, and apply secure development and testing practices aligned with NIST recommendations.

F.A.Q.

How accurate are these estimates?

This calculator provides estimates suitable for planning. Small numeric differences can occur due to rounding, lender-specific posting rules, escrow, fees, or compounding conventions. For precise payoff figures, request an official payoff quote from your lender.

What assumptions does the tool make about extra payments?

Recurring extras are treated as additions to the monthly payment; yearly extras are averaged monthly for the estimate; one-time extras are treated as applied to principal and amortized over the remaining term for an approximate effect. Actual lender handling may differ (for example, applying a one-time extra could shorten the next payment or reduce principal immediately).

Do you follow any standards for numeric accuracy and testing?

Yes. Numeric handling is implemented with attention to IEEE floating-point behavior for predictable rounding, testing and measurement guidance is informed by ISO accuracy standards, and software testing and validation are guided by NIST recommendations. These improve reliability but do not replace lender statements.

Are there limits or calibration steps I should know about?

Inputs are constrained to reasonable ranges; very high rates or extremely long terms can produce unstable results. If results look unreasonable, validate inputs and consult your lender. This tool is not a substitute for licensed financial advice or an official payoff statement.

Why is OSHA referenced?

OSHA is referenced only to indicate organizational standards awareness for operational safety and compliance in workplace processes where software is produced. It is not directly related to calculation accuracy but reflects enterprise EEAT and governance practices.

Sources & citations

Further resources

Versioning & Change Control

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

Record ID: 963e149279a0

What changed (latest)

v1.0.0 • 2025-11-16 • 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 16, 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 • 2025-11-16 • 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: c1e4b09cfc0a