Fidamen

Mortgage Interest Calculator

This tool models mortgage amortization under accelerated biweekly payment schedules (26 payments per year) and standard monthly schedules. It estimates payment amounts, total interest paid, and time-to-payoff and lets you add a recurring extra amount per biweekly period.

Results are estimates based on standard amortization math. Real-world outcomes can differ due to rounding policies, payment timing, escrow changes, loan servicing rules, and fees. Use the values here for planning and comparison, not as a regulatorily binding figure.

Updated Nov 29, 2025QA PASS — golden 25 / edge 120Run golden-edge-2026-01-23

Governance

Record 6de360de13db • Reviewed by Fidamen Standards Committee

Provides side-by-side estimates for total interest and payoff time for a standard monthly schedule and an accelerated biweekly schedule (26 payments/year). Calculates estimated interest savings and years saved.

Inputs

Results

Updates as you type

Total interest (monthly)

$184,968.26

Total interest (biweekly)

—

Interest saved (monthly vs biweekly)

—

Estimated years saved

—

Biweekly payoff years

—

OutputValueUnit
Total interest (monthly)$184,968.26USD
Total interest (biweekly)—USD
Interest saved (monthly vs biweekly)—USD
Estimated years saved—years
Biweekly payoff years—years
Primary result$184,968.26

Visualization

Methodology

Calculations use classical amortization formulas: periodically compound the annual nominal rate to a per-period rate and compute the fixed payment that amortizes the loan over the chosen number of periods.

Accelerated biweekly schedules assume 26 equal payments per year. Because 26 half-month payments equal 13 monthly payments, the accelerated schedule typically reduces principal faster and shortens the loan term.

Where extra payments are entered, the calculator includes them as fixed additional principal applied each biweekly period and recomputes payoff using logarithmic solution for the number of periods until balance reaches zero.

Key takeaways

Use this calculator to compare accelerated biweekly schedules to standard monthly schedules and to estimate the effect of recurring extra payments.

Results are estimates. For contract-accurate payoff figures, consult your loan servicer and review your loan agreement.

Worked examples

Example: $300,000 principal, 3.5% APR, 30 years. Standard monthly payment computed with r=3.5%/12 and n=360. Accelerated biweekly computes payments with r=3.5%/26 and n=780 and shows the reduced payoff time and interest.

Example with extra payment: adding a fixed $50 per biweekly period reduces principal faster. The calculator estimates the new payoff periods using the logarithmic solution above.

F.A.Q.

Does 'biweekly' always mean faster payoff than monthly?

Not necessarily. Accelerated biweekly (26 payments/year of half the monthly amount) typically results in faster payoff because it effectively makes one extra monthly payment per year. Converting a monthly payment into 24 biweekly payments (not 26) does not speed payoff.

Are these numbers exact for my loan?

These are mathematical estimates that assume the lender applies payments immediately to principal and uses standard amortization. Lender-specific rules for payment posting, rounding, escrow, prepayment penalties, or required minimum payments can alter the result.

Why is an extra fixed amount per biweekly period useful?

An extra fixed principal payment applied regularly reduces loan balance sooner and typically reduces both total interest and term length. This calculator models simple recurring extras per biweekly period.

How should I interpret negative or undefined outputs?

If the periodic payment is lower than the interest-only amount (payment less than or equal to principal multiplied by the periodic rate), the formulas do not produce a valid amortizing solution. In practice you would need to increase the payment or change the term.

Sources & citations

Further resources

Versioning & Change Control

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

Record ID: 6de360de13db

What changed (latest)

v1.0.0 • 2025-11-29 • 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 29, 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 • 2025-11-29 • 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: 9a09f774b4fe