Fidamen

Mortgage APR Calculator

This calculator compares a true bi‑weekly amortization schedule (26 payments per year) with a standard monthly schedule (12 payments per year). It computes periodic payments, total interest paid over the loan, effective annual rates, and estimated time and interest savings when switching to bi‑weekly payments.

Results are derived from standard amortization formulas for fixed‑rate loans and show both periodic payment amounts and aggregate totals. Use the inputs to model the loan amount, nominal annual rate (APR), loan term, and any consistent extra payment applied each period.

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

Governance

Record 9cbcdd6177ac • Reviewed by Fidamen Standards Committee

Compute both schedules and show interest and time differences. This method repeats the amortization formulas for direct comparison.

Inputs

Advanced inputs

Extra payment options

Results

Updates as you type

Bi‑weekly payment

$660.73

Monthly payment

$1,432.25

Total interest (bi‑weekly)

$215,369.52

Total interest (monthly)

$215,608.52

Interest savings (monthly → bi‑weekly)

$239.00

Estimated time saved

0

OutputValueUnit
Bi‑weekly payment$660.73USD
Monthly payment$1,432.25USD
Total interest (bi‑weekly)$215,369.52USD
Total interest (monthly)$215,608.52USD
Interest savings (monthly → bi‑weekly)$239.00USD
Estimated time saved0years
Primary result$660.73

Visualization

Methodology

We use fixed-rate amortization formulas where periodic_rate = nominal_annual_rate/periods_per_year and periodic_payment = (r*PV)/(1-(1+r)^-n) when the rate is positive. For zero interest the payment equals principal divided by the number of periods.

Effective annual rate (EAR) for k periods per year is computed as EAR = (1 + periodic_rate)^k - 1. Bi‑weekly schedules use k = 26, monthly uses k = 12.

Comparison is performed by computing both schedules from the same nominal annual rate and loan term, then subtracting total interest and comparing payoff durations. This reflects the mathematical difference from compounding frequency and payment timing; actual savings may vary with lender policies and exact payment application.

Worked examples

Example: $300,000 loan, 4.0% APR, 30 years. Monthly payment computed with periods_per_year=12; bi‑weekly payment computed with periods_per_year=26. The tool outputs both payments, total interest for each schedule, effective annual rate for each compounding frequency, and the interest/time difference.

F.A.Q.

Does the calculator compute the legal APR under consumer protection laws?

This tool computes amortization-based effective rates and compares schedules; it does not perform a full statutory APR disclosure calculation that may require fees, points, prepaid interest, or lender charges. For regulated APR disclosures consult your lender's Good Faith Estimate or Truth in Lending statement.

Why do bi‑weekly payments sometimes save interest?

When payments are made more frequently, interest is calculated on a slightly smaller principal between payments, and more frequent contributions toward principal reduce compounded interest. The calculator models true 26‑period compounding and payment timing to illustrate this effect.

Are results exact for every lender?

Results are mathematically exact for fixed-rate loans under the assumptions stated, but lenders may apply payments, fees, or rounding differently. Always confirm payoff and APR details with your lender.

How accurate is this calculator?

Calculations use standard numerical formulas suitable for consumer planning. Follow validation best practices (see citations). Small differences may appear due to rounding conventions, payment posting rules, fees, and escrow. Use results for planning and verification, not as a legal disclosure.

Sources & citations

Further resources

External guidance

Versioning & Change Control

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

Record ID: 9cbcdd6177ac

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: 4f01cf0062b6