Fidamen

Personal Loan Interest Calculator with Bi-Weekly Payments

This calculator computes bi-weekly (or any periodic) loan payments from a nominal annual percentage rate (APR), and shows total interest and total cost over the loan term. It also produces an equivalent monthly payment and the interest difference between the monthly schedule and the chosen periodic schedule.

Defaults assume standard amortizing loans with fixed nominal APR and equal periodic payments. Change the payment frequency to model weekly, semi-monthly, monthly, or custom schedules.

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

Governance

Record c6a4c6a5fc31 • Reviewed by Fidamen Standards Committee

Inputs

Results

Updates as you type

Payment per period

$138.53

Number of payments

130

Total amount paid (principal + interest)

$18,009.24

Total interest paid

$3,009.24

Equivalent monthly payment

$300.57

Total interest if monthly payments

$3,034.15

Interest difference (monthly minus bi-weekly)

$24.91

OutputValueUnit
Payment per period$138.53—
Number of payments130—
Total amount paid (principal + interest)$18,009.24—
Total interest paid$3,009.24—
Equivalent monthly payment$300.57—
Total interest if monthly payments$3,034.15—
Interest difference (monthly minus bi-weekly)$24.91—
Primary result$138.53

Visualization

Methodology

The tool uses the standard annuity formula for amortizing loans: periodic rate = APR / payments_per_year; payment = P * r / (1 - (1 + r)^(-n)) where n = term_years * payments_per_year. This assumes interest compounds at the payment frequency implied by the chosen payments_per_year.

Assumptions, rounding, and software quality follow best-practice guidance for numerical calculations and testing. Implementation and verification should observe guidance from ISO software quality standards (https://www.iso.org), IEEE floating-point and software engineering recommendations (https://www.ieee.org), and NIST guidance for algorithm validation (https://www.nist.gov).

F.A.Q.

Why are bi-weekly payments often advertised as saving interest?

Bi-weekly schedules make more frequent payments, which reduces principal slightly faster between interest accrual periods compared to monthly payments when both use the same APR and compounding frequency. Savings depend on exact timing and whether the lender applies bi-weekly compounding or converts payments to an equivalent monthly schedule.

Is the APR converted or the effective interest rate used?

This calculator uses the nominal APR and assumes compounding at the payment frequency you select. It does not convert to APRs that incorporate fees or use effective annual rate unless you enter an adjusted APR that includes those costs.

How accurate are the results?

Results use standard amortization formulas with double-precision arithmetic assumptions. Minor rounding differences may occur compared to lender statements due to daily interest conventions, day-count rules, or lender-specific posting rules. Verify final payoff figures with your loan servicer.

Can I model extra payments or variable rates?

This version models fixed-rate, fixed-payment amortization only. For extra payments, variable rates, or irregular schedules, use a full amortization schedule tool or consult a financial advisor.

Sources & citations

Further resources

External guidance

Versioning & Change Control

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

Record ID: c6a4c6a5fc31

What changed (latest)

v1.0.0 • 2025-11-25 • 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 25, 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 • 2025-11-25 • 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: edad3e5d375d