Mortgage Offset Health Check Tool

A lightweight, single-file HTML utility designed to audit monthly Australian home loan interest charges and detect unlinked or non-functioning mortgage offset accounts, aligned with ASIC 26-173MR regulatory findings.

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Overview

Following ASIC's Report 837, which revealed over $55 million in customer compensation due to bank failures in linking offset accounts, this tool provides home owners with a fast, zero-install method to verify if their offset balance is actively reducing home loan interest.

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Key Features

• Zero Installation: Single static HTML/JavaScript file runnable in any web browser on desktop or mobile.
• Private Calculations: All calculations run locally in your browser. The tool does not send or store any financial data. The site collects anonymous usage metrics through Google Analytics.
• Single-Month Focus: Requires only 5 input values from a monthly bank statement or mobile banking app.
• Automated Status Signal: Instantly categorises offset account health into clear visual badges based on mathematical bounds.

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Quick Start

1. Visit https://offset-check.arthurku.com/, or download index.html and open it in any web browser.
2. Input your home loan figures for the target month:
   • Average Loan Balance
   • Average Offset Balance
   • Annual Interest Rate (% p.a.)
   • Statement Cycle Days (28, 30, or 31)
   • Actual Interest Charged (from banking app)

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Status Classification Matrix

Status	Condition	Meaning
🟢 SAFE: Offset Applied	$|I_{\text{actual}} - I_{\text{expected}}| \le \text{Tolerance}$	Offset account is properly linked and actively saving interest.
🔴 ALERT: Offset Failed	$|I_{\text{actual}} - I_{\text{full}}| \le \text{Tolerance}$	Interest charged reflects full loan balance without offset deduction. Contact bank immediately.
🟡 UNCERTAIN / PARTIAL	$I_{\text{expected}} < I_{\text{actual}} < I_{\text{full}}$	Interest charged differs from static balance estimates (often due to daily balance fluctuations).

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Calculation Formula

The tool calculates expected monthly interest using the standard Australian daily accrual convention:

$$I_{\text{expected}} = \max(0, \text{Loan Balance} - \text{Offset Balance}) \times \left(\frac{\text{Interest Rate}}{365}\right) \times \text{Cycle Days}$$

$$I_{\text{full}} = \text{Loan Balance} \times \left(\frac{\text{Interest Rate}}{365}\right) \times \text{Cycle Days}$$