Mortgage Payment Calculator Overlay

WHAT IT DOES

Mortgage Payment Calculator Overlay places a floating calculator on your screen that computes monthly mortgage payments, total interest, and a detailed amortization schedule based on loan amount, interest rate, and loan term. Sliders for property price and down payment percentage provide real-time updates as you adjust values, making it easy to explore different scenarios. A visual pie chart shows the distribution of total costs between principal and interest, giving you an immediate sense of how much you'll pay in interest over the life of the loan. The calculator also displays a breakdown of principal vs interest for each payment, helping you understand how your payments shift from interest-heavy to principal-heavy over time. Optionally, the calculator can detect property prices and mortgage rates on the page and auto-fill the values, streamlining your analysis.

WHERE IT RUNS

This script operates as a userscript within the BotGentz extension on any webpage that contains real estate listings, mortgage information, or property prices, including Zillow, Realtor.com, Redfin, and other real estate sites. It functions on any modern browser that supports the BotGentz extension, including Chrome, Firefox, Edge, Safari, and Opera. The script runs entirely client-side, processing all calculations locally without sending any information to external servers. It works across all domains and adapts to different site structures automatically.

HOW TO USE

After installation, the calculator appears as a floating overlay in the bottom-right corner of your screen. Use the sliders to adjust property price, down payment percentage, interest rate, and loan term—the results update instantly. The monthly payment display shows your estimated payment, while the pie chart visualizes the total principal vs interest breakdown. Click the dropdown arrow to view the amortization schedule, showing the first 20 payments with principal, interest, and remaining balance. Open the panel to customize default values, toggle compact mode, enable or disable auto-detection of property prices, or change the calculator's position on screen. Click the "✕" button on the calculator to temporarily hide it for one hour.

THE MECHANISM THAT MAKES IT WORK

The calculator uses the standard mortgage payment formula: M = P * r * (1 + r)^n / ((1 + r)^n - 1), where P is the principal, r is the monthly interest rate, and n is the number of payments. The amortization schedule is generated iteratively, calculating the interest portion of each payment as the current balance multiplied by the monthly rate, with the remainder going to principal. The pie chart uses SVG path generation with trigonometric calculations to create segments proportional to the principal-to-interest ratio. The auto-detection feature scans the page using multiple regex patterns and DOM selectors to find property prices (e.g., "$350,000") and interest rates (e.g., "6.5%"), extracting them from text content and structured data elements. All calculations are performed with double-precision floating-point arithmetic and formatted for display using localized currency formatting.

THE PANEL

The script panel is built using BotGentz's BGPanel framework and provides comprehensive control over the calculator's behavior. The panel opens as a floating overlay that can be dragged anywhere on your screen, with a clean dark theme matching the calculator's aesthetic. It includes dropdown menus for default interest rate, loan term, and down payment percentage, and toggle switches for enabling the calculator, compact mode, amortization display, and auto-detection of property prices. A dropdown menu lets you set the calculator's default position to top-left, top-right, bottom-left, or bottom-right. Action buttons allow you to save settings or reset all settings to defaults. The panel collapses to a small icon when not in use and expands when clicked, remaining accessible but unobtrusive. Press the Escape key at any time to close the panel.

PLEASE NOTE

This script requires the free BotGentz extension to be installed and activated in your browser. The extension is available from the official BotGentz website. The calculator provides estimates only and should not be used as a substitute for professional financial advice. Actual mortgage payments may vary based on property taxes, insurance, PMI, and other factors not included in this calculator. The auto-detection feature may not work on all sites and may occasionally misread values; you can always adjust the sliders manually. All calculations and settings are processed locally and not shared with any third-party services. The amortization schedule shows only the first 20 payments to keep the display readable; use external tools for full amortization tables.