Loan Payment Calculator Injector

WHAT IT DOES

This script automatically scans any page for loan amounts, interest rates, and loan terms—then injects a floating calculator that computes your monthly payment, total interest, and total payment. The calculator features a full amortization schedule showing each payment's breakdown, and a what-if analysis for extra payments that shows how much you can save and how many months you can shave off your loan. A floating badge displays the estimated monthly payment at a glance. Perfect for mortgage shopping, auto loans, personal loans, and any financing decision.

WHERE IT RUNS

The script works on any page with loan information—banking websites, mortgage calculators, auto dealer pages, loan comparison sites, credit card applications, and any page displaying loan amounts, interest rates, or terms. It detects data using pattern matching for currency amounts, percentages, and term keywords (months, years). The script works on any website regardless of framework.

HOW TO USE

After installation, the script automatically scans the page for loan data. A floating badge shows the estimated monthly payment. Click the badge to toggle the full calculator. The calculator lets you adjust the loan amount, interest rate, term, and extra monthly payment—all updates in real time. The amortization schedule shows the first 24 months with payment breakdown and remaining balance. Open the panel to customize: enable or disable the calculator and badge, choose calculator position (top-right, top-left, bottom-right, bottom-left), adjust opacity, select dark or light theme, set currency symbol, default loan term, enable or disable amortization display, set default extra payment, and choose badge position. Click "Scan & Calculate" to manually re-scan the page.

TECHNICAL MECHANISM

The script uses a multi-stage loan detection and calculation pipeline. First, it scans the DOM for loan data using regex patterns: currency amounts using the configured currency symbol, interest rates using percentage patterns, and loan terms using month/year keywords. Second, it validates the extracted data and applies defaults if needed. Third, it calculates the monthly payment using the standard loan payment formula: P × r × (1+r)^n / ((1+r)^n - 1), where P is principal, r is monthly interest rate, and n is number of months. Fourth, it generates an amortization schedule by iterating through each month, calculating interest (balance × monthly rate) and principal (payment - interest), and tracking the remaining balance. Fifth, it supports extra payments by reducing the principal each month and recalculating the payoff timeline. Sixth, it renders the calculator with real-time input binding and updates the badge with the current monthly payment. All calculations use double-precision floating-point arithmetic.

THE PANEL

The settings panel is fully draggable and can be positioned anywhere on your screen. It automatically snaps to the nearest edge when released, keeping your workspace organized. The panel can be collapsed to a small bubble showing only a gear icon, and restored with a single click. Its position is remembered across page reloads using GM_setValue. The panel also responds to the Escape key, which instantly hides it until you click the bubble again. All settings are saved immediately when changed and persist across browser sessions.

PLEASE NOTE

This script requires the free BotGentz extension to run. Loan detection relies on text pattern matching and may not capture all formats. The script does not access, store, or transmit any personal data, financial information, or browsing history. All processing happens locally in your browser and never leaves your device.