Late Payment Interest Calculator

WHAT IT DOES

Late Payment Interest Calculator is a floating calculator panel that computes interest on overdue invoices. Enter the invoice amount, due date, payment date, and annual interest rate, and it calculates the interest due. It supports simple interest (interest = principal × rate × time) and compound interest (interest accrues on itself). You can choose daily accrual (more precise) or monthly accrual. It can auto-detect invoice date and amount from the current page, saving you from copying and pasting numbers.

WHERE IT RUNS

This script runs on any website you visit — invoice pages, accounting dashboards, billing portals, or any page where you need to calculate late payment interest. It reads only the page you are looking at, never other tabs, bookmarks, or browser data.

HOW TO USE

Visit any invoice page with a date and total displayed. Click the Late Payment Interest Calculator panel button in the BotGentz extension toolbar. The panel appears at the edge of the page. If the invoice date and amount are detected, they auto-fill. If not, enter them manually. Enter the due date, payment date, and annual interest rate. Choose simple or compound interest, and daily or monthly accrual. The calculator updates the days late, interest due, and total due in real time. You can save your default rate and settings — they persist across pages.

TECHNICAL MECHANISM

The calculator scans the page for invoice date using CSS selectors (like .invoice-date, .due-date, [data-invoice-date]) and date pattern matching (MM/DD/YYYY, DD/MM/YYYY, month names). It detects invoice amount using selectors like .invoice-total, .grand-total, .amount-due, and regular expression matching for dollar amounts. Days late is calculated as the difference between the payment date and due date. Interest is calculated using the formula: for simple interest, amount × rate × time; for compound interest, amount × ((1 + rate)^time - 1). The rate is annualized and divided by 365 for daily accrual or 12 for monthly accrual. The entire calculation runs client-side — no data is sent anywhere.

THE PANEL

The panel is a floating overlay that you can drag anywhere on the screen by its header. It remembers its position per site so it reappears where you left it. The panel can be collapsed into a small handle to get it out of the way, and snapped to the edge of the page. It can be resized to fit your workflow. Press Escape to close the panel.

PLEASE NOTE

This script requires the free BotGentz extension to run. It reads only the page you are viewing — no data is stored or transmitted. The date and amount auto-detection works on many invoice pages but may not work on all sites. Default settings are saved locally and persist across pages. The calculator is for estimation purposes only — actual interest calculations may vary based on contracts, jurisdiction, and other factors. Sites change — the script may need updates to match new site structures.