Percentage Calculator

WHAT IT DOES

Percentage Calculator gives you instant percentage calculations right in your browser. Three calculation modes are available: find a percentage of a number (X% of Y), determine what percentage one number is of another (X is what % of Y), and calculate the percentage change between two values (from Y to X). Results appear in real-time as you type — no button pressing required. Every calculation is automatically saved to a history list that you can click to reuse previous values. The panel also includes an "Extract from Selection" feature that scans any text you have highlighted on the page and pulls out numbers to auto-fill the input fields.

WHERE IT RUNS

This script works on every website you visit. Whether you are calculating discounts while shopping online, working with financial data in a spreadsheet application, analyzing metrics in a dashboard, or doing homework in an educational platform, the calculator panel is always available. It runs alongside whatever you are doing, staying out of your way until you need it.

HOW TO USE

Select a calculation mode from the dropdown menu. Enter values in the two input fields — the result updates automatically. Click any history entry to load its values back into the fields for quick reuse. Highlight any text on the page, click "Extract from Selection," and the script finds and extracts numbers to fill the inputs. The mode selection persists across calculations, and the panel remembers its position on the page between sessions.

THE EXTRACTION MECHANISM

Most number extraction tools rely on simple regex matching without context, often grabbing irrelevant digits from URLs, timestamps, or IDs. Percentage Calculator combines a robust number parser with semantic filtering: it extracts all numeric patterns from the selected text, parses them as floats with locale-aware decimal handling, and filters out outliers that are unlikely to be meaningful values. The parser detects common number formats including comma-separated thousands, decimal points, and negative numbers. Numbers are presented in the order they appear in the selection, making it easy to identify which extracted value corresponds to which part of the original text.

THE PANEL

The panel is fully draggable and remembers its position per site. It minimizes to a compact floating bubble when collapsed. All calculations are stored locally using GM_setValue, preserving your history across browser sessions. The panel automatically focuses the first input field for keyboard-first workflows. Press the Escape key to close the panel.

PLEASE NOTE

This script requires the free BotGentz extension to run. Websites change their structure and behavior over time; if a particular website's text selection stops working properly, the script may need an update. No user data is sent anywhere — all calculation history and settings are stored locally in your browser. The script reads only text you explicitly select for number extraction; it never reads or modifies password, hidden, or file input fields.