# Viewport Tester Watch your browser viewport size live as you resize the window, and see which responsive breakpoint is active. Perfect for testing responsive layouts. Runs in your browser. ### How it works The **viewport** is the visible area of the page — `window.innerWidth × innerHeight` — which is what CSS media queries respond to. It differs from your screen resolution because it excludes browser chrome and changes as you resize. The active breakpoint is highlighted using the standard Tailwind CSS widths (sm 640, md 768, lg 1024, xl 1280, 2xl 1536). Everything is measured locally. --- *Source: [ocxly.com/viewport-tester.html](https://ocxly.com/viewport-tester.html) — OCXLY, free 100% client-side privacy-first tools. Free viewport tester. Watch your browser viewport width and height live as you resize, and see the active responsive breakpoint. Great for testing responsive design. Runs in your browser.*