Developer Essentials · Free Tool

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.

Resize your window — these update in real time

Common breakpoints (Tailwind)

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.