// preview designs at any screen size instantly
Test your website at common screen resolutions and breakpoints. Preview responsive designs, check viewport sizes, and simulate device widths instantly in the browser.
Enter a URL above and click LOAD
Or select a preset device to resize your browserThe live bar at the top shows your current viewport and screen dimensions in real time as you resize the browser.
Type any URL into the input field and click LOAD to preview that site inside the resizable frame.
Click a device preset button or enter a custom width × height to simulate that screen resolution instantly.
The Screen Resolution Tester displays your current browser viewport dimensions in real time, and lets you preview any URL inside a simulated frame at popular device resolutions — all without leaving the browser.
Screen resolution is your physical display size in pixels (e.g. 1920×1080). Viewport is the visible area of your browser window — it changes when you resize the window or open DevTools. This tool shows both.
Many websites block embedding via the X-Frame-Options or Content-Security-Policy headers. If the preview stays blank, try opening the page directly in a new tab. This is a security restriction enforced by the target site, not by this tool.
DPR is the ratio between physical pixels and CSS pixels. A Retina/HiDPI screen typically has DPR 2 or 3, meaning 2–3 physical pixels per 1 CSS pixel. A value of 1 means a standard display.
Click any mobile preset (e.g. iPhone SE at 375×667) — the tool will resize the preview frame to that dimension. For full browser-level simulation including user agent strings, use your browser's built-in DevTools device emulator.
Common Tailwind CSS breakpoints: sm (640px), md (768px), lg (1024px), xl (1280px), 2xl (1536px). Bootstrap uses: sm (576px), md (768px), lg (992px), xl (1200px), xxl (1400px).
Yes. All resolution detection runs entirely in your browser — no data is sent to any server. The URL preview frame loads content directly from your browser to the target site.
A screen resolution tester is a browser-based utility that instantly reports your current viewport and screen dimensions, and lets you simulate how a website appears at different device widths and heights. Unlike browser DevTools, which require opening a secondary panel, a dedicated resolution tester gives designers and developers an always-visible, at-a-glance readout of every dimension metric that matters for responsive web design.
JLV DevTools' Screen Resolution Tester goes further by combining live resolution monitoring with a URL preview iframe, device presets, a visual breakpoint ruler, and secondary metrics like device pixel ratio, orientation, and touch capability — all on one page, all free, no sign-up required.
Modern websites must work beautifully across an enormous range of screens — from a 320px-wide budget Android phone to a 5K ultrawide monitor. CSS media queries allow developers to apply different styles at different widths, but knowing exactly when those queries trigger requires precise viewport measurement. A single pixel difference can mean a layout breaks unexpectedly on a real device.
Common pain points include:
By knowing your exact viewport dimensions at any moment, you can pinpoint which media query range applies and debug layout issues with precision.
These two terms are often confused but refer to different things:
CSS media queries respond to viewport width, not screen resolution. That's why the viewport measurement is what truly matters for responsive design work.
Device Pixel Ratio (DPR) describes how many physical device pixels correspond to a single CSS pixel. A standard display has DPR 1 — one physical pixel per CSS pixel. Retina and HiDPI displays (like those on iPhone and MacBooks) have DPR 2 or 3, meaning they pack four or nine physical pixels into each CSS pixel for sharper rendering.
DPR matters for:
srcset attribute: HTML's responsive image feature uses DPR hints to serve the right image file.This tool includes eight presets covering the most common device categories used in web development:
Different CSS frameworks define breakpoints differently. Here are the most widely used systems:
For a thorough responsive QA pass on any website, follow this workflow:
While this tool is powerful for quick checks, it's important to understand its limitations. The iframe preview cannot simulate a mobile user agent string, so server-side user agent detection won't behave as it would on a real device. Additionally, some websites block iframe embedding for security reasons. For full device emulation including user agent strings, touch events, and network throttling, your browser's built-in DevTools (F12 → Device toolbar) is the appropriate next step. This tool is best used for rapid viewport measurement and quick layout spot-checks.