A responsive breakpoint tester lets you preview how any website looks at different device widths without leaving your browser. Test mobile, tablet, and desktop layouts instantly by selecting a device preset or entering custom dimensions.
How to Use the Responsive Breakpoint Tester
Testing responsive designs at different viewport sizes is essential for shipping websites that work well on all devices. This free responsive breakpoint tester loads any URL in a resizable iframe at common device dimensions.
Step 1: Enter a URL
Type or paste any URL including local development URLs (http://localhost:3000). Press Load URL to load it in the preview. If the site blocks iframe embedding, click Open tab to view it in a new browser tab.
Step 2: Select a Device Preset
Click any device preset button to resize the preview. Presets include common iPhones, iPad, common laptops, and desktop. The current dimensions are shown below the preset buttons.
Step 3: Test Landscape Mode
Toggle the Landscape switch to swap width and height, simulating device rotation. This is especially important for tablet layouts where landscape mode often triggers a completely different layout.
Step 4: Use Custom Dimensions
Select the "Custom" preset to enter any specific width and height. Use this for testing at CSS breakpoints (640px, 768px, 1024px) or to match a specific device not in the presets.
FAQ
Is this responsive breakpoint tester free?
Yes, completely free with no limits or account required. The tool runs entirely in your browser.
Why doesn't the iframe load some websites?
Many websites set the X-Frame-Options or Content-Security-Policy headers to block embedding in iframes. This is a security feature, not a bug in this tool. Sites like Google, Twitter, and Facebook block iframe loading. Use the 'Open in new tab' button to test these sites in your browser instead.
What are common CSS breakpoints?
Common breakpoints: 320px (small mobile), 375px (iPhone SE/XR), 390px (iPhone 14), 430px (iPhone 14 Pro Max), 768px (iPad), 1024px (iPad Pro/laptop), 1280px (laptop), 1440px (desktop), 1920px (full HD). Tailwind CSS uses: 640px, 768px, 1024px, 1280px, 1536px.
What does viewport width mean?
Viewport width is the visible width of the browser window in CSS pixels. It's what CSS media queries like @media (max-width: 768px) test against. A device's physical screen width (in physical pixels) may be 2-3× larger due to device pixel ratio (Retina displays).
How do I test a localhost URL?
Enter your localhost URL (e.g., http://localhost:3000) in the URL field. It will load in the iframe since there are no X-Frame-Options restrictions on localhost. This makes the tool useful for testing local development sites across breakpoints.
Can I test a specific device's exact viewport?
Use the Custom option to enter any width and height. Common exact device viewports: iPhone 14 (390×844), iPhone 14 Pro Max (430×932), Samsung Galaxy S23 (360×780), iPad Air (820×1180), MacBook Air (1280×832).
What is the landscape toggle for?
The landscape toggle swaps the width and height dimensions to simulate rotating the device. Landscape mode shows websites in horizontal orientation, which is especially useful for testing tablet layouts.