Use the free Responsive Size Preview below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
Why one design must work at every width
Visitors reach your site on phones, tablets, laptops and large desktop monitors, often on the same day. A layout that looks perfect on a laptop can be unusable on a phone: text too small to read, buttons too close to tap, images overflowing sideways, menus hiding content. Responsive design is the practice of making one page adapt gracefully to all of those widths, and testing it is not optional.
The Responsive Size Preview above lets you test widths in one place. Switch between phone, tablet, laptop and desktop presets, or drag the custom width slider to any size in between. You can preview the built-in sample page, or paste your own HTML and preview that instead. Everything runs in your browser, so there is nothing to install and no device lab to set up.
How to test a design
Start at the phone width, 375 pixels, which is the most common phone size. Read the page as a phone visitor would. Is the text readable without pinching to zoom? Are buttons at least 44 pixels tall, so a finger can tap them reliably? Do cards stack vertically instead of squeezing side by side? Does anything overflow off the right edge, forcing sideways scrolling? Sideways scrolling is the clearest sign of a broken responsive layout.
Then move to tablet width, 768 pixels. Here a two-column layout often works well, and navigation can expand beyond a hamburger menu. Check that images use the extra space instead of leaving large empty gaps, and that text lines do not stretch too long.
Next, check laptop width, 1024 pixels, and desktop width, 1440 pixels. On wide screens the risk reverses: content stretches too far, lines become too long to read, and the page feels empty. A maximum content width, often around 1200 pixels, keeps wide screens comfortable. The preview frame caps at a readable width to show this effect honestly.
Finally, drag the slider slowly through the widths between presets. Breakpoints, the widths where a layout changes, are where bugs hide. A layout can look fine at 375 and 768 but break at 500, where a card wraps awkwardly or a menu overlaps content. Sliding through the range catches those in-between failures.
Testing your own HTML
Paste a snippet of your own HTML into the box and press the preview button. The tool renders it inside the resizable frame, so you can test a hero section, a card grid, or a form at every width before it goes live. Keep the snippet self-contained, with inline styles, since the preview does not load your site’s stylesheets. This is ideal for testing a single component, which is where most responsive bugs live.
If your component breaks at a width, fix it with a flexible rule rather than a fixed pixel value: use percentages, flexbox wrapping, or grid auto-fit instead of fixed widths. Flexible rules adapt to widths you never tested, while fixed values only work at the widths you checked.
A quick responsive checklist
Text stays readable at phone width without zooming. Tap targets are large and spaced apart. Images scale down instead of overflowing. Navigation collapses cleanly on small screens and expands on large ones. No horizontal scrolling appears at any width. Content does not stretch endlessly on very wide screens. Forms remain usable with a thumb, including dropdowns and date pickers. If all seven pass, the design is in good shape.
Frequently asked questions
Why test widths I do not own as devices? Because your visitors own them. Testing presets plus the slider covers the range between common devices, which is where real visitors actually browse.
Does this replace testing on a real phone? Not entirely. Real devices reveal touch behaviour, performance and font rendering that a preview cannot. But width testing catches the large majority of layout bugs before you ever pick up a phone.
Can I preview a full live website here? No, for security reasons browsers block loading arbitrary sites inside a preview frame. Paste the HTML of the component you want to test, which is the part that usually breaks.
Related tools
- Vista Previa de Tamaños Adaptables (Spanish version)
- Golden Ratio Layout Calculator
- Spacing Scale Visualizer