Online Tool Store
· 5 min read
Best 3 Viewport Size Tester Tools Compared
Heshan Fernando
Co-founder & COO
You’ve made layout changes and want to check how the page actually looks across common device sizes — iPhone, iPad, laptop, desktop — without physically grabbing every device or resizing your browser window by hand repeatedly.
Every tool here previews a page at set viewport widths; the differences are in how many device presets are available, whether multiple sizes render side by side at once, and whether you can define a fully custom width and height.
How to judge a viewport size tester tool
Covers the device sizes people actually use. Common phone, tablet, laptop, and desktop breakpoints matter more than an exhaustive list of obscure devices.
Shows multiple sizes at once, or switches fast. Comparing layouts side by side catches responsive breakpoint issues faster than testing one size, closing the tool, and testing another.
Supports a custom width and height. Preset devices don’t cover every case — a custom size matters when testing a specific breakpoint in your CSS.
Works for pages that aren’t publicly deployed yet. If you’re testing a local or staging page, check whether the tool can actually load it (embeddable/iframe-friendly) rather than only working on live public URLs.
The comparison
| Tool | Best for | Free tier | Watch out |
|---|---|---|---|
| Pixefy | 300+ device presets with synchronized scrolling across views | Free | Browser extension, not a standalone web page |
| UiLooker | Fast, zero-signup testing with 50+ presets and instant switching | Free, no signup | Fewer presets than Pixefy’s 300+ |
| OverlayQA | Five common breakpoints shown simultaneously by default | Free, no account required | Some sites won’t load due to iframe-blocking security headers |
| Viewport Size Tester | iPhone, iPad, laptop, desktop, or custom width/height | Free, no signup | No simultaneous side-by-side multi-device view |
Facts checked August 2026; tools change their plans.
Pixefy
Pixefy displays multiple device views simultaneously in one window with over 300 device presets, synchronized scrolling and clicking across all views at once, custom breakpoint definitions you can save, split-screen comparisons, user agent simulation across 100+ browser/OS combinations, and an AI-powered inspect feature for CSS and HTML.
It isn’t for someone who wants a plain web-based tool — it’s distributed as a Chrome or Firefox browser extension rather than a standalone page you visit.
UiLooker
UiLooker offers 50+ device presets across mobile, tablet, and desktop/widescreen categories, custom viewport size creation, and notably fast performance with a stated 0.2-second average load time and instant device switching, with no signup mentioned.
It isn’t for someone who wants the largest possible preset library — its 50+ presets are solid but well short of Pixefy’s 300+.
OverlayQA
OverlayQA renders a page at five common breakpoints simultaneously by default — iPhone SE (320px), iPhone 14 (375px), iPad (768px), laptop (1024px), and desktop (1440px) — with additional presets like Pixel 7, Samsung Galaxy, iPad Pro, or a 4K monitor available via dropdown, completely free with no account.
It isn’t for someone testing a site with strict iframe-blocking security headers — the tool explicitly notes some sites won’t load due to X-Frame-Options or Content Security Policy restrictions.
Viewport Size Tester
Our tool previews any embeddable page at common device viewport sizes — iPhone, iPad, laptop, desktop, or a custom width and height — entirely in your browser.
A real limitation: it shows one viewport size at a time rather than multiple side by side — for comparing several breakpoints simultaneously without switching, OverlayQA or Pixefy’s multi-view layout is the better fit.
Which one to pick
If you want a quick single-viewport preview with a custom size option, use our Viewport Size Tester.
If you want to compare several common breakpoints side by side at once, use OverlayQA.
If you want the widest preset library and are fine installing a browser extension, use Pixefy.
If you want fast switching between 50+ presets without installing anything, use UiLooker.
How to do it with Viewport Size Tester
- Open the Viewport Size Tester.
- Enter the page URL you want to preview.
- Choose a device preset or enter a custom width and height.
Browse the full tools directory for more free, browser-based SEO and web tools.
Frequently asked questions
Is there a free viewport size tester that doesn’t need an account?
Yes. Our Viewport Size Tester, UiLooker, and OverlayQA all work without requiring signup.
Why does testing on real device widths matter more than just resizing a browser window?
Resizing a desktop browser window changes the viewport width but doesn’t fully replicate mobile-specific behaviors like touch target sizing, font rendering, or how mobile browsers handle certain CSS units — testing against actual common device widths (like 375px for a typical phone) gives a more accurate picture of what real visitors on those devices will see.
What’s a common responsive design mistake these tools help catch?
A frequent issue is content that looks fine at a design tool’s preview width but breaks at an in-between size — like a navigation menu that overlaps text specifically between tablet and small-laptop widths — which is exactly what testing at multiple discrete breakpoints, or defining a custom width at the exact problem size, is built to catch before real visitors encounter it.
Final thought
Don’t just check the extremes (phone and desktop) — the breakpoints in between, like tablet and small-laptop widths, are often where responsive layouts actually break, so test a custom width there if your design has any complex breakpoint logic.