Free Responsive Website Tester
Free Responsive Website Tester
Check how your website or HTML/CSS layout behaves at different screen sizes with an interactive CSS viewport tester.
Viewport Studio Pro lets you preview layouts at common mobile, tablet and desktop viewport sizes, test custom dimensions, inspect individual elements, check rendered dimensions and experiment with different device pixel ratio settings.
No installation required.
Test Your Website at Different Screen Sizes
A website that looks perfect on a desktop screen can behave very differently on a phone or tablet.
Use the viewport controls above to test common CSS viewport sizes, including:
| Viewport | Device Type |
|---|---|
| 360 × 800 | Mobile |
| 375 × 667 | Small mobile |
| 390 × 844 | Modern iPhone-sized viewport |
| 414 × 896 | Large mobile |
| 768 × 1024 | Tablet |
| 1024 × 1366 | Large tablet |
| 1280 × 800 | Desktop |
| 1366 × 768 | Common laptop |
| 1440 × 900 | Desktop |
| 1920 × 1080 | Full HD |
| 2560 × 1440 | QHD |
You can also enter your own width and height when you need to test a specific viewport.
What Does a Responsive Website Tester Do?
A responsive website tester helps you see how a webpage changes when its available viewport width and height change.
This is useful for finding problems such as:
- Text wrapping unexpectedly
- Buttons becoming difficult to tap
- Navigation overflowing horizontally
- Images extending beyond the viewport
- Cards becoming too narrow
- Fixed-width containers breaking on mobile
- Excessive padding on smaller screens
- Headings becoming too large or too small
- Content being hidden or pushed below the fold
- Horizontal scrolling caused by CSS
Testing several viewport sizes before publishing can help catch layout problems that may not be obvious on your own screen.
How to Test Responsive Design
1. Add your HTML and CSS
Paste your HTML/CSS into the editor.
2. Choose a viewport
Select a mobile, tablet or desktop preset, or enter custom dimensions.
3. Inspect the layout
Use the preview to see how the page fits inside that viewport.
4. Turn on Inspector Mode
Hover over elements to view their rendered dimensions and computed spacing.
5. Test different sizes
Switch between several viewport widths rather than checking only one mobile and one desktop size.
This is especially useful around your site's responsive breakpoints.
CSS Viewport vs Screen Resolution
These two terms are often confused.
Screen resolution describes the physical or logical pixel dimensions associated with a display.
CSS viewport size is the coordinate space that a webpage uses for layout.
For responsive web development, CSS viewport dimensions are generally more useful than simply asking whether a device has a 1920 × 1080 display.
For example, your CSS may respond to a viewport width using media queries such as:
@media (max-width: 768px) {
.navigation {
display: none;
}
}
The important question is therefore not simply "What resolution does the device have?" but "What CSS viewport does my page receive?"
Common Responsive Breakpoints
There is no universal set of breakpoints that every website should use.
However, developers commonly test around ranges such as:
| Device Type | Example Viewport |
|---|---|
| Small mobile | 360 × 800 |
| Mobile | 390 × 844 |
| Large mobile | 414 × 896 |
| Tablet | 768 × 1024 |
| Large tablet | 1024 × 1366 |
| Laptop | 1366 × 768 |
| Desktop | 1440 × 900 |
| Full HD | 1920 × 1080 |
| QHD | 2560 × 1440 |
These are testing presets, not rules. Your actual breakpoints should be based on where your layout needs to change.
What Is CSS Device Pixel Ratio (DPR)?
Device Pixel Ratio, commonly called DPR, describes the relationship between physical device pixels and CSS pixels.
For example, a display may have a DPR of 2, meaning one CSS pixel can correspond to two physical pixels in each dimension.
This is one reason why CSS viewport size and physical screen resolution should not be treated as the same thing.
Viewport Studio Pro includes DPR controls so you can experiment with different density settings while testing your layout.
Important: DPR simulation inside a browser-based tool is not identical to testing on a physical device. For final QA, real devices and browser developer tools should still be used.
What Is an Element Inspector?
The Inspector Mode in Viewport Studio Pro lets you examine individual elements inside the preview.
When you hover over an element, the tool can show information such as:
- Rendered width and height
- X and Y position
- Font size
- Line height
- Padding
- Margin
The measurements are based on the element's rendered layout and getBoundingClientRect().
This makes the tool useful when you are trying to understand why an element does not appear where you expect it to.
Responsive Design Testing Checklist
Before publishing a responsive webpage, check:
Mobile
- No horizontal scrolling
- Navigation fits the available width
- Headings wrap naturally
- Buttons are easy to use
- Images scale correctly
- Forms fit within the viewport
- Important content is not hidden
Tablet
- Two-column layouts still work
- Cards have sufficient width
- Navigation does not become crowded
- Images maintain their proportions
- Spacing does not become excessive
Desktop
- Content does not become excessively wide
- Maximum-width containers work correctly
- Typography remains readable
- Large empty areas are intentional
Across breakpoints
- No unexpected layout jumps
- Media queries activate at the intended widths
- No element causes horizontal overflow
- Fixed and absolute elements remain positioned correctly
Why Test More Than One Mobile Size?
Checking only 375px or 390px is not enough.
A layout can work at 390px and still break at 360px because a heading wraps differently, a button becomes too wide, or a navigation item no longer fits.
Likewise, a layout that works at 768px may need different treatment at 1024px.
For this reason, responsive testing is better treated as a range of viewport conditions, rather than a single "mobile test."