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For the ordinary layout viewport, open your browser’s developer console and run window.innerWidth and window.innerHeight. The two integer values are CSS pixels used for layout, and include a rendered scrollbar. If you need the content area without scrollbars, use document.documentElement.clientWidth and document.documentElement.clientHeight instead.
Measure the layout viewport in seconds
The fastest method works in Chrome, Edge, Firefox, Safari and most Chromium-based browsers:
- Open the page you want to inspect.
- Open Developer Tools (for example,
F12orCtrl+Shift+Ion Windows/Linux;Cmd+Option+Ion macOS). - Select the Console tab.
- Evaluate
window.innerWidthand thenwindow.innerHeight.
window.innerWidth
window.innerHeightThe returned numbers are the layout viewport’s width and height in CSS pixels. They describe the page’s layout area, not your monitor’s physical resolution and not the browser window including tabs, toolbars or borders.
To get both values in one object, run:
({ width: window.innerWidth, height: window.innerHeight })Resize the browser and run the expression again. The result changes when the layout viewport changes.
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Choose the right viewport measurement
“Viewport size” can mean several different rectangles. Name the one you report so another developer can reproduce it.
| Question | Code | What the value means |
|---|---|---|
| What width and height does the page use for layout? | window.innerWidth, window.innerHeight |
Layout viewport in CSS pixels. A rendered vertical or horizontal scrollbar is included. |
| What is the layout area excluding scrollbars? | document.documentElement.clientWidth, document.documentElement.clientHeight |
Root element’s client area. The corresponding scrollbar is omitted. |
| What part is currently visible during mobile zoom or browser UI changes? | window.visualViewport.width, window.visualViewport.height |
Visual viewport. It can be smaller than the layout viewport. |
| How large is the whole outside browser window? | window.outerWidth, window.outerHeight |
Window dimensions including browser chrome; not the page viewport. |
Scrollbar-inclusive versus scrollbar-exclusive values
On a desktop browser with a classic scrollbar, innerWidth can be wider than clientWidth by the scrollbar’s width. Overlay scrollbars may consume little or no layout space, so the numbers can match. Neither result is universally “correct”: use innerWidth when you are discussing the browser’s layout viewport, and clientWidth when you need the root content area available beside scrollbars.
CSS pixels versus device pixels
These APIs return CSS pixels. A high-density display maps several physical device pixels to one CSS pixel, and browser zoom changes the relationship. Do not describe a CSS-pixel viewport as a phone’s physical screen resolution. If you need pixel density or a screenshot’s bitmap dimensions, record that as a separate measurement.
Read the live values while resizing
A one-time console expression is enough for a quick check. For breakpoint debugging, install a small live logger:
function showViewportSize() {
console.log({
width: window.innerWidth,
height: window.innerHeight,
clientWidth: document.documentElement.clientWidth,
clientHeight: document.documentElement.clientHeight,
visualWidth: window.visualViewport?.width,
visualHeight: window.visualViewport?.height,
});
}
showViewportSize();
window.addEventListener("resize", showViewportSize);
window.visualViewport?.addEventListener("resize", showViewportSize);The resize listener refreshes layout values when the window changes. The optional visualViewport listener catches visual-viewport changes such as mobile pinch zoom. The optional-chaining operator prevents an error in browsers that do not expose visualViewport.
To stop the listeners, reload the page or use named handler functions and pass those functions to removeEventListener. Logging every resize event can be noisy; for automated diagnostics, write the values to a panel or collect only when they cross a breakpoint.
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Understand mobile viewport behavior
Use a viewport meta element
A responsive document normally includes this in its <head>:
<meta name="viewport" content="width=device-width">width=device-width tells the mobile browser to use the device’s CSS width for the page viewport. Without a suitable declaration, some mobile browsers use a much wider virtual layout viewport (MDN documents 980 CSS pixels as an example) and scale the page down. A narrow-looking phone can therefore report a wide layout value and produce unexpected media-query results.
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Pinch zoom changes how much content is visible without necessarily changing the layout viewport. The visual viewport may become smaller while the layout viewport remains at the width used to calculate the page’s CSS layout. Read both when investigating a zoom or fixed-positioning problem:
({
layout: {
width: window.innerWidth,
height: window.innerHeight
},
visual: window.visualViewport && {
width: window.visualViewport.width,
height: window.visualViewport.height,
scale: window.visualViewport.scale
}
})On-screen keyboards add another complication. Depending on browser and platform behavior, the keyboard can resize the visual viewport, resize page content, or overlay the content. The CSS interactive-widget behavior is commonly described as resizes-visual, resizes-content or overlays-content; treat keyboard results as an interaction state, not as the phone’s permanent viewport size.
Use viewport measurements for responsive design
Check the widths where your content fails
CSS media-query width and height conditions are evaluated against the viewport, so test the values at which your own layout becomes cramped: navigation wrapping, cards becoming too narrow, tables overflowing or controls becoming hard to use. A device model’s advertised screen resolution is not a universal breakpoint.
Use flexible grids, fluid images and relative units, then add breakpoints where the content needs them. Record the measured CSS width, height, browser zoom and whether a scrollbar was present. That record is much more useful than saying “tested on a laptop.”
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Compare CSS and JavaScript readings
A media query can be inspected from JavaScript to confirm which CSS branch is active:
const narrow = window.matchMedia("(max-width: 767px)").matches;
console.log({ width: window.innerWidth, narrow });If the query and your visual expectation disagree, check the viewport meta element, browser zoom and whether you are looking at the layout or visual viewport. Keep breakpoint logic in CSS where possible; use JavaScript only when behavior truly needs a script.
Capture a reproducible viewport record
When filing a responsive bug, include:
- URL and page state (for example, whether a menu or keyboard was open).
- Browser and operating system.
- Browser zoom level.
- Layout width and height from
innerWidth/innerHeight. - Client width and height if scrollbar space matters.
- Visual-viewport values and scale on mobile.
- Device-pixel ratio when comparing a bitmap screenshot.
This separates a layout bug from a zoom, scrollbar, keyboard or screenshot-scaling issue.
Troubleshooting inaccurate or surprising results
“My width is smaller than the window I see.”
You are probably comparing the page viewport with the outer browser window. Tabs, toolbars, borders and side panels are outside the page. Compare innerWidth with outerWidth only when you intentionally need both.
“The width changes by about a scrollbar’s size.”
Check whether you read innerWidth or document.documentElement.clientWidth. The former includes a rendered scrollbar; the latter excludes it. Overlay-scrollbar settings can also make the difference appear or disappear.
“A phone reports a desktop-like width.”
Inspect the document’s <meta name="viewport"> declaration. Without width=device-width, a mobile browser may use a wide virtual viewport and scale the page. Test again after correcting the markup and reloading.
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“Pinch zoom did not change innerWidth.”
That is expected in many mobile scenarios: zoom changes the visual viewport while the layout viewport remains stable. Read window.visualViewport.width, height and scale for the currently visible region.
“The value changes when I open the keyboard.”
Record whether the change is in the layout viewport, visual viewport or both. Interactive-widget behavior differs by browser and platform, so design the form for each state rather than assuming one keyboard rule.
“The console says visualViewport is undefined.”
Use the layout APIs, or guard access with optional chaining as in the live logger. A missing visual-viewport API does not prevent ordinary layout measurement.
“A screenshot has different dimensions from my console reading.”
Check whether the screenshot tool used a different CSS viewport, device scale factor, full-page stitching or browser zoom. A bitmap’s pixel dimensions are not automatically its CSS viewport dimensions.
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FAQ
Can I measure viewport size without opening Developer Tools?
Yes. Add the same JavaScript to a page bookmarklet, a temporary script, or your site’s diagnostic panel. The values are browser APIs, so Developer Tools is only the quickest access method.
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Should I report width first or height first?
Report width × height and label the pair, for example “layout viewport: 1365 × 768 CSS px.” Also state whether scrollbars are included.
Does full-page scrolling change the viewport height?
No. Scrolling changes the document’s scroll position. The viewport dimensions describe the viewing rectangle; use document measurements such as scroll height when you need the page’s total content extent.
Why can two browsers at the same nominal window size report different values?
Scrollbar policy, browser zoom, operating-system scaling, mobile UI and viewport emulation can differ. Compare CSS-pixel readings under the same state instead of relying on the device or monitor label.
Frequently Asked Questions
What is the quickest command to check a page viewport?
Run ({ width: window.innerWidth, height: window.innerHeight }) in the browser console.
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Use window.visualViewport.width and window.visualViewport.height; they describe the visual viewport, which can be smaller than the layout viewport.
How do I exclude the scrollbar from the width?
Read document.documentElement.clientWidth and clientHeight instead of the window’s inner dimensions.
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