A website screen size checker should report more than a monitor’s resolution. For reliable responsive testing, record the CSS viewport width and height, device-pixel ratio (DPR), orientation, active media-query breakpoint, and the space left after browser chrome or scrollbars. The quickest method is Chrome DevTools Device Mode: open it, enter exact dimensions, test just below and above each breakpoint, and inspect the layout at every state.
Screen resolution, browser window, and CSS viewport are different
“What is my screen resolution?” and “What is my browser viewport size?” are related questions, but they do not have the same answer.
Physical or display resolution
Display resolution is the panel’s pixel grid, such as 1920 × 1080. Operating-system scaling and high-density screens can make that number a poor guide to the space available to CSS.
Screen width in CSS pixels
MDN defines the Screen.width read-only property as the screen width in CSS pixels. Browser and operating-system controls can occupy part of that screen, so the entire screen is not necessarily available to a page.
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Browser window size
The browser window includes tabs, address bars, borders and other browser UI. Maximizing the window does not guarantee that the page receives every screen pixel.
CSS viewport
The viewport is the coordinate space used by layout rules and media queries. It is the number that normally explains why a breakpoint activates, why a navigation menu collapses, or why a two-column grid becomes one column. A page can therefore look different on two displays with the same resolution if their browser zoom, UI, scaling, viewport, or DPR differs.
Check a site with Chrome DevTools Device Mode
- Open the page in Chrome.
- Open DevTools with F12 or Ctrl+Shift+I on Windows/Linux, or Cmd+Option+I on macOS.
- Activate the device toolbar with Ctrl+Shift+M (Windows/Linux) or Cmd+Shift+M (macOS), or select the phone/tablet icon in DevTools.
- Choose Responsive and enter a width and height, or select a device preset. Chrome documents common widths including 320px, 375px, 425px, 768px, 1024px, 1440px and 2560px.
- Set the device pixel ratio when your test requires it, then rotate between portrait and landscape.
- Drag the viewport or type values around every CSS breakpoint. Testing only the named device presets can miss the exact width where a rule changes.
- Reload the page and inspect navigation, text wrapping, images, forms, fixed elements and horizontal overflow.
Device Mode also exposes media-query breakpoints. Use the breakpoint indicators to jump between layout states, then write down the exact width at which each state begins.
A repeatable responsive test checklist
- Viewport dimensions: record CSS width and height, not just display resolution.
- Orientation: test portrait and landscape; the same device can cross a breakpoint after rotation.
- DPR: note device-pixel ratio when evaluating raster images, canvas output or screenshots.
- Available space: account for scrollbars, browser chrome and safe-area constraints.
- Breakpoint state: identify which media-query branch is active at each width.
- Content behavior: check wrapping, clipping, truncation, tap targets, menus, dialogs and sticky headers.
- Overflow: look for a horizontal scrollbar and inspect unusually wide children in the Elements panel.
Useful widths to test
Start with 320, 375, 425, 768, 1024, 1440 and 2560 CSS pixels, then add values 1–10 pixels on either side of your project’s actual breakpoints. A layout that works at 768px can still fail at 767px if a flex item, image or long word does not shrink.
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In DevTools, open the Elements panel to inspect live HTML and the Styles pane to see which declarations win. You can temporarily change widths, grid columns, padding, font sizes and overflow rules; the change appears immediately in the emulated viewport. These edits are diagnostic and disappear on reload unless you save them in your source.
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Confirm the viewport meta tag on narrow screens
Before blaming a breakpoint, inspect the document’s <head> for an appropriate declaration:
<meta name="viewport" content="width=device-width, initial-scale=1">MDN warns that some narrow devices otherwise use a wider virtual viewport and shrink the page to fit. In that situation, narrow media queries may never activate, making a correctly written responsive rule appear broken. Add or correct the declaration, reload, and measure again. Avoid adding restrictive zoom settings solely to hide a layout problem; accessible users should be able to zoom.
Read measurements from the browser
For a quick diagnostic, open the DevTools Console and run:
({
screen: { width: screen.width, height: screen.height },
viewport: { width: window.innerWidth, height: window.innerHeight },
document: { width: document.documentElement.clientWidth, height: document.documentElement.clientHeight },
dpr: window.devicePixelRatio,
orientation: screen.orientation?.type || "not available"
})window.innerWidth includes the layout viewport and can include the vertical scrollbar; document.documentElement.clientWidth is often closer to the content area after scrollbar space. Compare both when a one-pixel breakpoint difference matters. The result is a measurement of the current browser context, not a guarantee about every physical device.
Why the same resolution can produce different layouts
Zoom and operating-system scaling
Browser zoom and OS display scaling change CSS-pixel calculations. Reset zoom to 100% for a baseline, then test the zoom levels your users rely on.
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Browser UI and scrollbars
Address bars, side panels and scrollbar behavior reduce usable space. Overlay scrollbars may consume no layout width while classic scrollbars do.
Viewport meta configuration
Without the correct viewport hint, a phone can report a virtual width that is much wider than its physical display and scale the result down.
Dynamic mobile browser chrome
On real phones, URL bars can expand or collapse while scrolling. A screenshot or emulation pass may not reproduce those changing heights, especially for full-height elements.
Fonts, content and rendering engines
A different font, language, browser engine or font-loading state changes line breaks and element heights even when the viewport numbers match.
Emulation is not a real-device test
Chrome describes Device Mode as a simulation. It cannot reproduce every difference caused by mobile hardware. Use it for fast breakpoint and layout checks, then verify important flows on physical phones and tablets.
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- Test touch gestures, keyboard behavior, focus handling and virtual keyboards on hardware.
- Check performance, memory pressure, battery impact and network conditions on representative devices.
- Verify browser UI effects, safe areas, camera permissions and device-specific rendering.
- Repeat on the mobile browsers your audience actually uses, not only the desktop browser’s emulation.
Automate screenshots at exact sizes
For regression checks, capture the same URL at a fixed matrix of viewport sizes and compare the files in CI. Keep the URL, width, height, DPR, color scheme, locale and authentication state in the test record. Wait for a stable selector or network idle before capture, and load lazy images before judging the result. A screenshot proves what was rendered; it does not by itself prove that touch, scrolling or performance are correct.
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ScreenshotNeo is a website screenshot API and MCP server. One request returns PNG, JPEG, WebP or PDF, with viewport, device presets, DPR, full-page capture, lazy-image loading, CSS-selector element capture, dark mode and custom CSS or JavaScript options. It also supports waits, click-before-capture actions, hidden selectors, blocked ads or trackers, headers, cookies, user agents, authorization, timezone, geolocation, transparent backgrounds, resizing, chosen cache TTLs, signed links, asynchronous webhooks, bulk capture of up to 100 URLs per call, usage reporting and an OpenAPI specification.
Its cleanup is useful for responsive checks: it accepts cookie or consent banners like a visitor and removes more than 60 known consent platforms, newsletter popups and chat widgets before the shot; each step can be disabled. Bot checks or CAPTCHAs, blank pages, timeouts, failed loads and cache hits are not billed, and response headers identify the page verdict and billing status.
cURL
See the complete parameter reference in the ScreenshotNeo documentation.
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
r.raise_for_status()
open("shot.webp", "wb").write(r.content)Node.js
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);
if (!res.ok) throw new Error(`${res.status} ${res.statusText}`);
require('fs').writeFileSync('shot.webp', Buffer.from(await res.arrayBuffer()));ScreenshotNeo includes an MCP server with take_screenshot, get_page_info and capture_pdf tools for Claude, Cursor and other MCP clients. The Free plan includes 1,000 shots per month with no card; paid plans start at $5 for 3,000 shots. Every feature is available on every plan. Create a free ScreenshotNeo account to start.
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The site always appears desktop-sized on a phone
Check the viewport meta tag first, then reload without a cached document. Inspect window.innerWidth and confirm that your media query uses the expected units.
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A horizontal scrollbar appears
Use the Elements panel to find a child wider than the viewport. Common causes are fixed-width images, long unbroken strings, transforms, negative margins and flex items missing min-width: 0. Fix the offending component rather than applying global overflow-x: hidden, which can conceal clipped content.
The breakpoint changes one pixel earlier
Compare innerWidth with clientWidth, check scrollbar mode and reset zoom. Test both sides of the breakpoint and document the value that your CSS actually receives.
Images or fonts differ between runs
Wait for the relevant selector or font load, disable animations for the capture, and ensure the same network, authentication and locale state. A screenshot taken before lazy content arrives is not a valid layout comparison.
What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
Emulation looks correct but the phone fails
Move the test to hardware. Investigate touch targets, dynamic browser chrome, software keyboards, safe-area insets, GPU rendering and performance; these are outside a basic desktop simulation.
FAQ
Is screen resolution the same as viewport size?
No. Resolution describes the display; viewport size describes the CSS space available to the page.
Which width should I use for a responsive test?
Use your project’s real breakpoints plus nearby values, and include representative widths such as 320, 375, 425, 768, 1024, 1440 and 2560px.
Can DevTools prove a site works on every phone?
No. It is a useful simulation, but hardware testing is required for device-specific behavior, touch, performance and browser UI effects.
What should an automated screenshot record?
Record the URL, CSS viewport, height, DPR, orientation, browser or user agent, state and readiness condition so later images are comparable.
Quick Recap
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