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CSS pixels describe the browser’s layout

Web pages use CSS pixels to lay out content. Display scaling and pixel density can mean that a screen with many hardware pixels reports a smaller CSS-pixel size. The checker labels these values explicitly.

The browser window is a different measurement

Your screen dimensions and the page’s available window dimensions are separate. Resizing the window changes how much room a website has to work with. Browser controls, side panels and zoom can also affect layout.

Device pixel ratio helps explain density

The ratio describes the relationship between device pixels and CSS pixels in the current environment. It can change with zoom or display settings. Multiplying the screen reading by this ratio is not a reliable way to prove a monitor’s native resolution.

Which number should you send to support?

Send the labelled screen and window values, together with any unusual zoom or scaling settings you know about. A screenshot can show clipping or overlap that the numbers alone cannot explain.

Need the display’s native resolution?

Use your operating system’s display settings and your monitor’s specifications. The browser does not provide a definitive hardware inventory.