Your display and window measurements, live. Resize the window or rotate the device and the numbers follow.
Your screen resolution
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Three different sizes matter and are often confused: the screen resolution (the physical display), the available screen (minus taskbars and docks), and the browser window’s inner size (what a web page can actually use). Device pixel ratio explains why a "4K" phone still reports a modest CSS resolution — many physical pixels map to one layout pixel.
Why is my resolution lower than the box said? Display scaling. A 4K screen at 150% scaling reports about 2560×1440 of usable space — the checker shows the usable (CSS) figures plus the true physical pixels.
What size do screenshots come out at? Usually the physical pixel count: CSS size multiplied by the device pixel ratio.