What your screen is telling us
This page reads your display directly from the browser and shows what it reports. The screen resolution is the logical size the operating system exposes; the physical pixels multiply that by your device pixel ratio, which is how a "Retina" phone can advertise 1170 by 2532 real pixels while the browser lays out at a smaller logical size.
The browser viewport is the actual space your web page has to work with, always a little smaller than the screen because of toolbars. DPR (device pixel ratio) tells you how many physical pixels make up one CSS pixel, and adding your screen's diagonal turns all of this into a real PPI figure.
Why it is useful
Knowing your true resolution helps when you set a wallpaper, record your screen, report a display bug, or check whether a monitor is running at its native resolution. For the on-screen size story behind these numbers, see the PPI calculator and the screen DPI explainer.