SpeedXG
Screen Resolution Test
Read native screen size, available workspace, browser viewport and pixel ratio. Useful when a UI looks blurry or zoomed.
Resolution vs viewport
Screen resolution is the panel. Viewport is the browser chrome. Device pixel ratio of 2 means a “1920 CSS pixel” layout is drawing more physical pixels — Retina-style.
FAQ
Why does it not match the advertised 4K?
OS scaling, a laptop running a low-res HDMI mode, or a browser zoom other than 100% will change these numbers.
What is a good resolution for this test?
Whatever the panel is sold as. The test reports facts; it does not change the mode.