screenresolutiontest

The transition to high-density pixel displays that began with smartphones and tablets has spread to the computer monitors. 4K PC screens appeared in 2014, and understanding pixel density has become important when choosing a product, along with screen size and resolution. There are two standards for 4K resolution, “DCI 4K” and “UHD 4K”. Question of what is my screen resolution can be answered as: the display resolution of a digital television, computer monitor, tablet, smartphone or display device is the number of distinct pixels in each dimension that can be displayed.

4K is not standardized description of a number of physical pixels. It’s an approximation of the number of pixels wide something is (as opposed to the previous convention of how many pixels tall as in 480p, 720p, 1080p, etc.). Something with somewhere around 4000 pixels wide will be called 4K. So you have a variety starting from 3840 pixels up to at least 5120 pixels on “ultrawide 4K” displays, broadcasts, or video.
The takeaway should be that at a certain point, a physically smaller display requires physically fewer pixels to remain usable at HiDPI (assuming similar view distances). Cramming “4K” pixels into that small of a space is worse than just using fewer pixels, even if you don’t get the marketing-friendly 4K term.
Pixel density is the specification that defines how detailed a display is. It is expressed in units of DPI (Dots Per Inch) or PPI (Pixels Per Inch). DPI is typically used for OS and applications, and PPI is typically used for LCD panel specs.
The number of device pixels that make up a CSS pixel in one direction is its Device Pixel Ratio (DPR). You can interpret this as the width (or height) of the grid of device pixels that fit inside one CSS pixel. Every device has a different DPR