Pixel density is a measurement of the resolution of electronic image devices, indicating how many pixels are grouped within a space. As the industry typically adopts imperial units for this estimation, the pixel density will often be referred to as "PPI" (pixels per inch), and quite rarely, as "PPC" (pixels per centimeter). Within this context, more pixels within an area translate into sharper images, so the general rule for pixel density is "the more, the merrier!"
Although factors such as personal eyesight, distance, and luminosity must be taken into account, scientists have suggested that the human eye can't see details beyond a pixel density of 300 PPI. Whichever the case, consumer electronics manufacturers are continually investing on a higher PPI count: this goes to electronic display, image scanning, and camera technology, as pixel density not only addresses the resolution of screens but also of scanner and image sensor quality. Top-notch smartphones, for instance, are packing an impressive number of pixels per inch: Google's resources for Android developers currently refers to densities of up to 640 PPI, or "xxxhdpi" (extra-extra-extra-high-density).
Surprisingly, the current PPI frontrunner was released quite a while back, in 2015: the Sony Xperia Z5 Premium, with 806 PPI. We bet, however, that it won't take long for another smartphone to feature even higher pixel density.
When it comes to electronic displays, the calculation of pixels per inch takes into account the screen's diagonal resolution in pixels, as well as its size in inches. You must first determine the diagonal resolution (in pixels) using the Pythagorean theorem, and then divide it by the screen's diagonal size (in inches). Unless you are crazy about maths, you may want to check out our data on PPI instead of racking your brain, or you can use one of the various pixel density calculators available online. Also, note that you may find this information listed as "dots per inch" (or DPI) on some websites, although this measurement is not accurately applied to all electronic devices.