If you want to quickly and easily calculate the pixels per inch of a display, here’s a useful tool:
While this tool calculates the pixels per inch for you, it also calculates the width and height of a screen. If you have a bunch of scanned documents and intend to use a tablet or e-reader to view them, this will help you determine the size of display that best matches the size of your originals. I can read a scanned magazine page on a 7-inch tablet, but I wouldn’t call it comfortable. Even a 10-inch tablet falls short, but if you rotate it to landscape mode you can view about half a page at a time and read it comfortably.
Among the things I found: A pricey 27-inch WQHD 2560×1440 display delivers 108.79 pixels per inch. A relatively cheap 21.5-inch 1920×1080 display delivers 102.46 pixels per inch. So if you’re just after a sharp display, a pedestrian 21.5-inch 1080p display, commonly available for less than $150, is surprisingly good for that.
If you’re in the market for a tablet or a monitor, this tool is worth bookmarking.