How to use it
Enter the resolution in pixels (or pick a common one) and the diagonal size of the screen in inches, which is how screens are sold. The result is the pixel density in pixels per inch (PPI): the diagonal in pixels divided by the diagonal in inches. The page also shows the size of one pixel (the dot pitch), the real width and height of the picture area, the aspect ratio and the total number of pixels. A 24-inch Full HD monitor has about 92 PPI, a 27-inch 4K monitor about 163 PPI, and a modern phone often has 400 or more.
What PPI means for you
The higher the PPI, the smaller each pixel and the sharper text and pictures look at the same viewing distance. Sharpness also depends on how far away you sit: a TV with a low PPI looks fine from a sofa, while the same density on a desk monitor looks coarse. Around 100 PPI is typical for desktop monitors for years, and 160 or more is high enough that text looks smooth. Many systems scale the interface on dense screens, so a 4K screen at 27 inches usually shows things at the size you would see on a 1440p screen, but sharper. Note that PPI is for screens; for printed pages, the equivalent measure is dots per inch (DPI).
Questions
Is the diagonal the whole screen or just the picture? The visible picture area, measured corner to corner, which is what makers quote. Does a curved screen change it? Slightly, since the diagonal is measured along the curve, but the figure is close. Why is the aspect ratio shown as 16:10 for 1920 × 1200? 8:5 and 16:10 are the same shape; the common name is used. To size paper in pixels, see the Paper Sizes Chart.