PPI stands for pixels per inch. It measures the density of pixels on a screen, calculated by dividing the number of horizontal (or vertical) pixels by the physical width (or height) of the display in inches. A higher PPI value means pixels are packed more tightly together, resulting in a sharper, more detailed image.
PPI matters because it affects how crisp text and images appear at normal viewing distance. A smartphone screen viewed 10 inches from your eyes needs a higher PPI than a monitor viewed 24 inches away to look equally sharp. Most people find text comfortable to read when PPI exceeds 100, though the threshold varies by viewing distance.
For example, a 6-inch phone with 1080 x 2400 pixels has roughly 440 PPI, making individual pixels invisible to the naked eye at normal reading distance. A 27-inch desktop monitor with the same resolution has only about 163 PPI, so individual pixels may be faintly visible if you sit very close.
When buying a display, compare PPI alongside panel type and brightness. Two screens with identical resolutions will have different PPI if their sizes differ. A 5.5-inch screen and a 6.7-inch screen both at 1440 x 3120 will have noticeably different sharpness. For phones and tablets, aim for 300+ PPI; for laptops, 150+ is generally acceptable; for desktop monitors, 100+ suffices for most users.