Screen resolution refers strictly to the total number of physical pixels arranged horizontally and vertically on a display. For example, a standard 4K (UHD) display has a resolution of 3840 x 2160 pixels, totaling about 8.3 million pixels.
However, resolution alone tells you nothing about how sharp the image will appear. A 65-inch television and a 6-inch smartphone can both have a 4K resolution. Because the smartphone packs the exact same number of pixels into a fraction of the physical area, its pixels are microscopically small, leading to vastly different visual clarity.
Calculating Pixels Per Inch (PPI)
Display density is measured in Pixels Per Inch (PPI). It quantifies how tightly the pixels are packed. The formula uses the Pythagorean theorem to find the diagonal pixel count, divided by the physical diagonal screen size in inches: PPI = sqrt(width² + height²) / diagonal_inches.
For a 27-inch 4K monitor (3840x2160), the diagonal pixel count is sqrt(3840² + 2160²) = 4405.8 pixels. Dividing by 27 inches yields 163 PPI. Doing the same math for a 6-inch 4K smartphone yields a staggering 734 PPI. The smartphone screen is over four times as dense.
The role of viewing distance (Retina displays)
More PPI is not infinitely better because human visual acuity has limits. The 'Retina' concept coined by Apple relies on the angular resolution of the human eye. At a normal smartphone viewing distance of 10-12 inches, the eye generally cannot distinguish individual pixels beyond 300 PPI.
However, as viewing distance increases, the required PPI drops. A desktop monitor viewed from 2 feet away only needs about 110-160 PPI to appear completely smooth. A living room TV viewed from 10 feet away looks perfectly sharp at just 50 PPI. This is why 4K is necessary for close-up monitors but often overkill for small TVs across the room.