Both the size of the screen and its resolution have an effect on the calculation of the number of pixels per inch in a display. Calculators for determining the pixels per inch of a given display account for these factors as part of the calculation.
Smaller displays with higher resolutions have higher numbers of pixels per inch than larger, lower-resolution displays. For example, a 4-inch, 1080p diagonal smartphone screen has approximately 550 pixels per inch, while a typical 55-inch LCD television has only about 40 pixels per inch at the same resolution. The same television would have 80 pixels per inch at the 3840x2160 4K resolution.