If the calculation of the display ppi

Know that the display aspect ratio, resolution to know, know that the diagonal size, I would like to ask how to calculate the accuracy of the display ppi

such as mobile phones, Apple iphone, the official website of ppi to 163 on the subject, I would like to ask is how to calculate.

ppi expressed by every inch is owned by the pixel (Pixel) number.For example, iphone screen parameter is 3.5 -Inch TFT, a resolution of 320 × 480.It's ppi is the operator, first of all calculated equivalent diagonal screen pixel = 576.888 pixels, and then divided by 3.5 inches diagonal, to be ppi = 164.825.