Have the screen resolution (width and height) in pixels and the diagonal length in inches, and want to calculate the PPI? Pretty simple really. The iPad 4 has a screen resolution of 2048 by 1536 pixels, and a diagonal length of 9.7 inches.

We can calculate its PPI as

The calculation above was performed using the wonderful MyScript Calculator, and the result captured by taking a screenshot.

The same formula can be written, expressed using , in a more generalized form as . Where, is the screen width in pixels, is the screen height in pixels, and is the diagonal length of the screen in inches.