With this handy application you can calculate the PPI (pixels per inch) of all the screens you want. In addition to the PPI you can get information about aspect ratio and the measurements of the screen:
* Height
* Width
* Area
All the data are available in either inches or centimeters.

To make your life easier there is also a list of standard resolutions and many common devices. Tap the item you want to preset the data without having to write anything.