Screen Resolution and Screen Size are two dimensions that are very confusing. The Screen Resolution of a display device is the number of distinct pixels in each dimension that can be displayed. Screen Size is measured in inches and is the physical size of the screen, knowing both of these is required to figure out the PPI (Pixels Per Inch). The PPI is slightly complicated which I hopefully can simplify below.

Using The iPhone 3GS and iPhone 4 for examples. Both devices have a screen size of 3.5 inches. The iPhone 4 has a resolution of 960 x 640 pixels where as the iPhone 3GS is 320 x 480 pixels.

To determine the PPI the following steps must be followed:

1. Take the sum of the squares of the resolution (960² + 640² = 1,331,200)
2. Take the square root of the previous answer (√1,331,200 = 1,153.78)
3. Finally, divide the previous answer by the diagonal size of the screen (1,153.78/3.5=329.65)

The iPhone 4 advertises at 326 PPI, so we weren't far off. Feel free to figure out the PPI for the iPhone 3GS or, if your brain is hurting like mine is now, you can just look it up and discover that the 3GS has it's PPI at 165. No big surprise why the iPhones 4 retina display is so crisp and clean.

Screen Resolution is not the only factor in determining how good an image is displayed. If you take the iPhone 4's retina display resolution and apply it to an iPad the PPI would be only 118 or a 15 Inch mac book pro would have 76 PPI, both of these would result in a miserable computing experience.

Image from blog.gsmarena.com