DPI icons based size

I'm having some problems on the icon DPI. For example, when I make an icon for an iPhone application 6 I know that the icon will be 326 DPI. But if I do a 256x256 icon that can be used on iPhone, Android and on the Web at different screen resolutions. Wich one DPI I use? Is there a "formula" to calculate the DPI based on the size of the image?

OBS .: I want to make one set of icons for sale, but I want to avoid creating multiple equal with different DPI so I can sell unopened packages. I am using Photoshop CC as yet dont acquainted with Illustrator