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Samsung Galaxy S4 resolution:
Real resolution: 1080X1920
CSS resolution: 360X640

How to acquire CSS resolution? In Wikipedia you have all mobile devices resolution table (tablet and mobile devices). http://en.wikipedia.org/wiki/List_of_displays_by_pixel_density
The calculation is very easy, you just need to divide the pixel screen in the CSS pixel ratio number, like this: 1080/3 = 360px (portrait - width) 1920/3 - 640px (portrait - height)