in iPhone App How to detect the screen resolution of the device

CGRect screenBounds = [[UIScreen mainScreen] bounds];

That will give you the entire screen’s resolution in points, so it would most typically be 320×480 for iPhones. Even though the iPhone4 has a much larger screen size iOS still gives back 320×480 instead of 640×960. This is mostly because of older applications breaking.

CGFloat screenScale = [[UIScreen mainScreen] scale];

This will give you the scale of the screen. For all devices that do not have Retina Displays this will return a 1.0f, while Retina Display devices will give a 2.0f and the iPhone 6 Plus (Retina HD) will give a 3.0f.

Now if you want to get the pixel width & height of the iOS device screen you just need to do one simple thing.

CGSize screenSize = CGSizeMake(screenBounds.size.width * screenScale, screenBounds.size.height * screenScale);

By multiplying by the screen’s scale you get the actual pixel resolution.

A good read on the difference between points and pixels in iOS can be read here.

EDIT: (Version for Swift)

let screenBounds = UIScreen.main.bounds
let screenScale = UIScreen.main.scale
let screenSize = CGSize(width: screenBounds.size.width * screenScale, height: screenBounds.size.height * screenScale)

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