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In Google Analytics, "Screen Resolution" is a default dimension under Audience > Technology > Browser & OS reports, but pixel density is nowhere to be found.

How can I use Google Analytics to determine how many of my visitors are using Retina or other high-def displays?

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Google Analytics does not support pixel ratio by default but you can add it with script for a custom variable. See https://github.com/tysonmatanich/GetDevicePixelRatio for more details.

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Lets turn that around and ask why would you care?

Assuming it's to determine if image/icon size is an issue, I've recently tossed all the multiple-resolution icons and replaced them with SVG graphics. Logos and larger non-vector images are all 2x size in a plain <img> tag.

Positive side effects are faster loads (the SVG is in the main style sheet) and easier modification. Haven't encountered any negative side effects yet.

If IE8 compatibility is required I will run one sprite file - make one page with all your icons and you can just screencapture it for instant sprites.

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You should care as if there's no high pixel ratio users, then sending everyone 2x sized images is a grand waste of everyone's time and resources. Also if everyone happens to be using 3x pixel ratio devices, then you still aren't providing full res pictures to your user base. – andrewb Mar 26 '15 at 22:30

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