How to calculate device pixel ratio

You can find many of your screen parameters by visiting https://www.mydevice.io/

you can find the pixel ratio using javascript window.devicePixelRatio

Device Pixel Ratio (DPR) is the relationship between physical hardware-based pixels and Device-Independent Pixels which are an abstraction. The value of the device pixel ratio is not something which can reliably be calculated as it's based on how the device manufacturer intends to map one type of pixel system to the other. To reliably obtain this information, you will need to request it from the device.

According to this article

http://www.html5rocks.com/en/mobile/high-dpi/

The magic number seems to be 150.

Dividing the physical ppi by the ideal ppi of 150, gives the device pixel ratio.

E.g. a device with 445ppi would have a dpr of 3 ->> 445 / 150

The simple calculation seems to hold up okay for many items on this list,

http://en.wikipedia.org/wiki/List_of_displays_by_pixel_density