Here’s the deal: at the time I’m writing this, early 2013, there’s no way to accurately design for the Web using physical units, nor will there be for a very long time. But there is a way to design while knowing the physical characteristics of the device — or, at least, there will be in the very near future.
Go read the whole thing and use the VERY HANDY device resolution charts: here. As a bonus, you will get a geeky kind of humor as shown below.
A typographic, designological, visulicious extravaganza!