What is the precise accuracy of W3W versus GPS on current cell phones?

I feel much depends on location and possibly even your phone itself. Here, sadly, I have had the blue dot be inaccurate by as many as eleven, yes eleven w3w squares.  It gives me no pleasure to tell you this. I do worry it might influence new users' opinions on w3w.

The generally accepted accuracy using the GPS network in open air on a recent smartphone is 10 meters or 3 W3W squares. There are claims that the accuracy can reach to a couple of feet. In the end, the best way to find out is to test your location by standing at a location recognisable by satellite and see if W3W agrees. There is also a faint blue circle around your location that offers an idea of accuracy. Personally, I'm usually placed in exactly the right square or an adjacent one.

Hey @Bill Hayward8, thanks for your question! 

Any navigational, geocoding or mapping tools on phones will rely on the GPS accuracy of the device. In the case of what3words: GPS gives your device latitude/longitude coordinates and then what3words converts those numbers into a what3words address. 

GPS accuracy on devices can vary if the sky is blocked by trees/ you are indoors etc. You can zoom out of the grid map to see if your blue dot is in the correct square (it will be more or less around the same place) and if not, you can tap the closest, most appropriate square and share that what3words to be more specific about your exact location.

I hope this helps!