Re: 48px vs 44px target sizing

Jonathan wrote:

> “Yes, I agree, that's what I was trying to say.  That is why I was saying we can't assume that the 20mm referred to in the report was actually 50 CSS pixels on the device used.”



We can’t assume that, but it is probably a lot closer than that 75px David was suggesting due to using a CSS-mm to CSS-px based translation instead of a physical-mm to CSS-px translation.





> “There are slight variations as you have noted but generally they are close.”



7.5 to 9.5mm is a 25% difference, but we are discussing the difference between 44 & 48px? (9%). If you consider that close, then we might as well use 44px and not clash with platform guidelines.





> “I guess this makes the assumption that larger devices will have high device width and smaller devices will have lower device widths in scale that is acceptable.  Currently an iPhone reports the width as 320 with a device pixel ratio of 2.  What if they decided to change the width to 640 and have a device pixel ratio of 1.”



They would have an unusable device, for almost everyone.



At the end of the day it is up to the manufacturer to choose a reasonable size of CSS pixel, and up to the user to choose a suitable device (and people do, those with sight or mobility issues who come into testing generally have larger devices or another adaptation).



This overlaps a lot with the discussion of visual sizing and using pixels, the other factor is how close you are intended to hold the device. Phones are assumed to be hand-held quite close, tablets are more arms-length and therefore use larger sizing. A useful adaptation for some people is to pick a larger device and hold it closer.





> “ I do have a concern that things could change in the future - it just seems like a holdover to have an iPhone 6 report it's width as 320.”



The iPhone 6 is larger than the previous models, and it’s CSS pixel width is wider at 375px [1]. There is a very predictable relationship between physical size, intended viewing distance and CSS pixel size [2]. That is the most reliable size measure we have on the web.



Cheers,



-Alastair



1] http://dpi.lv/#750×1334@4.7″


2] https://alastairc.ac/2013/02/how-to-hold-your-ipad/