web - using css pixel calculating screen size, where is wrong? -


in css w3(here),

1px = 0.75pt, 1pt = 2.54/72 cm,so 1 css pixel:1px = 0.26mm

for example, css pixel of iphone4 320px * 480px(not device pixel),
in width ,the length should be: 

320 *0.26 = 83.2mm

but iphone4 3.5"，so length in width 60mm,

it dont match result calculated above
wrong?
thanks

a css pixel not 0.26mm

from css w3:

the reference pixel visual angle of 1 pixel on device pixel density of 96dpi , distance reader of arm's length. nominal arm's length of 28 inches, visual angle therefore 0.0213 degrees. reading @ arm's length, 1px corresponds 0.26 mm (1/96 inch).

the iphone4 not 96dpi. device pixel ratio of 2, "css dpi" around 163. since 96dpi devices render 320px @ 83.2mm, mathematically, 163dpi devices should render 320px @ 49mm.


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