I've been reading since a long time and I don’t recall any website talking about this and apparently 99% of people don’t know the fact that the actual size of the display changes with the aspect ratio, so for example 34 inch 21:9 ultra wide display equals 31.5 16:9, we are using the diagonal length as the size of the display which is accurate in the old days when there was only one aspect ratio which was 4:3 or when you're comparing displays of the same aspect ratio, I wanted to be as comprehensive as possible so I decided to explain this by taking rectangles all of them have the same actual size so when you multiply the length and the width you're gonna get 8, and each one have different aspect ratio (dimensions), and you're gonna notice the wider the rectangle the longer the diameter of the rectangle gets.
Aspect ratio 1:1 Length:2.82 Width:2.82 Diameter:4
AR 5:4 L:3.16 W:2.53 D:4.048
AR 4:3 L:3.266 W2.45 D:4.0825
AR 16:10 L:3.577 W:2.236 D:4.219
AR 16:9 L:3.26 W2.121 D:4.326
AR 18:9 L:4 W:2 D:4.5
AR 21:9 L:4.3205 W1.85 D:4.7
AR 32:9 L:5.333 W:1.5 D:5.54
Like I said all these rectangles got the same exact size if you multiply length and width you're gonna get 8, which is the accurate way to measure the size of rectangles(displays), but as you could notice the diameter which is used to indicate the size of the display changes with aspect ratio. So a 4:3 display with 4.1 inch size display equals 4.7 inch 21:9, I'll mention few more realistic examples, 5:4 19" display with resolution 1280X1024 equals 20.3 16:9 display, 29" old CRT TV equals 30.73 16:9 TV, and 27" 16:9 equals 29.3 21:9, 31.5 display 16:9 equals 34.2, 21:9, 35" 16:9 equals 38" 21:9, 6.2 inch phone 18.5:9 equals 5.95 inch 16:9, 6" 18:9 equals 16:9 5.8 so these phones looks smaller not just because they are bezel less but because the actual size of the display is smaller as well, and finally double full HD 49" 32:9 equals 38" 16:9 displays.
I also want to mention since the size of the display is measured by the diameter of the display that when you double the size of the display the actual size double 4 times for example 8" display is actually 4 times bigger than 4" display, the same goes for the way we measures pixel density which measured by pixel per inch AKA PPI so 200 PPI display has 4 times more pixels than 100 PPI display, How do we calculate PPI? We use Pythagorean equation of triangles which is length^2+width^2=diameter^2
So for example if you want to measure the pixel density of display FHD 27" 1920^2+1080^2=4852800 then you get the root of that number to get the diameter pixels 4852800^0.5=2202.9 then divide that number to the diameter of the display which is in this case is 27 so 2202.9/27=81.5 PPI, a 4k 27" which got 4 times more pixels has 163 PPI.
I didn’t want turn tech site into math class but after double full HD (32:9) displays came out and apparently most people were getting the wrong idea that's when I decided to write this.