What is the ratio between aspect ratio and pixels?

Hi all,

I'm trying to figure out what is the ratio between the aspect ratio and the pixels. For example; I made my UI depending on the 1920x1080 resolution, which is the aspect ratio of 16:9. If I put an image that covers 600 pixels on the screen (and set its anchor correctly - stretch), and change the aspect ratio to 21:9, the image covers ~687 pixels, which is not something I understand. The ratio between 16 and 21 is 1.3125, so I was expecting the image to be ~787 pixels. However, the image extends as if the ratio is 1.145.

How should I approach when try to find out the ratio? 16:9 is my reference point. If someone plays the game with an aspect ratio of 21:9, I need to change a value of a parameter depending on the aspect ratio, but I couldn't find any relation.

 

I do not fully understand your question but I believe you are looking for this answer: Learn how to use anchors.
Here is a video I made for my asset Better UI, but it contains also helpful information about anchors in general: