I want to add black widescreen bars, or change my project settings, equilant to the 21:9 format, for more cinematic videos.
https://en.wikipedia.org/wiki/21:9_aspect_ratio

I think it's 1920x808 for full HD, but what is the height when working with 3840px wide footage?

Try using a calculator

It is 1920 x 1080 for full HD, and 3840 x 2160 for 4K, 16:9 screen ratio. You cannot set your own screen aspect ratio, but you can add black bars in post production. Why you would do it though beats me as it just annoys viewers. I am also puzzled as to why you ask, as it is all spelled out in the link you gave.

Geebax Posted at 2018-4-8 15:11
It is 1920 x 1080 for full HD, and 3840 x 2160 for 4K, 16:9 screen ratio. You cannot set your own screen aspect ratio, but you can add black bars in post production. Why you would do it though beats me as it just annoys viewers. I am also puzzled as to why you ask, as it is all spelled out in the link you gave.

It doesn't annoy me. And absolutely every Hollywood movie does it, so.. and I can set the ratio manually in Premiere, what do you mean "cannot set your own screen aspect ratio"?
And I didn't find it in the link haha..

Labroides Posted at 2018-4-8 14:41
Try using a calculator

I got that the height should be 1617px, and it looks correct. But i don't know if it is 1 or 2 pixels off. I want it to be as correct as possible

Alekzk Posted at 2018-4-8 23:34
It doesn't annoy me. And absolutely every Hollywood movie does it, so.. and I can set the ratio manually in Premiere, what do you mean "cannot set your own screen aspect ratio"?
And I didn't find it in the link haha..

You cannot set the aspect ratio on the Phantom camera is what I meant. It offers a limited choice of aspect ratios, and 21:9 is not one of them. That ratio is a Cinemascope ratio, and will always be displayed with bars at the top and bottom. Absolutely every Hollywood movie does not use that ratio, only those shot in Cinemascope, which is an obsolete format.