Can a YouTube Short be square? Rendering 1080x1080 in Sume Timeline

Yes. YouTube's Shorts help page, read on 2026-10-06, says a Short needs a square or vertical aspect ratio and a length of up to three minutes. In Sume Timeline, set output.width and output.height to 1080 and 1080, both even integers in the 256 to 2160 range.

What happens to a 9:16 source in a square frame?

video[].fit decides. cover (the default) fills the square and crops the top and bottom of a vertical source, which can cut a face or a caption. contain keeps all of it with bars, blur fills the bars with a blurred copy, and stretch distorts. For talking heads, cover with a source framed for the center is usually enough; for text-heavy clips use blur.

Is square better than vertical?

The page only says both qualify. Vertical fills a phone screen and is the default Timeline frame, so choose square when the same file must also work in a feed. Whichever you pick, stay at 180 seconds or less.

{
  "audio": {"mode": "silence", "duration_seconds": 30},
  "video": [{"source_url": "https://media.sume.com/artifacts/artf_demo/talk.mp4",
             "start": 0, "duration": 30, "fit": "blur"}],
  "output": {"width": 1080, "height": 1080}
}