Is a 4:5 or 3:4 video a YouTube Short? Render 1080x1350 or 1080x1440

YouTube's help says a Short is a video with a square or vertical aspect ratio of up to three minutes, so a 4:5 or 3:4 video, which is taller than it is wide, is portrait by plain reading. The help page does not list ratios, though, so if the Shorts shelf matters, 9:16 at 1080x1920 is the safe choice. Sume Timeline renders 1080x1350 (4:5) and 1080x1440 (3:4) when you set output.width and output.height.

The rule is on Understand three-minute YouTube Shorts (read 2026-10-06): "Videos uploaded on or after this date with a square or vertical aspect ratio up to three minutes in length will be categorized as Shorts". The Shorts series page uses "portrait or square".

What do the sizes look like?

Timeline takes even integers from 256 to 2160 for width and height, so all of these are valid.

What happens to a 9:16 clip in a 4:5 frame?

The slot's fit decides. The default cover scales the clip to fill the frame and crops the overflow, so a 9:16 clip in a 1080x1350 frame loses the top and bottom 25 percent or so of its height. contain keeps all of it with bars, and blur fills the bars with a blurred copy. Pick contain when a face or caption sits near the edge.

If you are generating clips, Sume's video catalog lists 3:4 and 2:3 beside 9:16 and 1:1, but each model shows only the subset it accepts in supported_aspect_ratios, so check the model before requesting 3:4.

Which one should I pick?

Choose 9:16 unless you have a layout reason, because it is the one shape YouTube's own pages use as the vertical example. Set output explicitly for any other ratio so the render never surprises you.