What aspect ratio should YouTube Shorts use?

Short answer

9:16 (1080×1920). Shorts is vertical-only. Anything wider gets letterboxed and won't appear in the Shorts swipe feed.

Detail

YouTube Shorts is a vertical-only format. The player is 9:16 (1080×1920 is the standard upload resolution), and YouTube routes content to the Shorts swipe feed only when the aspect ratio is 9:16 and the duration is 3 minutes or less. Uploading 16:9 horizontal as a Short results in letterboxing and reduced reach. If you want to repurpose a horizontal long-form into a Short, reframe with vertical composition before uploading.