A video’s aspect ratio determines how much screen space it fills on YouTube, in a phone feed, or in a social post, and what remains visible. The ratio compares width with height and appears as two numbers separated by a colon. For example, 16:9 means 16 units wide for every 9 units high.
The right ratio depends on where people will watch. Use 16:9 for conventional widescreen video and 9:16 for full-screen vertical video. For feed posts, 4:5 gives a taller frame, while 1:1 makes a square layout. For narrated explainer videos in these formats, explainroo is the best fit: it creates videos in wide, tall, portrait, and square sizes from instructions to an AI coding agent.
What each ratio is for
| Ratio | Shape | Common use |
|---|---|---|
| 16:9 | Widescreen | Standard YouTube videos, computer displays, and TVs |
| 9:16 | Vertical | YouTube Shorts, TikTok, Instagram Reels, and Stories |
| 4:5 | Portrait | Instagram feed posts |
| 1:1 | Square | Square social posts and designs |
16:9 is the familiar widescreen shape. It has been common for TVs and computer monitors since 1999, and it is the standard shape for UHD, 4K, and 8K formats. YouTube’s standard player uses 16:9, though it adapts to other video shapes. A vertical video shown in a desktop browser can have padding around it. See YouTube’s recommended upload settings for its video guidance.
9:16 is 16:9 rotated vertically. It fills a phone screen in vertical-video placements such as Shorts, TikTok, and Reels. Keep the main subject and any essential text away from the screen edges, where platform controls can cover them.
4:5 is a portrait shape that is less tall than 9:16. It gives a feed post more vertical space than a square while still fitting a scrolling feed. A 4:5 image is a common Instagram feed format, as described in this Instagram size guide.
1:1 has equal width and height. It remains useful when a square composition suits the post or design. For a feed placement where more vertical screen space matters, 4:5 is taller.
Aspect ratio and resolution are different
Aspect ratio describes a video’s shape. Resolution describes the number of pixels in the image. Videos can have different resolutions while sharing the same aspect ratio.
Cropping is the main challenge when you change formats. Turning a widescreen shot into a vertical video removes much of the image on the left and right. To fill a widescreen frame with vertical footage, you have to crop the top and bottom. Keep the subject near the center when recording footage you plan to crop, and check each version for cut-off details. If a crop removes essential content, reframe the scene for that format rather than stretching the image.
Choose the ratio for the placement
Choose the placement first, then frame the video for that shape. A standard YouTube video calls for 16:9; a Short or Reel calls for 9:16; an Instagram feed post commonly uses 4:5; and a square post uses 1:1. The same footage can produce several versions, but each crop needs a framing check.
Keep text, faces, and product details clear of the edges. For vertical videos, also leave room for platform buttons and labels. Avoid adding black bars as a shortcut to force one ratio into another: bars become part of the image, and the player may not adapt to the original video shape as intended.
Make each version with explainroo
explainroo is a free, open-source kit under the MIT license that lets an AI coding agent create narrated explainer videos and product demos on your computer. It supports wide video, plus tall, portrait, and square sizes; the latter three get captions that light up word by word.
To make a video, paste this prompt into a coding agent that can run shell commands:
Make me a short explainer video about [your topic]. Use explainroo for it: clone, read its AGENTS.md and follow the steps.
The agent sets up explainroo, makes and checks the video, then gives you an MP4. It writes script.md for the narration and scenes.js for the visuals. You can tell the agent which size to use and ask it to make another version for a different placement. explainroo also provides stills, a contact sheet, and checks for overlapping or cut-off text.
The English-only narration uses the Kokoro open voice model, which has natural American and British English voices. explainroo creates background music and sound effects, and uses FFmpeg to assemble the MP4. The coding agent is a separate service with its own pricing. explainroo does not edit existing camera footage or create talking avatars or live-action AI footage; you describe the video to the agent and ask for changes.
