Aspect ratio
Aspect ratio is the width-to-height proportion of a generated image or video, e.g. 1:1, 16:9, or 9:16.
Aspect ratio sets the shape of the output frame independent of resolution — 1:1 square, 16:9 widescreen, 9:16 vertical, 4:5 portrait. In image and video models it is usually passed as a flag (Midjourney's --ar 16:9) or as the output dimensions. It matters beyond cropping: models are trained on certain ratios, so an unusual ratio can change composition, subject framing, and even content (a 9:16 prompt tends to produce full-body or stacked compositions; 16:9 favors landscapes and establishing shots). Pick the ratio for the destination — Reels and Stories want 9:16, YouTube and banners 16:9, feeds often 4:5 or 1:1.
When to use aspect ratio
- Matching a target placement (9:16 Reels/Stories, 16:9 YouTube, 4:5 feed).
- Steering composition — vertical for full-body or stacked subjects, wide for scenes.
When not to use aspect ratio
- Expecting an exotic ratio to look as clean as common ones; far-from-trained ratios degrade.
Example
Input: Midjourney: "city skyline at dusk --ar 16:9" Output: A widescreen landscape composition framed for video, not a square crop of one.
Common mistakes
- Generating square then cropping to 9:16 and losing the subject — set the ratio up front.
- Pushing extreme ratios that the model handles poorly.
- Confusing aspect ratio (shape) with resolution (pixel count).
FAQ
What is aspect ratio?
Aspect ratio is the width-to-height proportion of a generated image or video, e.g. 1:1, 16:9, or 9:16.
When should I use aspect ratio?
Matching a target placement (9:16 Reels/Stories, 16:9 YouTube, 4:5 feed). Steering composition — vertical for full-body or stacked subjects, wide for scenes.
What are the most common mistakes with aspect ratio?
Generating square then cropping to 9:16 and losing the subject — set the ratio up front. Pushing extreme ratios that the model handles poorly. Confusing aspect ratio (shape) with resolution (pixel count).
Related terms
- Outpainting — Outpainting generates new, coherent image content beyond the original canvas edges, extending a picture outward.
- Seed — A seed is an integer that initializes the random number generator inside an image, video, or audio model, making generation reproducible.
- Prompt engineering — Prompt engineering is the practice of designing input text that reliably steers a large language model toward a specific output.
- Negative prompt — A negative prompt is text that tells an image, video, or audio generator what to avoid producing — the opposite of the main prompt.
Sources
Last updated: 2026-06-02. Raw markdown: https://promtable.com/glossary/aspect-ratio.md.