Every AI image model supports a handful of aspect ratios, but each platform has its own ideal. Generating a square image for a YouTube thumbnail or a 16:9 image for a Pinterest pin wastes credits and produces awkward crops. This page is a quick reference for which ratio fits each common platform and which AI models handle non-square ratios best.
When to use it
Common ratios: 1:1 (Instagram square), 4:5 (Instagram portrait), 9:16 (TikTok, Reels, Stories), 16:9 (YouTube thumbnails, presentation slides, blog hero images), 3:4 (magazine portrait), 21:9 (cinematic widescreen). Midjourney's --ar flag accepts any custom ratio. Imagen 4 and Flux 2 also support arbitrary ratios. Stable Diffusion works best at 512x512 multiples — non-standard ratios sometimes need ControlNet to avoid composition drift.
When to use it
Pick the ratio before you generate — most AI image models will crop or distort if you change ratio after. Instagram feeds (1:1, 4:5), YouTube thumbnails (16:9), and TikTok/Reels (9:16) all reward content that fits natively. Print and large-format outputs need 3:2 or 2:3.
How to use it (step by step)
- Enter your target dimensions. Type the width and height in pixels above. The calculator simplifies to the smallest whole-number ratio (e.g. 1920×1080 → 16:9).
- Or click a preset. Choose from Instagram, YouTube, TikTok, X, and AI-native presets. Each preset matches the platform's recommended dimensions.
- Use the ratio in your prompt. On Midjourney use `--ar 16:9`. On Nano Banana 2 / GPT Image-2 specify "16:9 landscape" in the prompt. On Flux 2, set width/height in the model parameters.
Common mistakes to avoid
- Generating square then cropping to vertical — you lose subject detail and resolution.
- Using non-standard ratios like 17:11 — they confuse models and produce inconsistent results.
- Forgetting that Stable Diffusion has trained on specific bucket sizes (512×512, 768×768, etc.). Off-bucket ratios degrade quality.
Frequently asked questions
What aspect ratio is best for Instagram?
Square (1:1) for the grid, 4:5 for in-feed posts (takes more vertical space), and 9:16 for Stories and Reels.
Does aspect ratio affect AI image quality?
Yes. Models are trained on specific ratios. Standard ratios (1:1, 4:5, 16:9, 9:16) generally produce the best results.
Can I change aspect ratio after generating?
You can crop, but you cannot add content. It's better to generate at the correct ratio. Use outpainting (Photoshop Generative Expand, Midjourney Zoom Out) to extend an image.
Which AI model supports the widest range of ratios?
Midjourney v7 supports any ratio via `--ar` flag. Flux 2 supports arbitrary width/height. GPT Image-2 accepts ratio descriptions natively.