If you generate an image at the wrong aspect ratio, you are either losing pixels to cropping, wasting tokens on black bars, or posting an image that gets auto-cropped to something unrecognizable on the platform.
This is the reference cheatsheet for picking the right aspect ratio across the four primary AI image models (Nano Banana 2, GPT Image 2, Seedream 5.0 Lite, Midjourney) and the major platforms you publish to (Instagram, YouTube, TikTok, X, LinkedIn, Pinterest, print, blog header).
Below: what each model supports, what each platform wants, the decision matrix, and the pre-flight checklist.
What "aspect ratio" actually means here
Aspect ratio is width-to-height, written as width:height. So 16:9 is wide (16 wide for every 9 tall), 9:16 is tall (9 wide for every 16 tall), 1:1 is square.
The unit does not matter — it is the ratio the model and the platform care about. 1024×1024, 2048×2048, and 4096×4096 are all 1:1.
The 4-model support matrix
| Model | Aspect ratios supported | Resolution range | Notes |
|---|---|---|---|
Nano Banana 2 (gemini-3.1-flash-image) | 1:1, 1:4, 1:8, 2:3, 3:2, 3:4, 4:1, 4:3, 4:5, 5:4, 8:1, 9:16, 16:9, 21:9 | 512 (Flash only), 1K, 2K, 4K | 14 named ratios. Use uppercase K for resolution ("4K", not "4k"). Default is 1K. |
GPT Image 2 (gpt-image-2) | Any, with constraints | 655,360 – 8,294,400 pixels | Max edge ≤ 3840px, both sides multiples of 16, ratio ≤ 3:1. Common presets: 1024×1024, 1024×1536, 1536×1024. |
Seedream 5.0 Lite (bytedance/seedream-5-lite) | 1:1 to 21:9 | 2K, 3K | Max 4704×2016. Named ratios include 1:1, 4:3, 3:4, 16:9, 9:16. Replicate API: aspect_ratio string. |
| Midjourney v7 | Any value via --ar | Defaults to ~1024px on long edge | All values possible from v5+. Common convention is --ar 16:9 at the end of the prompt. |
Key takeaways:
- All four models support the standard ratios (1:1, 16:9, 9:16, 4:3, 3:2, 4:5). Pick from the standard set unless you have a specific reason to go custom.
- Nano Banana 2 has the widest named set (14 ratios) including extreme formats (1:8, 8:1, 21:9).
- GPT Image 2 is the most flexible mathematically (any ratio) but constrained by pixel count, edge multiples, and the 3:1 ceiling.
- Midjourney v7 has the loosest API — any value via
--ar— but you trade off resolution and quality at unusual ratios. - Seedream 5.0 Lite is the strictest — 2K or 3K only, with a fixed set of named ratios.
The platform cheatsheet
These are the target aspect ratios and pixel dimensions for the major platforms in 2026. Generate at the platform's aspect ratio so your image does not get auto-cropped or letterboxed.
| Platform | Format | Aspect ratio | Pixel size |
|---|---|---|---|
| Instagram Feed (post) | square | 1:1 | 1080×1080 |
| Instagram Feed (post) | portrait | 4:5 | 1080×1350 |
| Instagram Carousel | portrait (preferred) | 4:5 | 1080×1350 |
| Instagram Story / Reel | vertical | 9:16 | 1080×1920 |
| Instagram Story ad | vertical | 9:16 | 1440×2560 |
| Reels cover (on grid) | square crop of 9:16 | 1:1.55 (≈ 4:5) | 420×654 |
| YouTube thumbnail | wide | 16:9 | 1280×720 |
| YouTube Shorts | vertical | 9:16 | 1080×1920 |
| TikTok | vertical | 9:16 | 1080×1920 |
| X (Twitter) post | wide | 16:9 | 1200×675 |
| X (Twitter) post | square | 1:1 | 1200×1200 |
| X (Twitter) header | wide | 3:1 | 1500×500 |
| LinkedIn post | wide | 1.91:1 | 1200×627 |
| LinkedIn cover | wide | 4:1 | 1584×396 |
| Pinterest Pin | vertical | 2:3 | 1000×1500 |
| Facebook post | wide | 1.91:1 | 1200×630 |
| Facebook cover | wide | 2.7:1 | 851×315 |
| Blog header | wide | 16:9 or 21:9 | 1920×1080 / 2560×1080 |
| Print poster (A2) | portrait | √2:1 (≈ 1:1.414) | custom — usually 4:5 or 2:3 |
| Print postcard | landscape | 3:2 | custom — usually 6×4 inch |
| Mobile wallpaper (vertical phone) | tall | 9:16 or 9:19.5 | 1080×1920 / 1170×2532 |
Three things to notice:
- Vertical is the new default. Stories, Reels, Shorts, TikTok, Pinterest all want 9:16 or 2:3. If you only have time to optimize for one format, optimize for vertical.
- The Reels cover crop is sneaky. You upload a 9:16 (1080×1920) cover image, but Instagram only shows the central 420×654 (1:1.55) portion on your profile grid. Center the important subject matter.
- LinkedIn and Facebook want 1.91:1. Most models do not have a named preset for 1.91:1 — you have to use a custom ratio. 1200×627 is the safe upload.
The decision matrix: "I want X, use this"
| I want a... | Use this aspect ratio | On this model | At this size |
|---|---|---|---|
| Square Instagram post | 1:1 | Any of the 4 | 1024×1024 (NB2) or 2048×2048 upscale |
| Instagram portrait post | 4:5 | Any of the 4 (NB2 named) | 1080×1350 |
| Instagram Story / Reel | 9:16 | Any of the 4 (NB2 / Midjourney --ar 9:16) | 1024×1536 (NB2 2K) or 1080×1920 upscale |
| YouTube thumbnail | 16:9 | Any of the 4 | 1280×720 (NB2 1K 16:9) or 1536×1024 (GPT Image 2) |
| YouTube Shorts / TikTok | 9:16 | Any of the 4 | 1024×1820 (NB2 2K) |
| X (Twitter) post | 16:9 or 1:1 | Any of the 4 | 1536×1024 (GPT Image 2) or 1024×1024 (NB2) |
| LinkedIn post | 1.91:1 (1200×627) | GPT Image 2 or Midjourney (--ar 1.91:1) | Custom size on NB2 (e.g. 1200×628 → ratio ≈ 1.91:1) |
| Pinterest Pin | 2:3 | Any of the 4 (NB2 named) | 1024×1536 (NB2 2K) or 1000×1500 final |
| Blog header | 16:9 or 21:9 | Any of the 4 | 1920×1080 (NB2 2K 16:9) or 2560×1080 (NB2 2K 21:9) |
| Mobile wallpaper | 9:16 or 9:19.5 | GPT Image 2 (custom) or Midjourney (--ar 9:19.5) | 1024×1820 (NB2 2K) or custom |
| Print poster (A2) | 2:3 or 4:5 | Any of the 4 (NB2 named) | NB2 2K or 4K, GPT Image 2 1536×2048 |
The "On this model" column is the easiest model for that ratio. All four models can technically produce any of these ratios — the table is just showing the path of least resistance (named preset, default size, no custom math).
The pre-flight checklist
Before you generate, do this. Skipping it is the most common reason for "why is my image cropped wrong."
- Confirm the platform target ratio. Look it up in the cheatsheet above. Do not guess.
- Confirm the model supports the ratio. All four support the standard set. Custom ratios (1.91:1, 9:19.5) need a model with flexible sizing (GPT Image 2, Midjourney).
- Check the resolution math. GPT Image 2: aspect ratio ≤ 3:1, both sides multiples of 16, total pixels between 655,360 and 8,294,400. Seedream 5.0 Lite: 2K or 3K. Nano Banana 2: 1K / 2K / 4K. Midjourney v7: default 1024px on long edge unless you upscale.
- Mind the model-specific gotchas. Nano Banana 2 rejects lowercase
4k— must be4K. GPT Image 2 above 2560×1440 is "experimental reliability." Seedream 5.0 Lite does not generate below 2K. Midjourney--ar 9:19.5works but be ready to upscale after. - Generate at 2x the final size, then downscale. If your platform target is 1080×1080, generate at 2K or 4K and downscale. The downscaled version looks sharper than the originally-generated 1080×1080.
The summary
Aspect ratio is not a creative choice. It is a platform constraint first, a model constraint second, and a creative choice third. Pick the platform target first, then the model that supports it with the least friction, then the resolution.
- All four primary models support 1:1, 16:9, 9:16, 4:3, 3:2, 4:5. Use these unless you have a reason not to.
- Nano Banana 2 has the widest named preset set (14 ratios including 21:9, 4:5, 1:4).
- GPT Image 2 is the most flexible mathematically (any ratio under 3:1, any size up to 3840px on long edge) but constrained by pixel count and 16-multiples.
- Midjourney v7 is the most permissive at the prompt level (
--araccepts any value) but you trade resolution at unusual ratios. - Seedream 5.0 Lite is the strictest (2K or 3K, named ratios only).
- The Reels cover crop is the most common gotcha — design for the central 1:1.55 portion, not the full 9:16.
- Generate at 2x the final size and downscale. The downscaled version is sharper than the natively-generated smaller size.
The model-agnostic craft post covers how to write the prompt. This cheatsheet covers what shape to ask for. Both work together — pick the right ratio first, then write the prompt for that ratio.
