Free tool · MP4 · Runs in your browser
Convert video to 21:9
Short answer
Choose a video and the tool scales it until it fills a 2560 × 1080 frame, then cuts the excess from the center: nothing is stretched and there are no black bars. A 16:9 clip keeps 75% of its picture, a clip that is already 21:9 keeps all of it. In our tests a 3 second 1080p clip took 17 to 27 seconds in the browser engine and came out at 1.19 MB to 1.40 MB. Nothing is uploaded.
Batch is free and unlimited - files are processed one after another on your device, with no queue or daily cap. Maximum 100 MB per file; the first run loads a ~30 MB engine once per visit.
Key takeaways
- The crop is a centered cover crop: scale by the larger of 2560 divided by the width and 1080 divided by the height, then cut the excess. A 16:9 source (1920 × 1080) is scaled by 1.33 and keeps 75% of its area.
- Six source shapes, all converted: the share of the picture kept ranged from 24% to 99%, and the encode scored SSIM 0.98 to 0.99 against a Lanczos crop of the same picture (1.0 would be identical).
- Time: 17 to 27 seconds for a 3 second 1080p clip on the single-thread engine, so a one minute clip takes roughly 6 to 9 minutes. Trim first to keep it short.
- Size: about 23.77 MB per minute for a source that is already 21:9, and 27.97 MB per minute at most in our tests.
- No platform in our registry lists 21:9 as its preferred shape, but it is a common editing shape.
Measured: six source shapes to 21:9
The page's own crop arguments (scale to cover, then crop; H.264 at the shared fast preset; AAC audio) on the same WebAssembly engine your browser downloads, on 3 second excerpts of Big Buck Bunny, an animated film. The 16:9 source is the original excerpt; the other five were cut from it to the shape shown, and the 9:16 source was made by enlarging a center strip, so it is softer than phone footage. Times are from a headless run; your device will differ.
| Source shape | Source pixels | Scale before the crop | Picture kept | Result size | Time | SSIM |
|---|---|---|---|---|---|---|
| 16:9 | 1920 × 1080 | 1.33× | 75% | 1.19 MB | 17.8 s | 0.98 |
| 9:16 | 1080 × 1920 | 2.37× | 24% | 1.33 MB | 26.8 s | 0.99 |
| 1:1 | 1080 × 1080 | 2.37× | 42% | 1.36 MB | 21.5 s | 0.99 |
| 4:5 | 864 × 1080 | 2.96× | 34% | 1.40 MB | 23.1 s | 0.99 |
| 4:3 | 1440 × 1080 | 1.78× | 56% | 1.30 MB | 19.5 s | 0.99 |
| 21:9 (same shape) | 1920 × 822 | 1.33× | 99% | 1.19 MB | 17.0 s | 0.98 |
2560 × 1080.
A scale below 1 means the source is larger than the frame and is reduced; above 1 it is enlarged, which adds no detail. For 16:9, 9:16, 1:1, 4:5, 4:3, 21:9 sources the picture is enlarged to fill the frame.
How the crop works
The tool runs one ffmpeg filter chain: scale=2560:1080:force_original_aspect_ratio=increase, which scales the picture until it covers the frame on both sides, then crop=2560:1080, which keeps the middle. FFmpeg's documentation describes force_original_aspect_ratio=increase as scaling up (or down) so the output is at least the requested size while keeping the source's shape. The audio is encoded as AAC and the picture as H.264.
| Source shape | What is cut |
|---|---|
| 16:9 (1920 × 1080) | The top and bottom: the picture is 25% shorter after the crop |
| 9:16 (1080 × 1920) | The top and bottom: the picture is 76% shorter after the crop |
| 1:1 (1080 × 1080) | The top and bottom: the picture is 58% shorter after the crop |
| 4:5 (864 × 1080) | The top and bottom: the picture is 66% shorter after the crop |
| 4:3 (1440 × 1080) | The top and bottom: the picture is 44% shorter after the crop |
| 21:9 (1920 × 822) | Nothing: the shape already matches |
Crop or add bars?
Cropping fills the frame but loses the edges: from a 16:9 source, 21:9 keeps 75% of the picture. Bars keep everything but leave empty space. If the subject sits in the middle, crop; if the whole frame matters (a screen recording, a wide shot), add bars or use the precise crop tool to choose which part is kept.
Free tools:Crop a video by exact pixelsCompress a videoTrim a video by timestampConvert an image to 21:9Convert video to 9:16Convert video to 16:9Convert video to 1:1
Where 21:9 is used
Ultrawide cinematic - used for trailers and letterboxed film looks.
None of the platforms in the registry names 21:9 as its preferred shape, so check the destination's own spec before you export.
Tips
- Trim before you convert: the time grows with the length of the video.
- Keep faces and text away from the edges of the source, since the crop cuts the sides or the top and bottom.
- Do not enlarge: a source smaller than the frame is stretched to fill it and looks soft. Start from the largest file you have.
- Platforms re-encode what you upload, so keep the original and use the result for the upload only.
Questions people ask
How do I convert a video to 21:9?
Choose the video above and press the button. The tool crops it to 2560 × 1080 and gives you an MP4.
Will my video be stretched?
No. The picture is scaled to cover the frame in proportion and the excess is cut from the center.
Why is part of my video missing?
A different shape cannot fill the frame without cutting something. A 16:9 clip loses 25% of its area in 21:9.
How long does it take?
A 3 second 1080p clip took 17 to 27 seconds in our tests. Time grows with the length, so trim first.
Is my video uploaded?
No. The conversion runs in your browser.
What is the file size limit?
Up to 100 MB per file.
Can I choose which part is kept?
Not here: this tool always keeps the center. Use the precise crop tool to set the box and offset.
What format does it write?
An MP4 with H.264 video and AAC audio, which every platform documents first.
Related
Sources and how we checked
- FFmpeg Filters Documentation: scale (force_original_aspect_ratio) and crop (read 2026-10-09)
- Blender Foundation: Big Buck Bunny (CC BY 3.0), the clip used for the video measurements (read 2026-10-09)
Reviewed October 9, 2026. The measurements ran the page's own ffmpeg arguments on the same single-thread WebAssembly engine your browser downloads. The script and the results are in data/seo/hub/evidence/2026-10-09/aspect.