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Letterbox, blur-fill, or crop · No watermark · No upload

Change Aspect Ratio

Reframe a video for TikTok / Reels (9:16), Instagram square (1:1), Instagram portrait (4:5), or YouTube (16:9) — letterbox fit preserves all pixels.

100% freeNo file size limitNo watermarkNo sign-up
  1. 1Pick file
  2. 2Configure
  3. 3Download
Three fill modes: letterbox keeps every pixel with bars, blur-fill puts a blurred copy behind the video (the Reels / TikTok look), and crop zooms to fill the frame, trimming the edges.
  • Files never leave your browser — processed entirely on your device
  • No upload, no queue, no waiting for a worker to free up
  • No file-size cap from us — limit is your device's RAM

About Change Aspect Ratio

AntiUpload's Change Aspect Ratio tool reframes a video for a different platform aspect — converting a horizontal YouTube clip to a vertical TikTok / Reels / Shorts video, a square Instagram post, an Instagram portrait 4:5, or a YouTube widescreen 16:9. The most common conversion is horizontal-to-vertical: phone videos shot in landscape orientation that need to fit the dominant social-media format (TikTok / Reels are vertical 9:16 by spec). The reverse (vertical-to-horizontal for YouTube long-form, desktop wallpaper, etc.) is also supported. Output dimensions are platform-canonical: 1080×1920 for 9:16, 1080×1080 for 1:1, 1080×1350 for 4:5, 1920×1080 for 16:9.

V1 ships letterbox fit only: the entire source frame is scaled to fit inside the target dimensions and the gap is filled with black bars on the empty axis. This preserves every pixel of the original — no content gets cropped — at the cost of visible bars (top/bottom for horizontal → vertical, left/right for vertical → horizontal). It's the lowest-risk default because the subject never gets cropped surprisingly. The trade-off is the bars: TikTok / Reels users sometimes prefer crop or blur-fill (TikTok-style blurred copy of the video behind the centred crop) for an edge-to-edge look. Those modes ship in follow-up PRs.

Behind the scenes: FFmpeg's `scale=W:H:force_original_aspect_ratio=decrease,pad=W:H:(ow-iw)/2:(oh-ih)/2:black` filter chain handles both the resize and the black-bar fill in one pass. Re-encoding via libx264 CRF 23 with `yuv420p` pixel format and `+faststart` muxer flag produces universally-playable MP4 output. The whole encode runs in your browser — no upload, no size limit, no watermark.

How it works

  1. Drop your videoAccepts MP4, MOV, WebM, MKV, AVI, M4V, FLV. Output is always MP4 because aspect-ratio change requires re-encoding the picture.
  2. Pick the target aspect ratio9:16 vertical for TikTok / Reels / Shorts (1080×1920). 1:1 square for Instagram square posts / Facebook ads (1080×1080). 4:5 portrait for Instagram's preferred non-Reel post format (1080×1350). 16:9 widescreen for YouTube / desktop (1920×1080).
  3. Click ReframeRe-encoding runs in WASM via libx264 — typically real-time-ish for short clips. Progress shows live frame= updates.
  4. Download the reframed fileOutput dimensions match the target ratio at canonical-size for that platform. Letterbox bars fill empty space; all source pixels are preserved.

When to use Change Aspect Ratio

Repurposing horizontal YouTube content for TikTok / Reels
Most content creators shoot landscape (16:9) for YouTube but social-media short-form is vertical 9:16. Letterbox the YouTube content into 9:16 frame for TikTok / Reels upload. Subject stays centred; black bars top and bottom.
Converting vertical phone video to YouTube-friendly 16:9
Phone clips shot vertically don't fill YouTube's horizontal player — they show with side bars. Reframe to 16:9 with our tool, get a video with the phone clip centred and grey side bars baked in.
Square posts for Instagram feed (1:1)
Square video is Instagram's legacy feed format and still gets reach. Reframe to 1:1 (1080×1080) for square posts that fill the visible area on mobile feeds without cropping.
Portrait posts for Instagram (4:5)
Instagram's 4:5 portrait is the highest-engagement post format (takes the most feed real estate per scroll). Reframe widescreen content to 4:5 for maximum feed footprint.
Fixing footage shot in the wrong orientation
Sometimes you shoot with the phone in landscape when you meant portrait, or vice versa. Reframing to the intended aspect with letterbox is the no-loss-of-content fix (vs cropping which would lose pixels).

Frequently asked questions

How to convert horizontal video to vertical for TikTok / Reels?
Drop your video into AntiUpload Change Aspect Ratio, pick 9:16, click Reframe. Output is 1080×1920 — TikTok / Reels / YouTube Shorts native dimensions. Black bars fill the top and bottom of the frame; the original content is centred and uncropped.
Why are there black bars in my output? Can I remove them?
Letterbox fit (V1 default) preserves every pixel of the source by adding black bars to fill the target ratio. The alternative would be cropping — losing content from the edges of the source's longer axis. Crop mode and blur-fill mode (TikTok's signature look — blurred version of the video behind the centred crop) are on the roadmap. For now, letterbox is the lowest-risk default — no subject ever gets cropped surprisingly.
Best free aspect ratio changer online with no watermark?
AntiUpload Change Aspect Ratio — no watermark, no signup, no size cap, no upload. Compare to Veed (watermarks free tier), Kapwing (watermarks free), Adobe Express (paywall after trial). All upload to their servers; ours runs in your browser.
What's the difference between 4:5 and 9:16?
9:16 is the vertical aspect of TikTok / Reels / Shorts — taller (1080×1920). 4:5 is Instagram's preferred feed-post aspect — less extreme (1080×1350). 4:5 fits in the Instagram feed without scrolling; 9:16 is for full-screen vertical players. If you're posting to both Instagram feed AND Reels, you usually want two versions.
Does this preserve audio?
Yes. Audio is re-encoded as AAC 128k alongside the video re-encode. Source audio quality is preserved up to AAC's quality ceiling at 128 kbps (which is transparent for typical content).
Can I crop instead of letterbox?
Not in V1 — letterbox only for now. Crop mode is on the roadmap; it'll let you pick a horizontal/vertical crop position (subject centred / left-biased / right-biased) so the subject stays in frame after cropping to the target aspect. For now, if you need crop output, use a desktop editor like DaVinci Resolve (free) for that one step.
How long does aspect-ratio conversion take?
Proportional to source length. A 1-minute 1080p source typically reframes in 1-3 minutes on a modern laptop. Re-encoding through libx264 in WebAssembly with `--disable-asm` is slower than native FFmpeg by 5-20×. WebCodecs hardware path ships in a follow-up PR.

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