Remove Black Bars from a Video
Detects letterbox and pillarbox bars automatically and crops the video to the real picture — right in your browser, no upload.
About this tool
Black bars appear when a video's aspect ratio doesn't match the frame it was packaged in: letterboxing (bars above and below, e.g. a widescreen film inside a 4:3 frame) and pillarboxing (bars left and right, e.g. a phone clip inside a 16:9 frame). They waste pixels and bitrate, look bad on re-uploads, and defeat auto-cropping thumbnails.
This tool removes them automatically, in two passes — the same technique video engineers use with ffmpeg by hand:
- Detect — ffmpeg's
cropdetectfilter scans every frame of the clip and keeps the union of all picture content it sees. Because the box only ever grows, a fade-from-black opening or a dark scene can't trick it into cropping real content. - Crop — the detected rectangle is applied with
crop=W:H:X:Yand the video is re-encoded with H.264 at CRF 18 (visually lossless tier). Audio is stream-copied untouched when the container allows it.
Everything runs in your browser via WebAssembly ffmpeg — the file is never uploaded.
Worked example
A 320×240 clip that's really a 320×180 widescreen picture with 30-pixel black bars top and bottom (classic 16:9-inside-4:3 letterbox):
- the detect pass settles on
crop=320:180:0:30— 320×180 of real picture, starting 30 px down; - the crop pass re-encodes just that rectangle;
- the result is a 320×180 video with the bars gone. If the clip had no bars at all, the tool says so instead of silently re-encoding it.
Leave the defaults (threshold 24, snap 2) for normal black bars. If your source was heavily compressed and the "black" is really dark grey, raise the threshold to around 48. Pick snap 16 if a downstream encoder or player prefers macroblock-aligned dimensions (the crop may keep up to 15 extra bar pixels per axis).
Limits and edge cases
- Input is capped at 25 MB in chat/CLI; in the browser the practical limit is your device's memory.
- The crop rectangle is one box for the whole clip — content that changes aspect ratio mid-video (e.g. a trailer switching between scope and flat) gets the union box, keeping everything visible.
- Threshold 0 is the strictest setting: typical "black" bars in limited-range video sit at luma 16, so threshold 0 usually reports no bars detected — that's the tool refusing to guess, not a bug.
- Threshold 255 makes every pixel count as black; the tool reports that the whole frame reads as black and asks you to lower the threshold.
- Cropping requires re-encoding (an ffmpeg stream copy cannot crop), so the video track is always re-encoded; CRF 18 keeps that visually transparent.
FAQ
How does it know where the bars are?
It runs ffmpeg's cropdetect filter over the whole clip. Each frame, the filter
measures the smallest rectangle that contains everything brighter than the black
threshold; with the running-maximum mode used here, those rectangles are merged into a
union box across all frames. The final box is the picture; everything outside it is
bars. The crop values are read from the detection log, then applied in a second pass.
Why did it say "no black bars detected"?
The detected picture box covered the entire frame. Either your video genuinely has no bars, or the bars aren't dark enough to count as black at the current threshold — heavily compressed sources often have noisy, dark-grey bars. Raise the black threshold (try 48, then 64) and run again. The tool prefers telling you over re-encoding a video it wouldn't change.
Does it lower my video quality?
The crop itself is pixel-exact, but the video track must be re-encoded (ffmpeg filters can't run in stream-copy mode). It re-encodes with H.264 at CRF 18, which is generally considered visually lossless. Audio is copied through untouched when the input container is MP4, MOV, M4V or MKV; other containers (e.g. WebM) are rewritten to MP4 with AAC audio.
Can it remove white or colored borders too?
No — detection is luma-based black detection (that's what cropdetect measures), which
covers the letterbox/pillarbox bars that video players and encoders add. For colored
borders you know the size of, use a manual crop tool and give it the exact rectangle.
What do the two settings actually do?
Black threshold (0–255) is how dark a pixel must be to count as bar: 24 (the ffmpeg default) matches true black bars; higher values catch grey-ish bars but risk eating into dark scenes. Snap crop to multiple of rounds the cropped width/height down to a multiple of 2, 4, 8 or 16: 2 removes the most bar while keeping dimensions H.264 legal, 16 yields the most encoder-friendly dimensions at the cost of possibly keeping a sliver of bar.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool video-autocrop-bars 'url=https://example.com/input' 'threshold=24' 'round=2'New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/video-autocrop-bars/?url=https://example.com/input&threshold=24&round=2Machine-readable descriptor: tool.json — title + parameters JSON Schema for agents.