How to add a blurred
background to a video.
The technique that turns a widescreen video into a vertical one without cutting anything out — what it is, why it works, and exactly how to do it.
Two copies of the same video, layered
A blurred background fill is simpler than it looks. You take the same source video twice:
- The background layer is scaled up until it covers the whole vertical canvas, cropped to fit, and blurred heavily.
- The foreground layer is the original, untouched, scaled to full width and centred on top.
The result fills a phone screen edge to edge while keeping every pixel of the original frame visible. The blurred layer reads as ambient colour rather than content, so your eye goes straight to the sharp centre.
Doing it with ffmpeg
Install ffmpeg
Available on macOS via Homebrew, on Windows via winget or the official builds, and in every Linux package manager. It's free and command-line based.
Build the background layer
Scale the source so it covers 1080×1920, crop the overflow, then blur it. Add setsar=1 so pixel aspect ratios can't distort the overlay.
Build the foreground layer
Scale the original to 1080 wide using scale=1080:-2. The -2 computes height automatically and keeps it even, which H.264 requires.
Overlay and export
Centre the foreground over the blurred background and export as H.264 with format=yuv420p.
Copy this
ffmpeg -y -i input.mp4 -filter_complex "\
[0:v]scale=1080:1920:force_original_aspect_ratio=increase,\
crop=1080:1920,boxblur=40:4,setsar=1[bg];\
[0:v]scale=1080:-2[fg];\
[bg][fg]overlay=(W-w)/2:(H-h)/2:format=auto,format=yuv420p[v]" \
-map "[v]" -map 0:a? -c:v libx264 -preset veryfast -crf 20 \
-c:a copy -movflags +faststart output.mp4Swap input.mp4 for your file. Everything else can stay as-is — these values are tuned for social platforms.
When it doesn't work
| Symptom | Cause | Fix |
|---|---|---|
| Won't upload to TikTok/Reels | Wrong pixel format | Add format=yuv420p at the end of the filter chain |
| Video looks stretched | Hardcoded output height | Use scale=1080:-2, never a fixed height |
| Overlay fails or distorts | Mismatched sample aspect ratio | Add setsar=1 to the background layer |
| No audio in output | Source had no audio stream | -map 0:a? — the ? makes audio optional |
| Background too sharp | Blur radius too low | Raise boxblur radius toward 40–50 |
Common questions
Do I need paid software for this?
No. ffmpeg is free and open source, and it's the tool doing the actual work in most paid apps that offer this feature.
What blur strength should I use?
A radius of 30–50 works well. Below 30 the background stays legible and competes with the foreground; above 50 it flattens into featureless mush.
Will this reduce my video quality?
The foreground is re-encoded once at CRF 20, which is visually near-lossless. Audio is copied through without re-encoding at all.
Can I do this on my phone?
Some mobile editors offer a blurred-fill preset, but quality and control vary. ffmpeg on a computer gives you exact, repeatable output.