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Javid Jamae
Javid Jamae

Posted on • Originally published at ffmpeg-micro.com

FFmpeg Scale Filter: Resize Video and Keep Aspect Ratio

#ffmpeg #video #api #tutorial

Originally published at ffmpeg-micro.com

You need to resize a video to 1080p, but the input is 4:3 and the output needs to be 16:9. You use -s 1920x1080 and get a stretched, distorted mess. The fix is FFmpeg's scale filter, and specifically, the -2 trick and force_original_aspect_ratio flag.

This guide covers every common scaling scenario: auto-calculation with -2, forced aspect ratios, letterboxing with pad, and how to do it all through an API instead of wrestling with CLI flags.

Quick Answer: Scale Without Distortion

The fastest way to resize a video while keeping its aspect ratio intact:

ffmpeg -i input.mp4 -vf "scale=-2:1080" output.mp4
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The -2 tells FFmpeg to calculate the width automatically based on the input's aspect ratio, rounding to the nearest even number (which video codecs require). Your 4:3 input becomes 1440x1080. Your 16:9 input becomes 1920x1080. No distortion.

What the -2 Value Actually Does

FFmpeg's scale filter takes width and height as arguments: scale=width:height. When you set either dimension to -1 or -2, FFmpeg calculates that dimension from the other one using the original aspect ratio.

The difference between -1 and -2:

  • -1 calculates the value, but it might produce an odd number (e.g., 1441)
  • -2 does the same calculation but rounds to the nearest even number

Odd pixel dimensions cause encoding failures with H.264 and H.265. Always use -2, not -1.

# Scale to 720p height, auto-calculate width
ffmpeg -i input.mp4 -vf "scale=-2:720" output.mp4

# Scale to 1280 width, auto-calculate height
ffmpeg -i input.mp4 -vf "scale=1280:-2" output.mp4

# Scale to 4K height
ffmpeg -i input.mp4 -vf "scale=-2:2160" output.mp4
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force_original_aspect_ratio: Fit or Fill

Sometimes you need the output to be exactly 1920x1080, no exceptions. But your input is 4:3. The force_original_aspect_ratio parameter handles this without stretching.

decrease shrinks the video to fit inside the target dimensions:

ffmpeg -i input.mp4 -vf "scale=1920:1080:force_original_aspect_ratio=decrease" output.mp4
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increase enlarges the video to fill the target dimensions:

ffmpeg -i input.mp4 -vf "scale=1920:1080:force_original_aspect_ratio=increase" output.mp4
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Letterboxing with pad

If you need exactly 1920x1080 with black bars (letterboxing), chain the pad filter:

ffmpeg -i input.mp4 -vf "scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2:black" output.mp4
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Common Resolution Targets

Target Command
480p (SD) scale=-2:480
720p (HD) scale=-2:720
1080p (Full HD) scale=-2:1080
2160p (4K) scale=-2:2160

Scale via the FFmpeg Micro API

If you don't want to install FFmpeg or manage a server, you can resize videos through an API call:

curl -X POST https://api.ffmpeg-micro.com/v1/transcodes \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "inputs": [{"url": "https://example.com/video.mp4"}],
    "outputFormat": "mp4",
    "options": [
      {"option": "-vf", "argument": "scale=-2:1080"}
    ]
  }'
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FFmpeg Micro is a cloud API that accepts the same FFmpeg filters as the CLI. No FFmpeg installation, no server management.

FAQ

Does ffmpeg scale preserve aspect ratio by default?
No. The default scale=width:height stretches the video to fit both dimensions. Use -2 for auto-calculation or force_original_aspect_ratio to prevent distortion.

What's the difference between -s and -vf scale in FFmpeg?
-s 1920x1080 sets exact output dimensions and can distort. -vf "scale=-2:1080" calculates one dimension from the other to keep the aspect ratio.

Can I resize video through an API without installing FFmpeg?
Yes. FFmpeg Micro is a cloud API that accepts the same FFmpeg filters. Send a POST to /v1/transcodes with options: [{"option": "-vf", "argument": "scale=-2:1080"}] and get back the resized video.

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