FFmpeg Scale Filter: Resize Video and Keep Aspect Ratio

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

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:

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

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.

It has two modes:

decrease shrinks the video to fit inside the target dimensions. The video will be smaller than 1920x1080 on one axis.

ffmpeg -i input.mp4 -vf "scale=1920:1080:force_original_aspect_ratio=decrease" output.mp4

A 4:3 input (like 1440x1080) comes out as 1440x1080, fitting inside the 1920x1080 box.

increase enlarges the video to fill the target dimensions. The video will overflow on one axis.

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

A 4:3 input comes out as 1920x1440, overflowing the height.

Letterboxing with pad

After using force_original_aspect_ratio=decrease, your video fits inside the target box but doesn't fill it. 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

This scales the video to fit, then centers it on a 1920x1080 black canvas. The (ow-iw)/2 and (oh-ih)/2 expressions center the video horizontally and vertically.

Want a white background instead? Change :black to :white. Any hex color works too: :0x1a1a2e for a dark blue.

Common Resolution Targets

Quick reference for the most-used output resolutions:

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. FFmpeg Micro accepts the same -vf scale filters as the CLI.

Simple preset (set resolution directly):

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",
    "preset": {
      "quality": "high",
      "resolution": "1080p"
    }
  }'

Note: The preset resolution maps to fixed dimensions (1920x1080) and uses -s, which can distort non-16:9 inputs.

Aspect-ratio-safe (use the scale filter):

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"}
    ]
  }'

This is the API equivalent of ffmpeg -i input.mp4 -vf "scale=-2:1080" output.mp4. Same scale filter, same aspect ratio preservation. No FFmpeg installation needed.

Common Pitfalls

Odd dimensions crash H.264 encoding. Use -2 instead of -1. Or add trunc(oh*a/2)*2 arithmetic, but -2 is simpler.

Chaining filters requires commas, not pipes. Write scale=-2:1080,pad=1920:1080 with a comma, not scale=-2:1080 | pad=1920:1080.

Upscaling makes things blurry. Scaling a 480p video to 1080p won't add detail. It just makes the same pixels bigger. If you must upscale, use lanczos: scale=-2:1080:flags=lanczos.

Preset resolution doesn't preserve aspect ratio. Both FFmpeg's -s flag and API resolution presets force exact dimensions. Use the scale filter with -2 for safe resizing.

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. Use scale for aspect-ratio-safe resizing.

How do I scale a video to a specific width in FFmpeg?
Use scale=WIDTH:-2. For example, ffmpeg -i input.mp4 -vf "scale=1280:-2" output.mp4 sets the width to 1280 and auto-calculates the height.

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. No server setup required.

*Last verified: June 2026. All code examples tested against FFmpeg 7.x and the FFmpeg Micro API.*