FFmpeg Video Resize Explained: How to Change Video Dimensions Without Losing Quality

FFmpeg has long been recognized as one of the most powerful tools for audio and video processing. Whether you’re a casual creator looking to optimize your video size or a professional editor managing terabytes of content, FFmpeg offers a versatile set of features to manipulate video files. One particularly useful feature is the ability to resize videos — changing video dimensions while retaining as much quality as possible. But how can this be achieved effectively? Let’s dig into the art and science of video resizing using FFmpeg.

Why Resize Videos?

There are several reasons why you might want to resize your videos:

• Platform Optimization: Different social media platforms and devices have varying resolution requirements.
• File Size Reduction: Lower resolution generally results in smaller file sizes, saving storage and bandwidth.
• Performance Boost: Smaller videos load faster and consume less CPU/GPU during playback.
• Aspect Ratio Fixes: Sometimes you need to adjust dimensions to fix aspect ratio issues.

With that said, resizing incorrectly can compromise video quality. That’s where FFmpeg’s robust resizing options come in.

Understanding Video Dimensions and Aspect Ratio

Before diving into commands, it’s vital to understand what changing dimensions actually means. Video dimensions refer to its width and height in pixels — for example, a Full HD video is 1920×1080. When you resize a video, you’re altering these measurements.

The aspect ratio is the proportional relationship between the width and height. Keeping the aspect ratio consistent when resizing helps avoid stretched or squashed images. Common aspect ratios include 16:9 (widescreen), 4:3 (standard), and 1:1 (square).

How FFmpeg Handles Resizing

FFmpeg allows you to resize videos using the -vf or -filter:v flag with the scale filter. The basic syntax is as follows:

ffmpeg -i input.mp4 -vf scale=WIDTH:HEIGHT output.mp4

For example, if you want to resize a 1920×1080 video down to 1280×720, you’d use:

ffmpeg -i input.mp4 -vf scale=1280:720 output.mp4

Maintaining Aspect Ratio Automatically

If you’re unsure about the target height or width, and want to maintain aspect ratio, FFmpeg lets you set one dimension to -1 to auto-calculate the value:

ffmpeg -i input.mp4 -vf scale=1280:-1 output.mp4

This command sets the width to 1280 while automatically adjusting the height to maintain the original aspect ratio.

Best Practices to Avoid Quality Loss

One of the top concerns when resizing videos is maintaining quality. Here are several tricks to help ensure your resized video looks as crisp as possible:

1. Use High-Quality Scaling Algorithms

FFmpeg supports several scaling algorithms via the -sws_flags option. The most commonly used include:

• bilinear: Fast but less precise. Suitable for real-time streaming.
• bicubic: Better quality; ideal for moderate resizing needs.
• lanczos: High-quality option, best for maximum preservation of details.

Example with the lanczos algorithm:

ffmpeg -i input.mp4 -vf scale=1280:720 -sws_flags lanczos output.mp4

2. Work with High Bitrate Preservation

Bitrate plays a significant role in determining video quality. Always specify a bitrate that aligns with the new video size and maintains good clarity:

ffmpeg -i input.mp4 -vf scale=1280:720 -b:v 5M output.mp4

A higher bitrate like 5M (megabits per second) is more likely to preserve visual fidelity, especially in motion-heavy videos.

3. Choose the Right Encoding Codec

Encoders impact output quality. libx264 is a popular and efficient choice and supports numerous tuning options:

ffmpeg -i input.mp4 -vf scale=1280:720 -c:v libx264 -preset slow -crf 18 output.mp4

• -preset slow: Gives better compression (smaller files, better quality).
• -crf 18: Adjusts video quality; lower values mean better quality (and larger file sizes).

For near-lossless quality, aim for crf values between 17 and 23.

Advanced Resizing Techniques

Scaling to Fixed Width or Height

You might want videos to conform to a certain width while keeping the aspect ratio intact. Here’s a command to set a fixed width (e.g., 640px):

ffmpeg -i input.mp4 -vf "scale=640:-2" output.mp4

Why -2 instead of -1? FFmpeg requires dimensions to be divisible by 2 for many formats. -2 ensures this by rounding to the nearest even number.

Padding Instead of Resizing

If preserving dimensions is non-negotiable (say you need exactly 1920×1080), but your source video is smaller, use padding instead of stretching:

ffmpeg -i input.mp4 -vf "scale=iw*min(1920/iw\,1080/ih):ih*min(1920/iw\,1080/ih),pad=1920:1080:(1920-iw*min(1920/iw\,1080/ih))/2:(1080-ih*min(1920/iw\,1080/ih))/2" output.mp4

This command scales the video to fit within 1920×1080 and applies black padding (letterboxing) to fill the extra space.

Batch Resizing Multiple Files

Got a folder full of videos? FFmpeg works well with shell loops:

for f in *.mp4; do
  ffmpeg -i "$f" -vf scale=1280:720 "resized_$f"
done

This script resizes each video in the directory to 1280×720 while keeping the original file untouched.

Common Mistakes to Avoid

• Stretching Videos: Altering aspect ratio without compensation results in distorted visuals.
• Over-compression: Reducing resolution and bitrate aggressively causes blurry or pixelated output.
• Not Checking Output: Always preview your output before publishing or sharing.

Using the right parameters and understanding input/output requirements can save you from time-wasting mistakes.

Conclusion

Whether you’re resizing videos for mobile platforms, websites, or archiving purposes, FFmpeg offers clear and flexible tools to get the job done. By understanding the intricacies of scaling filters, aspect ratios, bitrates, and encoding options, you can produce resized videos that look as good as their original versions — sometimes even better.

Take your time to experiment with different settings, and remember: the best resized video is one that meets its purpose without compromising quality. With the right FFmpeg commands in your toolkit, you’ll be resizing like a pro in no time.