ffmpeg usage example: How to reduce a 1 hour 50 minute 40GB video to 5GB while preserving video and audio quality?

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Introduction to ffmpeg

Calculating Bitrate

Assuming your video duration is 1 hour 50 minutes (i.e., 110 minutes = 6600 seconds), and the target file size is 5GB (5GB = 5000MB, actual 1GB = 1024MB)

Target Bitrate (kbps) = (File Size (MB) × 8192) ÷ Duration (seconds) = (5120 × 8192) ÷ 6600 ≈ 6355 kbps (For convenience, we’ll just use 6000k later)

First Encoding Pass

  • First pass: ffmpeg analyzes the entire video content, records the complexity and motion information of each frame, but does not generate the final file.
ffmpeg -y -i input.mp4 -c:v libx264 -b:v 6000k -preset slow -pass 1 -an -f mp4 /dev/null
  • -y: automatically overwrite existing files
  • -i input.mp4: input file
  • -c:v libx264: use H.264 encoder
  • -b:v 6000k: set video bitrate to 6000 kbps
  • -preset slow: slow encoding speed, high compression, better quality
  • -pass 1: first pass encoding (only analysis, no final video generated)
  • -an: no audio processing
  • -f mp4 /dev/null: output format mp4 but discard output (/dev/null)

Second Encoding Pass

  • Second pass: uses data collected in the first pass; ffmpeg can allocate bitrate more reasonably—more bitrate is assigned to complex scenes, less to simple scenes, thus maximizing overall quality within the file size limit.
ffmpeg -i input.mp4 -c:v libx264 -b:v 6000k -preset slow -pass 2 -c:a aac -b:a 192k output.mp4
  • -pass 2: second pass encoding (uses first pass analysis data to optimize bitrate allocation)
  • -c:a aac: audio encoding to AAC
  • -b:a 192k: audio bitrate 192 kbps
  • output.mp4: output filename

Adjusting Multi-Core Parallelism

By default, ffmpeg calls the maximum number of cores available
image

You can modify the number of cores with -threads
For example, for the first pass encoding, if we don’t want to fully utilize all 10 cores

ffmpeg -y -i input.mp4 -c:v libx264 -b:v 6000k -preset slow -pass 1 -an -f mp4 /dev/null

Change it to:

ffmpeg -y -i input.mp4 -c:v libx264 -b:v 6000k -thread 8 -preset slow -pass 1 -an -f mp4 /dev/null

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