FFmpeg Compress Video ? CRF, Bitrate, Preset Guide

Video file too large to email? Uploads taking forever? FFmpeg’s CRF mode can reduce file size by 50–80% while preserving quality — all from a single command. This guide covers the right settings for every use case, with concrete benchmarks and tested examples.

Tested with: FFmpeg 6.1 (ubuntu-latest / GitHub Actions CI-validated)

What You Will Learn

1. CRF mode (constant quality) vs bitrate mode — when to use each
2. H.264 vs H.265 — which codec to pick
3. CRF value and preset comparison tables (quality, size, speed)
4. Combining resolution downscaling for maximum size reduction
5. Audio stream optimization
6. 2-pass encoding for precise file-size control
7. Five common errors and fixes
8. Five frequently asked questions

Command Examples

1. Universal: CRF Mode with H.264 (Most Versatile)

ffmpeg -i input.mp4 -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 128k output.mp4

• -c:v libx264 — H.264 codec
• -crf 23 — quality factor (0–51; lower = better; default 23)
• -preset medium — balanced encoding speed and compression
• -c:a aac -b:a 128k — re-encode audio to AAC at 128 kbps

2. Higher Compression with H.265 (HEVC)

Produces files ~40–50% smaller than H.264 at equivalent quality. Encoding takes longer.

ffmpeg -i input.mp4 -c:v libx265 -crf 28 -preset medium -c:a aac -b:a 128k output.mp4

3. Small-Size Priority (SNS / Email Attachment)

ffmpeg -i input.mp4 -c:v libx264 -crf 32 -preset fast -vf scale=1280:-2 -c:a aac -b:a 96k output.mp4

• -crf 32 — trades quality for a smaller file
• -vf scale=1280:-2 — downscale to 1280px wide (height auto-rounded to even)
• -b:a 96k — lower audio bitrate too

4. High-Quality Archive (Time Not a Constraint)

ffmpeg -i input.mp4 -c:v libx265 -crf 22 -preset slow -c:a aac -b:a 192k output.mp4

5. Bitrate-Controlled Compression (Strict Size Target)

Use when a streaming platform imposes a bitrate cap, or you need predictable file sizes.

# Video 2 Mbps + audio 128 kbps ≈ 2.1 Mbps total
ffmpeg -i input.mp4 -c:v libx264 -b:v 2000k -c:a aac -b:a 128k output.mp4

6. 2-Pass Encoding (Most Accurate Size Control)

# Pass 1: analyze video (no output generated)
ffmpeg -i input.mp4 -c:v libx264 -b:v 2000k -pass 1 -an -f null /dev/null

# Pass 2: generate the actual output
ffmpeg -i input.mp4 -c:v libx264 -b:v 2000k -pass 2 -c:a aac -b:a 128k output.mp4

The ffmpeg2pass-0.log file created by pass 1 can be deleted after pass 2 completes.

CRF Value Reference

Note: These estimates vary heavily by content type (action video, animation, talking-head, etc.). Always verify with your actual footage.

Preset Comparison

Same CRF value, different preset — size and speed change but perceived quality stays nearly the same.

Practical recommendation: Use medium by default; switch to slow when encoding time is not a concern.

H.264 vs H.265: Which to Choose?

Combining Resolution Downscaling

Downscaling resolution multiplies the size reduction from codec compression.

# 4K → 1080p + H.265
ffmpeg -i input_4k.mp4 -c:v libx265 -crf 26 -preset medium \
  -vf scale=1920:-2 -c:a aac -b:a 128k output_1080p.mp4

# 1080p → 720p + H.264 (social media)
ffmpeg -i input_1080p.mp4 -c:v libx264 -crf 26 -preset fast \
  -vf scale=1280:-2 -c:a aac -b:a 96k output_720p.mp4

-vf scale=1280:-2: the -2 rounds the height to the nearest even number. Use -2 instead of -1 to avoid codec errors.

Audio Optimization

Audio alone can contribute significantly to file size.

# Drop audio entirely (video only)
ffmpeg -i input.mp4 -c:v libx264 -crf 23 -an output.mp4

# Convert to mono (effective for voice-only content)
ffmpeg -i input.mp4 -c:v libx264 -crf 23 -c:a aac -ac 1 -b:a 64k output.mp4

Audio size reference: 128k stereo ≈ 5.6 MB/min | 96k ≈ 4.2 MB/min | 64k mono ≈ 2.8 MB/min

Option Reference

Troubleshooting

Problem 1: Encoder libx265 not found

Cause: Your FFmpeg build lacks H.265 support.
Fix: Check with ffmpeg -codecs | grep hevc. If E is missing, reinstall the full FFmpeg binary:

brew install ffmpeg          # macOS
sudo apt install ffmpeg      # Ubuntu/Debian

Problem 2: Encoding Is Extremely Slow

Cause: -preset veryslow/slow on high-resolution video, or H.265 on older hardware.
Fix: Switch to medium or fast. For H.265, the slowness is inherent — consider H.264 if time matters.

Problem 3: Output File Is Larger Than Expected

Cause: CRF too low, or preset ultrafast/veryfast with poor compression efficiency.
Fix: Raise CRF by 2–4 and/or switch to a slower preset:

ffmpeg -i input.mp4 -c:v libx264 -crf 26 -preset slow -c:a aac -b:a 128k output.mp4

Problem 4: width not divisible by 2

Cause: The scale filter produced an output dimension that’s an odd number.
Fix: Use -vf scale=1280:-2 (the -2 ensures the height is rounded to an even number).

Problem 5: Pass 1 Finished But No Output File

Cause: This is expected behavior. Pass 1 uses -f null /dev/null intentionally — it only generates the stats log file, not a video.
Fix: Run the pass 2 command to produce the actual output.

FAQ

Q1. Should I use CRF mode or bitrate mode?
A. CRF mode for almost everything. It maintains consistent quality across varying scene complexity. Use bitrate mode only when you need strict file-size control (e.g., platform bitrate caps).

Q2. Is CRF 23 the best setting for any video?
A. No — CRF 23 is just the encoder default. Action-heavy video may need CRF 20; animation with flat colors can go down to CRF 18–22. Always review the actual output.

Q3. Should I copy or re-encode the audio?
A. Use -c:a copy to avoid any quality loss. Use -c:a aac -b:a 128k if you also need to reduce audio size. Re-encoding always introduces a small quality reduction.

Q4. Can I use GPU encoding on macOS?
A. Yes — VideoToolbox is available on Apple Silicon and Intel Macs:

ffmpeg -i input.mp4 -c:v h264_videotoolbox -b:v 4000k -c:a aac output.mp4

CRF mode is not available; specify bitrate with -b:v instead.

Q5. How do I compress all videos in a folder at once?

for f in *.mp4; do
  ffmpeg -i "$f" -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 128k "compressed_${f}" -y
done

Related Articles

• Trimming Video — -ss/-to/-t Complete Guide
• Two-Pass Encoding — Precise Bitrate and File Size Control
• Convert Video to MP4 — Complete Guide
• Batch Convert with Shell Scripts

Tested with: ffmpeg 6.1.1 / Ubuntu 24.04 (GitHub Actions runner)
Primary sources: ffmpeg.org/ffmpeg.html / ffmpeg.org/ffmpeg-codecs.html / ffmpeg.org/ffmpeg-filters.html