FFmpeg指定编码重编码省略码率参数的行为及默认码率来源咨询

Great question—let’s break down exactly what’s happening here, since this is a common point of confusion with FFmpeg’s default settings.

What’s going on when you skip the bitrate parameter? #

When you run the command ffmpeg.exe -i a.mp4 -c:v h264 c.mp4, FFmpeg defaults to using the libx264 encoder (the industry-standard open-source H.264 encoder) for H.264 video. Instead of locking into a fixed bitrate, libx264 uses a Constant Rate Factor (CRF) mode by default—this mode prioritizes keeping a consistent subjective quality level, rather than hitting a specific bitrate target.

Where does that 4467 kb/s bitrate come from? #

That output bitrate isn’t a universal standard value—it’s determined by three key factors:

• Default CRF value: libx264 uses a default CRF of 23, a setting calibrated to balance visual quality and file size (often called "visually lossless" for most everyday use cases).
• Original video content: The encoder dynamically adjusts bitrate based on how complex your source video is. For example:
  • Fast-moving, high-detail scenes (like action footage) will need a higher bitrate to maintain the CRF 23 quality bar.
  • Static or low-detail scenes (like a talking head or slideshow) will use a much lower bitrate.
• Encoder efficiency: libx264 is far more efficient than many other encoders (including some hardware-based ones). Your original video’s 17282 kb/s bitrate might have come from a less efficient encoder, or it could have been over-provisioned with bits that didn’t add meaningful quality. libx264 cuts that excess while preserving similar visual fidelity.

Will this bitrate be the same for all videos? #

Nope—you’d get wildly different results if you ran the same command on a 4K action clip vs. a 720P static slideshow. CRF mode keeps quality consistent, but bitrate shifts entirely based on the source content’s complexity.

How to control bitrate explicitly if you need to? #

If you want a fixed bitrate instead of quality-based encoding, use the -b:v parameter. For example:

ffmpeg.exe -i a.mp4 -c:v h264 -b:v 5M c.mp4

This forces an average bitrate of 5 Mbps. Alternatively, you can tweak the CRF value to adjust the quality/file size tradeoff: lower values (e.g., 18-20) mean higher quality and larger files, while higher values (e.g., 25-28) mean smaller files with subtle quality loss.

内容的提问来源于stack exchange，提问作者Rafael Lima