Set Target Average Bitrate in FFmpeg libvpx-vp9

This guide provides a straightforward explanation of how to set a target average bitrate when encoding video with the VP9 codec (libvpx-vp9) using FFmpeg. It covers the essential command-line flags for two-pass encoding, constrained quality mode, and constant bitrate mode to help you achieve your desired file size and video quality.

To set a target average bitrate in libvpx-vp9, you must use the -b:v flag followed by your desired bitrate value (such as 2M for 2 Mbps). Because VP9 is highly optimized for two-pass encoding, you should always perform a two-pass encode when aiming for a specific target average bitrate.

Method 1: Two-Pass Constrained Quality (Recommended)

This is the recommended mode for most VP9 encodes. It allows the encoder to use a target average bitrate while maintaining a maximum quality threshold using the Constant Rate Factor (-crf).

Run the following two commands in sequence:

Pass 1:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -crf 30 -pass 1 -an -f null /dev/null

(Windows users should use NUL instead of /dev/null)

Pass 2:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -crf 30 -pass 2 -c:a libopus -b:a 128k output.webm

In this mode, -b:v 2M sets the target average bitrate to 2 Mbps, and -crf 30 acts as a quality ceiling, preventing the encoder from wasting bits on scenes that do not require them.

Method 2: Pure Two-Pass Average Bitrate (ABR)

If you want the encoder to target an average bitrate without capping the quality with a CRF value, omit the -crf flag entirely.

Pass 1:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -pass 1 -an -f null /dev/null

Pass 2:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -pass 2 -c:a libopus -b:a 128k output.webm

Method 3: Constant Bitrate (CBR)

For live streaming use cases where you need a strict, unchanging bitrate, set the target bitrate (-b:v), the minimum bitrate (-minrate), and the maximum bitrate (-maxrate) to the exact same value.

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -minrate 2M -maxrate 2M -quality realtime -speed 5 -row-mt 1 -c:a libopus output.webm

This forces the libvpx-vp9 encoder to output a stream conforming strictly to the 2 Mbps target.