Two-Pass Encoding — Precise Bitrate and File Size Control

What You Will Learn

The difference between CRF mode and two-pass bitrate mode
How to run the two-pass pipeline: analysis pass then encoding pass
How to target a specific file size using bitrate math
The -passlogfile option for custom log file locations
H.264 and H.265 two-pass examples

Tested version: FFmpeg 6.1 (ubuntu-latest / CI-validated)
Target OS: Windows / macOS / Linux

CRF Mode vs. Two-Pass Bitrate Mode

FFmpeg’s libx264 and libx265 encoders offer two main quality/bitrate control modes:

Mode	Option	Use When
CRF (Constant Rate Factor)	`-crf 18–28`	You want consistent quality, file size varies
Two-pass bitrate	`-b:v N -pass 1/2`	You need to hit a specific file size or bitrate

CRF is simpler and often preferred for archiving or streaming. Two-pass is the right choice when you must deliver a file that fits within a specific size budget (e.g., email attachment limits, disc capacity).

How Two-Pass Works

Pass 1 (Analysis): FFmpeg encodes the video quickly, writing a log file that records the complexity of each frame. No output video is produced — only the log file matters.

Pass 2 (Encoding): FFmpeg reads the log file to understand which scenes need more bits and which need fewer, then encodes the final output with the bitrate distributed according to the analysis.

The log file is named ffmpeg2pass-0.log (and ffmpeg2pass-0.log.mbtree for H.264) in the working directory.

Basic Two-Pass H.264 Example

Pass 1 — Analysis only, no output:

ffmpeg -i input.mp4 -c:v libx264 -b:v 1M -pass 1 -an -f null /dev/null

-b:v 1M sets the target video bitrate to 1 Mbit/s
-pass 1 runs the analysis pass
-an suppresses audio (not needed in pass 1)
-f null /dev/null discards the output (only the log file matters)

Pass 2 — Actual encode with analysis data:

ffmpeg -i input.mp4 -c:v libx264 -b:v 1M -pass 2 -c:a aac -b:a 128k output.mp4

-pass 2 reads the log from pass 1 and encodes the final output
Audio is encoded in pass 2 only

Run both commands in the same directory — FFmpeg looks for ffmpeg2pass-0.log in the current working directory.

Targeting a Specific File Size

To calculate the bitrate for a target file size:

total_bitrate_kbps = (target_size_MB × 8192) ÷ duration_seconds
video_bitrate_kbps = total_bitrate_kbps − audio_bitrate_kbps

Example: Target 50 MB for a 4-minute (240-second) video with 128 kbps audio:

total = (50 × 8192) ÷ 240 ≈ 1707 kbps
video = 1707 − 128 = 1579 kbps ≈ 1.5M

Pass 1:

ffmpeg -i input.mp4 -c:v libx264 -b:v 1500k -pass 1 -an -f null /dev/null

Pass 2:

ffmpeg -i input.mp4 -c:v libx264 -b:v 1500k -pass 2 -c:a aac -b:a 128k output.mp4

Custom Log File Location

By default, FFmpeg writes the log to ./ffmpeg2pass-0.log. Use -passlogfile to specify a different path (useful in scripts or when encoding multiple files in parallel):

ffmpeg -i input.mp4 -c:v libx264 -b:v 1M -pass 1 -passlogfile /tmp/mylog -an -f null /dev/null

ffmpeg -i input.mp4 -c:v libx264 -b:v 1M -pass 2 -passlogfile /tmp/mylog -c:a aac output.mp4

Two-Pass H.265 (libx265)

Two-pass encoding with H.265 follows the same structure:

ffmpeg -i input.mp4 -c:v libx265 -b:v 800k -x265-params pass=1 -an -f null /dev/null

ffmpeg -i input.mp4 -c:v libx265 -b:v 800k -x265-params pass=2 -c:a aac output.mp4

Note: libx265 uses -x265-params pass=1 instead of -pass 1. The behavior is otherwise identical.

Adding Encoder Presets

You can combine two-pass with encoder presets. A slower preset gives the encoder more time to find efficient coding decisions:

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

ffmpeg -i input.mp4 -c:v libx264 -preset slow -b:v 1M -pass 2 -c:a aac -b:a 128k output.mp4

Use the same preset in both passes to ensure the log file and pass 2 analysis are consistent.

When to Use Two-Pass

Scenario	Recommendation
Archiving with consistent visual quality	Use CRF mode (`-crf 23`)
File must fit a specific size limit	Use two-pass bitrate mode
Streaming delivery with guaranteed bitrate	Two-pass (or constrained bitrate with `-maxrate`)
Quick transcodes or previews	CRF mode (single pass)

Common Pitfalls

Log File Not Found in Pass 2

If pass 2 runs in a different directory than pass 1, it cannot find ffmpeg2pass-0.log. Either run both in the same directory, or use -passlogfile /path/to/log in both passes.

Audio in Pass 1

Pass 1 with -an skips audio encoding, which is correct — the log only needs video complexity data. Including audio in pass 1 wastes time without benefit.

Same `-b:v` in Both Passes

Always use the same -b:v value in both passes. Using different values means the log from pass 1 targets a different bitrate than pass 2, producing unreliable results.

Tested with: ffmpeg 6.1.1 / Ubuntu 24.04 (GitHub Actions runner)
Primary sources: trac.ffmpeg.org/wiki/Encode/H.264 / ffmpeg.org/ffmpeg.html