How to Configure CRF in FFmpeg libx265

This article explains how to configure the Constant Rate Factor (CRF) value for the libx265 (H.265/HEVC) encoder in FFmpeg. It covers the exact command-line syntax, the default CRF settings, the recommended range for balancing quality and file size, and practical examples to help you optimize your video encoding workflow.

The CRF Parameter for libx265

Constant Rate Factor (CRF) is the default quality-setting mode for the libx265 encoder. It scales the file size and bitrate to maintain a visually consistent level of quality throughout the entire video.

To set the CRF value in FFmpeg, use the -crf option followed by your chosen numerical value.

ffmpeg -i input.mp4 -c:v libx265 -crf 23 output.mp4

Understanding CRF Values for H.265

The CRF scale for libx265 ranges from 0 to 51: * 0: Lossless compression (very large file sizes). * 51: Worst possible quality.

Unlike the H.264 (libx264) encoder, which has a default CRF of 23, the default CRF for the H.265 (libx265) encoder is 28. Because H.265 is significantly more efficient than H.264, a higher CRF value in H.265 yields a subjectively similar visual quality to a lower CRF value in H.264, but at a much lower bitrate.

Recommended Range

For most general encoding tasks, the recommended CRF range is 20 to 28: * CRF 18–21: High quality, visually near-lossless. Best for archiving or high-end streaming. * CRF 22–25: Excellent balance between visual quality and file size. * CRF 26–28: Good quality with highly optimized, smaller file sizes. Ideal for web distribution.

Every decrease of 6 units in the CRF value roughly doubles the bitrate, while an increase of 6 units halves it.

Combining CRF with Presets

When configuring the CRF in libx265, you should always pair it with the -preset option. The preset determines the encoding speed-to-compression ratio.

The available presets are: ultrafast, superfast, veryfast, faster, fast, medium (default), slow, slower, and veryslow.

Using a slower preset allows the encoder to use more advanced compression algorithms, resulting in a smaller file size for the exact same CRF quality level.

Example: High-Quality Slow Encode

This command uses a CRF of 22 and the slow preset to achieve high-quality output with an optimized file size:

ffmpeg -i input.mov -c:v libx265 -crf 22 -preset slow -c:a copy output.mp4