What is CRF (Constant Rate Factor)?
Definition
CRF (Constant Rate Factor) is the main quality dial in the x264 (H.264) and x265 (H.265) encoders. It targets a constant perceptual quality instead of a fixed bitrate - a lower CRF means higher quality and a bigger file, while a higher CRF means a smaller file with more artifacts.
CRF lets the encoder spend bits where they matter. Instead of holding bitrate steady, it holds visual quality steady, so calm scenes use fewer bits and complex motion uses more. You set one number and the encoder picks the bitrate for each frame. This is why CRF usually gives a better size-to-quality result than a fixed-bitrate (ABR) encode for offline files you encode once and keep.
The CRF scale runs from 0 to 51 in both x264 and x265, and lower means higher quality: 0 is lossless, while everyday values sit higher. Around 18-23 is visually near-lossless for most footage, and 24-28 gives a strong balance of small size and good quality. The default in x264 is 23 (x265 defaults to 28). As a rule of thumb, changing CRF by about +6 roughly halves the file size and -6 roughly doubles it.
A common confusion is that CRF is a fixed bitrate or a percentage - it is neither. The same CRF can produce very different file sizes depending on how hard the content is to compress. CRF values are also not comparable across codecs: CRF 23 in x265 (H.265) looks better and produces a smaller file than CRF 23 in x264 (H.264), because the two scales are tuned differently.
Quick facts
- CRF stands for Constant Rate Factor
- Used by the x264 (H.264) and x265 (H.265) encoders
- Scale is 0-51: lower = better quality and bigger file
- 0 is lossless; x264 defaults to CRF 23 and x265 to CRF 28; ~18-23 is near-lossless and ~24-28 balances size and quality
- Targets perceptual quality, not a fixed bitrate