# Quality controls: CRF, bitrate and presets

Canonical URL: https://encode.wiki/guides/quality-controls/
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Rate control decides how many bits each frame gets. Choose the mode by what has to stay fixed:
the quality, the file size, or the bitrate at every moment.

| Mode                   | You set                             | Result                                                  | Use it for                                            |
| ---------------------- | ----------------------------------- | ------------------------------------------------------- | ----------------------------------------------------- |
| CRF / constant quality | A quality value                     | Consistent quality; file size depends on the content    | Almost every encode you keep, archive or upload       |
| Capped CRF (CRF + VBV) | Quality value plus peak rate/buffer | Constant quality, except that peaks are limited         | Streaming and devices with a bitrate limit            |
| Two-pass VBR           | Average bitrate                     | Predictable size; quality close to CRF at the same size | Hitting a file size or an average-bitrate requirement |
| One-pass ABR           | Average bitrate                     | Roughly the right size; bits are spent poorly           | Live encoding where two passes are impossible         |
| CBR                    | Constant bitrate                    | Bitrate held constant, with padding if needed           | Broadcast and some live transport                     |
| Constant QP (CQP)      | A fixed quantizer                   | Same quantizer everywhere; wastes bits on hard scenes   | Codec development and testing, not delivery           |

## CRF and constant quality

CRF (constant rate factor) is quality-based variable bitrate. The encoder raises or lowers the
quantizer from frame to frame and block to block, spending fewer bits where loss is hard to see
(fast motion, complex texture) and more where it shows. At the same file size this looks better
than constant QP, which ignores where the eye notices errors.

CRF numbers are encoder-specific. They do not mean the same quality across encoders, and sometimes
not across versions of one encoder.

| Encoder (FFmpeg name) | Option | Default | Notes                                                                                              |
| --------------------- | ------ | ------- | -------------------------------------------------------------------------------------------------- |
| x264 (`libx264`)      | `-crf` | 23      | Lower is better; about ±6 halves or doubles the bitrate. 18–24 covers most uses.                   |
| x265 (`libx265`)      | `-crf` | 28      | Same 0–51 scale as x264, but the same number does not give the same quality.                       |
| SVT-AV1 (`libsvtav1`) | `-crf` | 35      | Range 1–70 (FFmpeg's `-crf` accepts up to 63). Upstream suggests 30 as a starting point for 1080p. |
| libaom (`libaom-av1`) | `-crf` | 32      | Range 0–63. `-crf` alone is constant quality; adding `-b:v` makes it constrained quality.          |
| libvpx (`libvpx-vp9`) | `-crf` | 32      | Range 0–63. In current FFmpeg, `-crf` alone is constant quality; the old `-b:v 0` is not needed.   |

Hardware encoders have their own constant-quality modes (NVENC `-cq`, Quick Sync ICQ, and others).
They are not interchangeable with software CRF values; see the
[hardware encoder pages](/tools/hardware-encoders/nvenc/).

**Presets** trade speed for compression efficiency and are also encoder-specific: x264 and x265 use
names (`ultrafast` … `placebo`), SVT-AV1 uses numbers where lower is slower and better. Change the
preset to fit your time budget, then adjust CRF until the result looks right. At the same CRF a
slower preset usually gives a smaller file or more detail.

## Cap the bitrate with VBV

VBV (video buffering verifier) models a decoder's input buffer: data arrives at up to `-maxrate`
and the buffer holds `-bufsize` bits. Combined with CRF it keeps constant-quality behaviour but
stops peaks from exceeding what a player or connection can take:

```sh
ffmpeg -i input.mkv -c:v libx264 -preset slow -crf 20 -maxrate 6M -bufsize 12M -c:a copy output.mkv
```

`-maxrate` is the peak rate and `-bufsize` the buffer it is averaged over; you need both. A larger
buffer allows short bursts; a smaller one enforces the limit more strictly. When the cap kicks in,
quality drops in those scenes, so set it from the real limit of the target, not lower. The same
`-maxrate`/`-bufsize` options work with libx265, and `-maxrate` with libsvtav1 turns its CRF mode
into capped CRF.

## Hit a file size with two passes

Work out the video bitrate from the size you want:

```text
video kbit/s = target MiB × 8388.608 / duration in seconds − audio kbit/s
```

For a 60-minute file of 1,500 MiB with 128 kbit/s audio: 1500 × 8388.608 / 3600 − 128 ≈ 3367 kbit/s.

```sh
ffmpeg -i input.mkv -c:v libx264 -preset slow -b:v 3367k -pass 1 -an -f null /dev/null
ffmpeg -i input.mkv -c:v libx264 -preset slow -b:v 3367k -pass 2 -c:a libopus -b:a 128k output.mkv
```

The first pass analyses the whole video and writes a stats file (`ffmpeg2pass-0.log`); its output
is discarded, and `-an` skips audio work in that pass. The second pass uses the stats to put bits
where they are needed. With x264, a two-pass encode looks about the same as a CRF encode
that happens to produce the same size, so use two passes when the size matters and CRF otherwise.

For SVT-AV1, multi-pass mainly serves VBR; upstream describes it as not critical in CRF mode. CBR is
always single-pass.

## Testing settings

Test on a short clip that includes the hardest material in the source, and change one thing at a
time. Compare the clips by watching them, not just by file size or a single metric score; the
[quality comparison guide](/guides/compare-encoding-quality/) covers metrics and how to use them
properly.