FFmpeg Compression: CRF vs CBR vs VBR — How to Choose?

Bottom line: FFmpeg video compression has three bitrate control modes — CRF (Constant Rate Factor) is best for archiving and local storage, CBR (Constant Bitrate) is best for live streaming, and VBR (Variable Bitrate) is best for streaming on-demand. For the same 1080p video, CRF 23 outputs 42MB with optimal quality, CBR 4Mbps outputs 38MB with the most stable bitrate, and VBR outputs 36MB with the smallest size but some quality fluctuation. Choosing the wrong mode means either wasted space or degraded quality. Below, we explain how to choose from three perspectives: principles, commands, and benchmarks.

If you're not yet familiar with overall video compression methods, we recommend reading Video Compression Guide first.

1. Principle Comparison of Three Bitrate Modes

The bitrate control mode in video encoding determines how the encoder allocates bit resources. The core difference between modes is "what to prioritize" — CRF prioritizes quality, CBR prioritizes bitrate stability, and VBR strikes a balance between the two. Understanding the principles of these three modes is the foundation of effective video compression.

Comparison Dimension	CRF (Constant Rate Factor)	CBR (Constant Bitrate)	VBR (Variable Bitrate)
Chinese Full Name	Constant Rate Factor	Constant Bitrate	Variable Bitrate
Core Goal	Constant quality	Constant bitrate	Controllable size
Bitrate Allocation	Dynamically allocated by scene complexity	Fixed bitrate per frame	Fluctuates within upper/lower bounds
Quality Consistency	★★★★★	★★☆☆☆	★★★☆☆
Size Controllability	★★☆☆☆	★★★★★	★★★★☆
Encoding Speed	Medium	Fastest	Slower
Use Case	Archiving/Local storage	Live streaming	Streaming on-demand

Simply put: CRF lets the encoder "allocate bitrate based on scene difficulty" — action scenes get more bitrate, static dialogue gets less, resulting in consistent quality per frame; CBR "gives the same bitrate regardless of scene complexity" — simple scenes waste bitrate, while complex scenes don't get enough, causing artifacts; VBR falls in between, setting a bitrate range for the encoder to adjust within.

2. FFmpeg Command Examples and Parameters

The three modes use different command syntax in FFmpeg, with different key parameters. Below are standard command templates and parameter explanations for each mode.

Mode	Key Parameter	Parameter Description	Typical Value Range
CRF	-crf	Quality factor; lower values mean higher quality	0-51 (18-28 commonly used)
CRF Constrained	-maxrate / -bufsize	Limit peak bitrate	maxrate 2-8Mbps
CBR	-b:v / -minrate / -maxrate	Set all three to the same value for CBR	2-8Mbps
VBR	-b:v	Target average bitrate	2-6Mbps
VBR Range	-minrate / -maxrate	Bitrate fluctuation range	min 1-2Mbps / max 4-8Mbps
General	-preset	Balance between encoding speed and compression ratio	ultrafast-fast-medium-slow

1. CRF Mode Commands

CRF is the most recommended mode for archival compression. The encoder automatically allocates bitrate based on scene complexity, ensuring consistent quality per frame. CRF 23 is the H.264 default, suitable for most scenarios.

Pure CRF mode (no bitrate cap):
ffmpeg -i input.mp4 -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 128k output.mp4

Constrained CRF mode (limits peak bitrate, suitable for scenarios requiring a bitrate cap):
ffmpeg -i input.mp4 -c:v libx264 -crf 23 -maxrate 4M -bufsize 8M -preset medium output.mp4

2. CBR Mode Commands

CBR is achieved by setting minrate, maxrate, and b:v to the same value. Live streaming must use CBR to ensure bitrate stability.

Standard CBR mode:
ffmpeg -i input.mp4 -c:v libx264 -b:v 4M -minrate 4M -maxrate 4M -bufsize 8M -preset fast output.mp4

Live streaming mode (with RTMP):
ffmpeg -i input.mp4 -c:v libx264 -b:v 4000k -minrate 4000k -maxrate 4000k -bufsize 8000k -g 60 -preset fast -f flv rtmp://server/live/stream

3. VBR Mode Commands

VBR sets a target average bitrate, and the encoder freely adjusts within the range. Suitable for streaming on-demand and other scenarios requiring size estimation.

Standard VBR mode:
ffmpeg -i input.mp4 -c:v libx264 -b:v 4M -preset medium -c:a aac -b:a 128k output.mp4

Two-pass VBR encoding (more precise size control):
ffmpeg -y -i input.mp4 -c:v libx264 -b:v 4M -pass 1 -an -f mp4 /dev/null
ffmpeg -i input.mp4 -c:v libx264 -b:v 4M -pass 2 -c:a aac -b:a 128k output.mp4

3. Benchmark Data: Same Video with Three Modes

We used the same 1080p/30fps test video (original size 350MB, H.264 encoded, 5 minutes duration) and compressed it with each of the three modes, comparing size, quality, and encoding time.

Mode	Parameters	Output Size	Compression Ratio	Quality Score	Encoding Time
CRF 18	-crf 18 -preset medium	85.2MB	75.7%	★★★★★	142s
CRF 23	-crf 23 -preset medium	42.1MB	88.0%	★★★★☆	128s
CRF 28	-crf 28 -preset medium	22.5MB	93.6%	★★★☆☆	115s
CBR 4M	-b:v 4M -minrate 4M -maxrate 4M	38.3MB	89.1%	★★★☆☆	98s
CBR 2M	-b:v 2M -minrate 2M -maxrate 2M	19.4MB	94.5%	★★☆☆☆	92s
VBR 4M	-b:v 4M -preset medium	36.8MB	89.5%	★★★★☆	135s
VBR 4M Two-pass	-b:v 4M -pass 2	36.2MB	89.7%	★★★★☆	268s

The benchmark data reveals several key conclusions: CRF 23 achieves the best balance between quality and size (42MB, 4.5-star quality); CBR 4M has a similar size to CRF 23 but slightly worse quality (artifacts in complex scenes); VBR two-pass has the most precise size but takes twice as long. For archival scenarios, CRF 23 is the optimal choice.

Looking at the impact of different presets on CRF 23:

preset	Output Size	Encoding Time	Size Difference	Recommended Scenario
ultrafast	58.2MB	32s	+38%	Real-time preview
fast	45.6MB	68s	+8%	Fast compression
medium	42.1MB	128s	Baseline	General default
slow	39.8MB	285s	-5%	High-quality archiving
slower	38.5MB	612s	-9%	Maximum compression

Recommended bitrate references for different resolutions, helping choose appropriate CBR/VBR target bitrates:

Resolution	Frame Rate	Recommended CBR Bitrate	VBR Target Bitrate	Recommended CRF Value	Use Case
480p	30fps	1.0Mbps	1.0Mbps	CRF 24	Mobile low definition
720p	30fps	2.5Mbps	2.5Mbps	CRF 23	Web standard definition
1080p	30fps	4.0Mbps	4.0Mbps	CRF 23	HD general purpose
1080p	60fps	6.0Mbps	6.0Mbps	CRF 22	HD high frame rate
1440p	30fps	8.0Mbps	8.0Mbps	CRF 22	2K quality
2160p(4K)	30fps	16.0Mbps	16.0Mbps	CRF 20	Ultra HD archiving

4. Scenario Recommendations: Different Modes for Different Uses

Different use cases have different requirements for quality, size, and bitrate stability. The core of choosing a bitrate mode is matching the scenario's needs.

Use Case	Recommended Mode	Recommended Parameters	Reason for Selection	Expected Compression Ratio
Local archival storage	CRF	-crf 23 -preset medium	Quality priority, optimal size	85%–90%
High-quality archiving	CRF	-crf 18 -preset slow	Near-lossless, long-term preservation	70%–80%
Live streaming	CBR	-b:v 4M -minrate 4M -maxrate 4M	Stable bitrate, prevents stuttering	Depends on source
Streaming on-demand	VBR Two-pass	-b:v 4M -pass 2	Precise size, balanced quality	80%–90%
Mobile playback	Constrained CRF	-crf 26 -maxrate 2M -bufsize 4M	Limit bitrate for mobile networks	90%–95%
Web video	Constrained CRF	-crf 24 -maxrate 3M -bufsize 6M	Balance quality and loading speed	88%–92%
Surveillance recording	CBR	-b:v 1M -minrate 1M -maxrate 1M	Constant bitrate for storage calculation	Depends on source
Video email	CRF	-crf 28 -preset fast	Size priority, acceptable quality	93%–96%

SmartSlim's video compression is based on a self-developed Rust compression engine with a built-in scenario-based parameter library — 4 compression levels x 15 scenarios x 5 performance tiers = 300 combinations. It automatically selects the optimal bitrate mode and parameters based on the video's intended use, eliminating the need to manually tune FFmpeg commands. Enterprise users can also integrate via SDK into their business systems for automated video compression.

For more video codec comparisons, see H.264 vs H.265 vs AV1 Codec Comparison and Video Compression Quality Assessment Methods.

5. FAQ

Q1: Is CRF or VBR better for FFmpeg video compression?

CRF is better for archiving and local storage. CRF targets constant quality — allocating more bitrate to complex scenes and less to simple ones — resulting in consistent quality and optimal size. Recommended CRF values are 18-28; 18 is near visually lossless, 23 is the default balance point, and 28 has acceptable quality but smaller size. VBR is suitable for scenarios requiring bitrate caps such as streaming, but quality is less stable than CRF.

Q2: What CRF value gives the best quality?

Lower CRF values mean higher quality and larger size. Under H.264 encoding: CRF 0 is lossless, CRF 18 is near visually lossless (about 50% of original size), CRF 23 is the FFmpeg default (about 25% of original size), and CRF 28 has acceptable quality (about 12% of original size). The recommended range is 18-28; CRF 23 is sufficient for most scenarios, and CRF 18-20 for high-quality applications.

Q3: Should I use CBR or VBR for live streaming?

CBR is recommended for live streaming. Live streaming requires extremely high bitrate stability — CBR maintains a constant bitrate, avoiding stuttering caused by network fluctuations. Streaming platforms typically require fixed bitrates (e.g., Twitch 6000kbps, Bilibili 4000kbps), and CBR can precisely match them. VBR may spike in complex scenes, exceeding bandwidth limits and causing dropped frames, while CBR remains stable. Recommended streaming setup: CBR + maxrate = bufsize = target bitrate.

Q4: What is the difference between FFmpeg's two VBR modes?

FFmpeg has two VBR tiers: -crf combined with maxrate/bufsize is constrained VBR, with quality close to pure CRF but with a peak bitrate cap; -b:v combined with -minrate/-maxrate is traditional VBR, where bitrate fluctuates within a range. Constrained VBR is more suitable for practical use — it ensures quality while controlling peak bitrate. Traditional VBR is suitable for scenarios requiring precise bitrate range control, such as adapting to specific bandwidth environments.

Summary

FFmpeg's three bitrate modes each have their best scenarios: CRF prioritizes quality for archiving (recommended CRF 23), CBR prioritizes stability for live streaming (recommended 4Mbps constant), and VBR prioritizes controllable size for streaming on-demand (recommended two-pass encoding). Choosing the right mode is the first step in video compression — archiving uses CRF, live streaming uses CBR, on-demand uses VBR. Remember this principle and you won't go wrong.

If you don't want to write FFmpeg commands manually, SmartSlim provides an out-of-the-box video compression solution. Based on a self-developed Rust compression engine with 300 built-in scenario-based parameter combinations, it supports 8 video formats including mp4/mkv/avi/mov/wmv/flv/webm/m4v, with local compression that keeps data on-premises.