Avidemux x264 & x265 Settings Explained: CRF, Presets, Two-Pass

Opening the Configure dialog

1. In the main window, set Video Output to MPEG-4 AVC (x264) or MPEG-4 HEVC (x265).
2. Click the Configure button next to the dropdown.
3. A dialog opens with tabs or sections for general encoding options, rate control, and advanced parameters.

Rate control: CRF vs single-pass vs two-pass

Rate control decides how the encoder spends bits. This is the single biggest quality/filesize lever you have:

Mode	How it works	Best for
CRF (Constant Rate Factor)	You pick a quality level; the encoder varies the bitrate scene by scene to hold that quality	Almost everything — archiving, uploads, personal encodes. One pass, predictable quality, sensible file sizes
Single-pass, fixed bitrate	You pick a bitrate (e.g. 8 Mbps); the encoder must hit it no matter what the scene needs	Only when a target size/bitrate is mandatory — streaming constraints, disc capacities, hardware limits
Two-pass	First pass analyzes the whole video; second pass distributes a fixed total bitrate optimally	When you need a specific final file size (e.g. “must fit 4 GB”) with the best quality that size allows

The practical rule: if nobody is forcing a file size on you, use CRF. It gives constant perceived quality — simple scenes get few bits, complex scenes get many — and it finishes in a single pass. Fixed bitrate is a compromise you accept only when the target size is non-negotiable.

CRF values: the quality dial

CRF is a scale where lower numbers mean higher quality and larger files, higher numbers mean lower quality and smaller files. This is general x264/x265 knowledge, not Avidemux-specific, and it behaves the same in every program that exposes these encoders.

• Start in the sensible middle of the scale and adjust from there — treat your first encode as a test, not a commitment.
• If the result looks worse than the source, lower the CRF (better quality, bigger file).
• If the file is bigger than you need and you can’t see a difference, raise the CRF (smaller file).
• As a sanity check, compare a short test clip at two CRF values side by side rather than re-encoding a whole movie to discover you chose badly.

Presets: the speed/quality trade-off

The preset controls how much effort the encoder spends searching for compression opportunities — not the target quality itself. The scale runs from the fastest to the slowest:

Preset direction	Encode speed	Compression efficiency	Use when
Fast end (e.g. ultrafast, veryfast)	Very quick	Worse — bigger files at the same CRF	Drafts, previews, or when you’re in a hurry
Middle (e.g. medium, slow)	Moderate	Good balance	The everyday sweet spot for most encodes
Slow end (e.g. slower, veryslow)	Very slow	Best — smallest files at the same CRF	Final archives you’ll keep for years; diminishing returns past a point

Two things beginners get wrong here:

• A slower preset does not mean higher picture quality — quality is set by CRF (or bitrate). The preset only changes how efficiently the encoder reaches that quality, i.e. the resulting file size.
• The returns diminish. The jump from the fast end to the middle saves a lot of size; the jump from the middle to the slowest preset saves much less while taking far longer. For most people, the middle of the scale is where the value is.

Two-pass encoding: when it earns its keep

Two-pass exists for exactly one situation: you have a hard file-size target. The first pass scans the entire video and logs which scenes are complex; the second pass spends the fixed bitrate budget where it matters most. Result: the best quality achievable within that exact size.

• Worth it for: fitting a movie on a size-limited medium, hitting a platform’s upload cap, producing a file that must be exactly X MB.
• Not worth it for: general archiving or uploading, where CRF gives equal or better perceived quality in half the time (one pass instead of two).
• Expect roughly double the encoding time — that’s the price of the analysis pass.

x264 vs x265: which encoder to pick?

	x264 (H.264/AVC)	x265 (H.265/HEVC)
File size at equal quality	Baseline	Roughly 30–50% smaller
Encode speed	Faster	Noticeably slower at the same preset
Compatibility	Plays on virtually everything	Fine on modern devices; weak on old hardware and some browsers
Best for	Maximum compatibility, quick encodes	Archiving and storage savings on capable hardware

Hardware encoders: the trade-off in one paragraph

Avidemux may also offer hardware encoders (GPU-based) alongside x264/x265. They encode much faster and use far less CPU, but at the same file size they typically deliver slightly lower quality than software x264/x265 at a medium-or-slower preset — and they expose fewer tuning options. Use hardware encoding when speed matters more than squeezing out the last few percent of efficiency (quick drafts, real-time capture); use software x264/x265 when quality per megabyte is the priority. See the hardware encoding guide for the full comparison.