How to Compress Video / Reduce File Size with Avidemux

Q: What CRF value should I use?

For x264: CRF 18-20 for high-quality archives, 21-23 for everyday use, 24-28 for smaller files. For x265, add roughly 2-3 points (CRF 23-26 is comparable to x264 CRF 21-23).

Q: Will compression noticeably hurt quality?

At CRF 21-23 most viewers cannot tell the difference on a normal screen. Visible softness appears mainly in dark scenes and fast motion. Keep the original if it is irreplaceable.

Q: Should I use x264 or x265?

Use x264 for maximum compatibility — it plays everywhere. Use x265 for smaller files when your devices support HEVC decoding. x265 encodes slower on older CPUs.

Q: How do I set the audio when compressing?

Set Audio Output to AAC (lav) at 128 kbps for stereo audio, or 96 kbps for dialogue. You can leave audio on Copy if the source AAC track is already fine.

Avidemux shrinks video files by re-encoding them with the x264 (H.264) or x265 (H.265/HEVC) encoder. You control the final size through the quality setting (CRF) or target bitrate, and optionally by lowering the resolution with the resize filter.

Heads-up: compression means re-encoding, and re-encoding always loses some quality — it is irreversible. If you only want to cut sections out without touching quality, use Copy mode for lossless trimming instead.

1. The Three Output Settings

After loading your video, set these three dropdowns in the left sidebar:

Setting	Recommended value	Notes
Video Output	Mpeg4 AVC (x264) or Mpeg4 HEVC (x265)	x265 compresses ~30–50% better but encodes slower and needs a compatible player
Audio Output	AAC (lav)	128 kbps stereo is plenty for most uses; 96 kbps works for speech
Output Format	MP4 Muxer or MKV Muxer	MP4 for maximum device compatibility; MKV if you need multiple audio tracks or chapters

2. CRF vs Bitrate: Picking Your Target

You set the size/quality trade-off in the encoder’s configuration (Configure button next to Video Output):

CRF (Constant Rate Factor) — quality-based. Lower number = better quality, bigger file. CRF 20–23 is the sweet spot for x264 (visually near-transparent for most content). Each +6 roughly halves the bitrate.
Average bitrate — size-based. Use when you need a specific file size. Rule of thumb: target bitrate (kbps) ≈ target size (MB) × 8 × 1000 / duration (seconds). Example: to fit a 2-hour (7200 s) movie into ~1 GB: 1000 × 8 × 1000 / 7200 ≈ 1100 kbps for video, plus ~128 kbps audio.

Tip: CRF gives more consistent quality across different scenes, while average bitrate gives a predictable file size. For personal archives, CRF 21–23 is usually the better choice.

3. Worked Example: 1080p Movie → ~1 GB

Load the movie and set Video Output to Mpeg4 AVC (x264).
Configure the encoder: choose CRF mode and set CRF 23 — or choose average bitrate and enter 2000 kbps if you want a guaranteed size.
Set audio: Audio Output → AAC (lav), configure to 128 kbps stereo.
Set format: Output Format → MP4 Muxer.
Save: File → Save (Ctrl+S). A typical 1080p film lands around 0.8–1.2 GB with these settings.

4. Optional: Shrink Further with the Resize Filter

Resolution is the biggest lever on file size. Dropping 1080p → 720p cuts the pixel count by more than half:

Go to Video → Filters.
In the Transform category, select swsResize and add it.
Set the width to 1280 (height 720 is calculated automatically for 16:9). Leave the defaults for everything else.

Combining 720p with CRF 23 can bring a movie down to ~500–700 MB — ideal for phones and tablets.

5. Two-Pass Encoding for Precise Sizes

Need to hit an exact file size (e.g. fitting a DVD or an upload limit)? Use two-pass mode in the x264/x265 configuration: the first pass analyzes the video’s complexity, and the second pass distributes bits where they matter most. It takes roughly twice as long as single-pass, but it is the most efficient way to spend a fixed bitrate budget.

FAQ

What CRF value should I use?

For x264: CRF 18–20 for high quality archives, CRF 21–23 for everyday use, CRF 24–28 for small files where some softness is acceptable. For x265 the equivalent values run about 2–3 points higher (CRF 23–26 ≈ x264 CRF 21–23 in quality).

Will compression noticeably hurt quality?

At CRF 21–23 most people cannot tell the difference from the source on a normal screen. Quality drops become visible mainly in dark scenes and fast action. Always keep the original if it is irreplaceable.

Should I use x264 or x265?

Use x264 if compatibility matters — it plays everywhere. Use x265 if you want the smallest file and your devices support HEVC decoding (most phones and TVs from the last several years do). x265 encodes noticeably slower on older CPUs.

How do I set the audio when compressing?

Set Audio Output to AAC (lav) at 128 kbps for stereo movie audio; 96 kbps is fine for dialogue-heavy content. If the source already has AAC audio and you are happy with it, you can leave audio on Copy to skip re-encoding it.