How to Split a Video into Multiple Parts in Avidemux (Lossless)
Splitting a video in Avidemux means saving the marked A–B selection as its own file. Avidemux has no “split into N parts” button: you mark a segment with the A and B markers, press Save, and the selection — and only the selection — is written to a new file. Repeat for each part. Done correctly, the whole job runs in Copy mode with no re-encoding and no quality loss. This guide covers the exact workflow, the traps that silently corrupt your splits, and the keyframe rules that make lossless splitting work.
The save-selection model (how splitting actually works)
Avidemux’s Save button does not save “the project” — it saves whatever lies between the A and B markers. Understanding this single fact unlocks splitting:
- Open your video in Avidemux.
- Move the playhead to where part 1 should start and press A (or click the “[” marker button).
- Move to where part 1 should end and press B (the “]” button).
- Set Video Output to Copy, Audio Output to Copy, and Output Format to Mp4 Muxer (or match your source container).
- Press Save and give the file a name with an extension, e.g.
part1.mp4. Avidemux does not add the extension for you. - Repeat steps 2–5 for part 2, part 3, and so on.
For the two-part case there is a shortcut: mark the first half, save it, then press Ctrl+Z (Undo) — Avidemux restores the full timeline — mark the second half and save it. This is the method experienced users on the Avidemux forum recommend.
The Reset Markers trap (do NOT do this)
This is one of those mechanics documented nowhere except the Avidemux forum’s Documentation & Tips board: Reset Markers was designed for a different purpose and is the wrong tool between split passes.
Keyframe rules: why your split point matters
In Copy mode Avidemux cannot decode and re-encode frames — it copies compressed data. That imposes strict rules on where a part may begin and end:
| Rule | Why |
|---|---|
| Every part must START on a keyframe (I-frame) | Frames after a keyframe depend on it. Starting mid-GOP produces a part whose first seconds are garbled or black until the next keyframe arrives. |
| Use the ↑ / ↓ keys to jump between keyframes | Place marker A on a keyframe you found with the keyframe-navigation buttons, not on an arbitrary frame. |
| Never end a cut right after a B-frame | B-frames reference frames on both sides. Cutting immediately after one can silently shift the actual cut point — your split lands a few frames off from where you marked it. |
| Prefer ending on, or just after, a P-frame | P-frames only reference the past, so the boundary stays where you put it. |
Practical approach: jump to a keyframe near your desired split point with ↑/↓, set marker A there, and accept a split point a second or two away from the “ideal” frame. Frame-accurate lossless cutting is a known hard problem — see our lossless cutting guide for the full explanation.
Splitting into many parts (TinyPy approach)
For a dozen or more parts, the manual method gets tedious. Avidemux’s built-in TinyPy scripting can automate it: a script loads the video, loops over a list of (start, end) timecodes, sets the markers with adm.setMarkerA() / adm.setMarkerB(), and saves each part — all in Copy mode. The forum’s best-practice thread documents this pattern; you supply the timecode list and the script does the repetitive marking. If you only need a handful of parts, manual marking is faster than writing the script.
After splitting: joining parts back together
Parts split in Copy mode with identical encoding parameters can be rejoined with File > Append — the same compatibility rules apply. See our merge/join guide for the 5-parameter checklist.
Frequently asked questions
Does splitting a video in Avidemux reduce quality?
Not if you stay in Copy mode for both Video Output and Audio Output — the streams are copied bit-for-bit. Quality loss only happens if you re-encode, which splitting does not require.
Why is the beginning of my split part garbled or black?
The part almost certainly starts on a non-keyframe. In Copy mode the decoder has no reference frame to build those first frames from. Re-do the split with marker A on a keyframe (use the ↑/↓ keyframe navigation keys).
Can Avidemux split a video into equal 10-minute parts automatically?
There is no built-in “split every N minutes” button in current 2.8.x builds. The practical options are manual A/B marking per part, or a TinyPy script that loops over a timecode list and saves each segment.
Why do my two parts not join seamlessly afterward?
Usually because Reset Markers was used between the two saves, which drops frames at the boundary. Use Undo (Ctrl+Z) to restore the timeline between passes instead.