untrunc guide

untrunc: what it is and how to repair an MP4 without the command line

Last updated June 13, 2026 · ~5 min read

If you've searched for how to fix a truncated or unfinalized MP4, you've probably run into untrunc. It genuinely works — but it's a command-line tool you have to compile yourself, and that stops a lot of people in their tracks. This guide explains what untrunc does, when it's the right choice, and a drag-and-drop alternative that uses the same repair method.

What untrunc actually does

untrunc is a free, open-source tool that repairs MP4 and MOV files whose moov atom — the index a player needs to read the file — was never written. This is exactly what happens when a recorder like OBS crashes before finalizing a recording.

Its clever trick is using a reference file: a healthy video recorded with the same device and settings. Because that reference contains a correctly written moov atom, untrunc can use it as a template to rebuild the missing index in your broken file. The footage was never gone — it just had no map.

How to use untrunc, step by step

If you want to run untrunc yourself, the process looks like this:

1. Get untrunc. On Linux/macOS you compile it from source (the anthwlock/untrunc fork is the most current); on Windows most people use an unofficial pre-built binary or run it under WSL.
2. Find a reference file. You need a healthy video recorded by the same device/app at the same settings as the broken one — it supplies the working moov atom.
3. Run the command. Point untrunc at the reference and the broken file: untrunc good.mp4 broken.mp4. It writes a new broken_fixed.mp4 next to the original.
4. Check the result. Open the output and confirm video and audio play in sync — there's no preview, so this is trial and error.

That's the whole tool. When it works, it works well — but every step above is a place non-technical users get stuck, which is the gap the rest of this guide covers.

Where untrunc gets hard

The repair logic is solid; the friction is everything around it:

• You have to build it. There's no official installer for most platforms — you compile from source or hunt for an unofficial binary.
• It's command-line only. You pass the reference and broken file as terminal arguments, with no visual feedback.
• The GUI fork is unmaintained. The community untrunc-gui exists but is difficult to get running on current systems.
• No preview. You don't know whether the repair worked until you open the output and hope.

For a developer comfortable in a terminal, none of this is a dealbreaker. For a streamer who just wants their VOD back, it's a wall.

untrunc vs. StreamSalvage at a glance

untrunc (CLI)	StreamSalvage
Compile from source	Download and run
Terminal commands	Drag and drop
No repair preview	Free preview before you pay
Manual reference setup	Guided 10-second reference clip
Local & private	Local & private
Free	$29 one-time to export

Which one should you use?

Use untrunc if you're technical, comfortable building software, and don't mind the command line — it's free and it works.

Use StreamSalvage if you'd rather drop in your file, follow a guided prompt to record the reference clip, and see the repaired result before paying anything. It performs the same reference-based repair, stays 100% local so your footage never uploads, and skips the build-and-terminal steps entirely.

Frequently asked questions