Untrunc online: what it is and a browser-based alternative

By the FixMP4 engineering team · Updated October 9, 2026 · Tested on real H.264, HEVC and AAC recordings

Untrunc is a well-known open-source command-line tool that repairs truncated MP4, MOV and M4A files by rebuilding their index from a working reference file recorded on the same device. People search for "untrunc online" because building or running a command-line tool is fiddly. There are two ways to avoid that: websites that run untrunc on their servers (you upload your video), or a browser tool like FixMP4 that does the same kind of index rebuild locally, without installing anything and without uploading your file. FixMP4 is our own implementation, written from scratch; it is not untrunc and contains no untrunc code. You can try it for free in the repair tool.

What untrunc does

Untrunc was created by Federico Ponchio and has active community forks. Given a truncated file and a healthy reference file from the same camera, it reads the codec configuration from the reference, scans the media data of the broken file to find where each sample starts and ends, and writes a new, playable file. It supports many codecs through FFmpeg's libraries and has proven its worth on GoPro, phone and camera footage for years. It is released under the GPL.

Three ways to use the untrunc approach

When to use untrunc itself

• You are comfortable with a terminal and building software, or you found a trustworthy binary for your system.
• Your file uses a codec FixMP4 does not support (for example certain professional or audio codecs that untrunc handles through FFmpeg).
• You are processing many files in a script.

When an upload site makes sense

Only when the footage is not sensitive, the file is small enough to upload comfortably, and you trust the operator. Read the site's privacy policy and retention period first.

When FixMP4 is the easier choice

• You want a point-and-click process with a preview of your own repaired file.
• The footage is private and you do not want to upload it.
• The file is large (multi-gigabyte OBS recordings, 4K drone clips).
• You are repairing an iPhone Voice Memo or another AAC M4A, where FixMP4 usually needs no reference.

How FixMP4 differs technically

FixMP4 parses the broken file's boxes, locates the media data, and walks through it sample by sample. Video frames are found from H.264 or HEVC NAL unit structure; AAC frames are measured with a full AAC bitstream parser (no guessing from silence or patterns); codec parameters come from the reference. It then computes timing, B-frame composition offsets and edit lists so playback starts correctly, marks keyframes for seeking, and writes a standard MP4 with the original media data untouched. More on the format: fix "moov atom not found".

Tips that apply to any of these tools

• The reference is everything: same device, same resolution, frame rate, codec and colour profile. See how to choose a reference file.
• Work on copies, never on the only original.
• Try the device's own recovery first: GoPro, DJI and Sony cameras often close unfinished files at power-on.
• 0-byte files cannot be repaired by any index tool; see MP4 is 0 bytes.

Honest limitations of FixMP4

FixMP4 supports MP4, MOV, M4V and M4A with H.264, HEVC and AAC. It does not support AV1, VP9, ProRes, RAW formats, Opus, ALAC or PCM-only audio, and it cannot recreate frames that are not in the file. Fragmented MP4 is not supported. If your file falls outside that list, untrunc or a desktop suite may be the better option.

Reading the result

Whichever tool you use, check the repaired file from start to end. Smooth playback with synced audio means success. Blocky frames throughout mean the reference does not match the broken file, most often because of a different codec, resolution or frame rate. A brief glitch followed by normal playback points to damaged data at that spot, which no index tool can restore. A result much shorter than the recording means the rest of the data never reached the file.

Licensing note

Because untrunc is GPL software, products that ship it must comply with the GPL. FixMP4 is an independent implementation and does not include untrunc code. Credits for third-party components we do use are listed in our notices.

Typical workflow comparison

With untrunc you would run something like "untrunc reference.mp4 broken.mp4" and receive a new file next to the broken one. With FixMP4 you drop the same two files into the page, read the diagnosis, watch the preview, and export. The input requirements are identical, so a reference you prepared for one tool works for the other.

If you are unsure which route to take, start with the free preview: it costs nothing and tells you within a minute whether the file can be rebuilt.

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