Repair an MP4 file that will not play

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

MP4 is the most common video format in the world, which also makes it the most common format to break. If an MP4 file plays for a split second and stops, shows a black screen, reports "this file is not playable", "unsupported format", "error 0xc00d36c4" or "moov atom not found", the file is probably structurally damaged rather than gone. FixMP4 repairs MP4 files locally in your browser and shows you a free preview before you pay.

Open the free repair tool to diagnose your MP4 now.

How an MP4 file is structured

Understanding the structure explains why repair is possible. An MP4 file contains three main parts:

• ftyp: a short header that says "this is an MP4 and these are the standards it follows".
• mdat: the media data, i.e. the compressed video and audio frames in the order they were recorded.
• moov: the index describing every track and every frame.

Damage to the ftyp or moov parts makes the whole file unreadable even though the mdat part, which is 99.9% of the file, is intact. That is the type of damage FixMP4 repairs.

Common MP4 problems and what they mean

What FixMP4 does to repair an MP4

The repair is done by our own MP4 engine, written from scratch for the browser:

1. Read the box structure. We walk through the top-level boxes and record what exists and where the structure breaks.
2. Keep a surviving index. If the moov box still exists, we reuse it, remove entries for frames that are no longer in the file, and write a fresh header. No reference file is needed in this case.
3. Rebuild a missing index. If moov is gone, we read the codec settings from a reference MP4 recorded with the same device and settings. Then we scan the media data:

• H.264 and HEVC video frames are found by following their length-prefixed NAL units and detecting where each picture begins;
• AAC audio frames are measured by walking the AAC bitstream, so every frame size is exact;
• keyframes are marked so that you can seek in the repaired file;
• picture order counts are decoded so that videos with B-frames play in the right order.

4. Write a new MP4. We output a fresh ftyp, a new moov index and the original frames unchanged.

Nothing is re-encoded, so there is no quality loss, and the process is fast: the browser only needs to read the file once.

Free preview first, then export

Every repair produces a playable 15-second preview on the page. If you are happy with it, a subscription unlocks the full export: $9 per month or $29 per year, cancel anytime from your account in one click. You are never asked to pay before you have seen your own file play.

Private by design

Most online MP4 repair services require you to upload the file. For a multi-gigabyte video that takes a long time, and it means your personal footage sits on someone else's server. FixMP4 processes the file inside your browser tab instead. The file never leaves your device, and there is no file size limit other than your device's memory.

Which MP4 files are supported

Supported:

• H.264 (AVC) and HEVC (H.265) video,
• AAC audio (AAC-LC, the format used by almost every phone and camera),
• files from iPhone, Android phones, GoPro, DJI drones, dashcams, OBS and other screen recorders that write standard MP4,
• missing index, truncated recordings, damaged headers, deleted moov atoms, short damaged regions in the middle of a file.

Not supported:

• AV1, VP9, MPEG-4 Part 2 (DivX/Xvid), ProRes,
• PCM, ALAC, Opus, AC-3 audio,
• fragmented MP4 and encrypted video,
• restoring the pixels of frames that were overwritten.

How to repair an MP4 file, step by step

1. Copy the broken MP4 from the camera or phone to your computer (keep the original untouched).
2. Copy one healthy MP4 recorded with the same device and settings. A few seconds is enough.
3. Open the repair tool and drop the broken file on the left, the healthy file on the right.
4. Press Repair. Watch the preview.
5. Subscribe and click Download full file to save the repaired MP4.

Tips to avoid MP4 corruption in the future

• Stop recordings with the stop button before turning a device off.
• Keep at least a few gigabytes free on the memory card.
• Use the "safely remove" option before unplugging cards and drives.
• In OBS, record to MKV or fragmented MP4, then remux, so a crash cannot destroy the index.

More guides: repair a corrupted video, fix "moov atom not found", repair a MOV file.

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