What is a corrupted MP4 file, really?

The three boxes that matter

ftyp sits at the very start and says “I’m an MP4, of this flavour.” It’s tiny. If it’s missing, the start of the file is gone.

mdat is the big one, usually 99% of the file. It holds the actual compressed video and audio frames, one after another, with no labels.

moov is the index. It lists every frame: where it starts in mdat, how big it is, when to show it. Without moov, mdat is just a few gigabytes of numbers that no player can make sense of.

Symptoms and what they usually mean

Won’t open anywhere, right file size: moov is missing. Recording didn’t finish properly. Very fixable. See fix a corrupted MP4.

Plays in VLC, not in other players: moov exists but has small errors, or the codec isn’t supported. A remux usually fixes it.

Plays the start, then freezes or ends early: the file is truncated, or mdat has damage partway through. See damaged MP4 repair.

Audio but black picture: often a codec issue (HEVC on an older PC), sometimes a damaged video track. See broken video files.

Shows as 0 bytes or a few KB: nothing was saved. Not repairable.

Why phones and cameras write moov last

The camera doesn’t know how long you’ll record, so it can’t write the index until you press stop. It writes the frames as they come, then writes moov at the end. If power dies before that last step, you get a full mdat and no moov. That’s why “my battery died while recording” is one of the most fixable kinds of corruption.

Checking it yourself

Run ffprobe broken.mp4. If it says “moov atom not found,” you know the case. If it lists streams with durations, the index is there and something else is wrong. Or skip the terminal and run a free diagnosis.

What a corrupted MP4 file looks like from the inside

It helps to picture an MP4 as a book. The ftyp box is the cover, saying what kind of book it is. The mdat box is the text, every page of video and audio. The moov box is the table of contents, listing where each page starts and how long it is. A player opens the cover, reads the table of contents, and then turns straight to the page it needs.

A corrupted MP4 is a book with something wrong in that arrangement. The table of contents may be missing, so the player has no idea where anything is. It may be wrong, pointing to pages that don’t exist. The cover may be torn off, so the player can’t even tell what kind of book it is. Or some pages may be smudged, so the text exists but parts of it are unreadable.

The first three are structural problems and can be fixed by writing a new cover or table of contents. The last is data damage, and the smudged pages stay smudged. Knowing which one you have is most of the work.

Identifying the damage with free tools

ffprobe corrupted.mp4 gives a fast answer. moov atom not found means a missing table of contents. Invalid data found when processing input with no other detail often means a damaged start. A normal stream listing followed by decode errors during playback means smudged pages.

A hex viewer shows the structure directly. Open the file in HxD on Windows or Hex Fiend on macOS. At byte 4 you should see the letters ftyp. Search for the text moov: if it appears near the end, the index exists; if it never appears, it’s missing. Search for mdat: it should appear near the start, followed by a long stretch of apparently random data, which is your compressed video.

A file that is mostly zeros in the hex viewer has been wiped or never written. A file whose first few kilobytes are zeros but the rest looks random has a damaged start, which a rebuild can usually work around.

The most common causes, ranked

Interrupted recording. By far the most frequent: battery died, app crashed, storage filled, phone overheated, card removed. The camera never wrote the index, so the file has video but no table of contents.

Interrupted transfer. A copy, upload, or download that stopped partway leaves a truncated file. Sometimes the index survives at the start (in web-optimised files) and the file plays until the cut. More often the index was at the end and never arrived.

Failing storage. Cards and drives with bad sectors corrupt whatever sits on those sectors. Damage is usually scattered and appears as glitches rather than a total failure to open.

Software bugs. Editors, converters, and recorders occasionally write malformed files, especially if they crash mid-export. Re-exporting is the simplest fix when the source still exists.

File-system errors. Unsafely ejected drives, especially FAT32 and exFAT cards, can record the wrong file size, cutting a file off or padding it with someone else’s data.

Fixing a corrupted MP4 file with a missing index

This is the case where most of the footage is safe. The fix is an index rebuild: a tool walks through mdat, identifies every video frame and audio chunk, and writes a new moov describing them.

Modern rebuild tools can identify H.264 and HEVC frames directly because each frame carries a recognisable structure. AAC audio is harder because its frames have no clear markers inside MP4, which is why a reference file from the same camera helps: it shows the typical audio chunk size and position.

Our engine scans frames natively first and falls back to reference-based rebuilding when needed. Open the repair page, check the free 15-second preview, and unlock the full file for $9/month or $29/year if it looks right.

Fixing a corrupted MP4 with a damaged start

When the first few kilobytes are overwritten, the ftyp box and sometimes the start of mdat are gone. Players can’t identify the file at all. The rest of the video is often intact.

The fix is to graft a valid header onto the file. Take a healthy file from the same device, copy its first few hundred bytes (up to the start of its mdat data), and replace the damaged start of the broken file with them. Then rebuild the index as above. This is fiddly by hand and best done by a tool that understands the box layout, but it works more often than people expect.

Expect to lose the very first second or so of video, because the first keyframe may have been in the overwritten region. Video resumes cleanly at the next keyframe, usually within one or two seconds.

Fixing a corrupted MP4 with scattered damage

If the file opens but shows blocks, smears, or freezes, the frames themselves are damaged. Rebuilding the index won’t help because the index isn’t the problem.

Make the most of what is left. Remux with ffmpeg -err_detect ignore_err -i in.mp4 -c copy out.mp4 to get a clean container that skips unreadable packets. Then decide whether to cut out the damaged moments or re-encode the whole clip so the decoder conceals them.

If damage is spread evenly through the file, the storage was probably failing while recording. Stop using that card immediately; it will corrupt the next file too.

Corrupted MP4 files from specific sources

iPhone: usually an interrupted recording or a partial iCloud download. Check that iCloud has finished downloading the original before repairing. See repair an MP4 file for device-specific notes.

GoPro: the last chapter of a long recording is usually the broken one. Earlier chapters are fine, and the LRV proxy may cover the gap.

Drones: check the controller app’s cache for a lower-resolution copy before repairing the card file.

Screen recorders: switch to MKV or fragmented MP4 recording to avoid losing future sessions to a crash.

Downloads: download again. A corrupted download almost never needs repairing if the source is still available.

How long a corrupted MP4 stays recoverable

On its own, a corrupted file doesn’t get worse over time. The data sits there until something overwrites it. The risk is what you do with the storage in the meantime: recording more, letting a phone free up space, or formatting a card all reduce your chances.

If the file lives on a phone with low storage, copy it off promptly. Phones aggressively clear caches and temporary files, and a half-written video may be classed as a temporary file.

Once you have a safe copy, there is no rush. A repair next month works just as well as one today.

Corrupted MP4 file checklist

Use this as a quick reference whenever an MP4 won’t behave.

1. Copy the file to a safe location and work on the copy.

2. Check the size against the expected size (bitrate × duration).

3. Try VLC. If it plays, remux for other players rather than repairing.

4. Run ffprobe. Note whether it reports a missing moov atom, invalid data, or a normal listing.

5. Missing moov: rebuild the index, with a same-device reference if possible.

6. Invalid data at start: graft a header from a healthy file, then rebuild.

7. Normal listing but glitches: remux with error skipping, then cut or re-encode around damage.

8. Verify the result in two players and with an FFmpeg decode check.

9. Back up the repaired file in two places.

10. Test or replace the storage that held the original.

Corrupted MP4 versus unsupported MP4

MP4 is a container that can hold many codecs. A perfectly healthy MP4 can be unplayable on a device that doesn’t support what’s inside. HEVC, AV1, 10-bit colour, HDR, and high frame rates are frequent culprits.

The test is simple: if VLC on a reasonably modern computer plays the file smoothly, it isn’t corrupted. If you need it to play elsewhere, convert it to H.264 8-bit in an MP4, which works on virtually everything.

If VLC fails too, check ffprobe output. A healthy unsupported file lists its streams normally. A corrupted file reports errors.

What a repaired file keeps and loses

Kept: every complete video frame and audio packet that exists in the file, at original quality; resolution, frame rate, and codec settings; correct playback timing.

Sometimes lost: the last fraction of a second of recording; creation date and GPS location if they were only stored in the missing index; extra tracks such as telemetry, timecode, or depth data; chapter markers.

Always lost: data that was never written or was overwritten. No tool can recreate it.

To restore the date, copy the original file’s modified time onto the repaired file, or set it with ExifTool. Our repair page preserves what metadata it can and shows a free 15-second preview before you unlock the full file for $9/month or $29/year.

Corrupted MP4 files in professional work

For paid shoots, a corrupted MP4 is as much a communication problem as a technical one. Tell the client early, explain what is recoverable, and give a realistic timeline. Most container damage is fully repairable, so the honest message is often reassuring.

Build redundancy into your workflow: cameras with dual card slots recording simultaneously, a second camera on key moments, and an external audio recorder. With redundancy, a corrupted file becomes an inconvenience rather than a missing moment.

Document every repair: which file, what was wrong, what tool and reference were used, what was lost. If questions come up later, you have clear answers.

Why some repair tools fail on the same file

Repair tools differ in which codecs, audio formats, and camera layouts they understand. A tool that handles iPhone HEVC perfectly may fail on a GoPro with its extra telemetry track or a camera recording PCM audio. Failure on one tool doesn’t mean the file is beyond repair; it may just be outside that tool’s experience.

Our engine combines a native frame scanner with reference-based rebuilding and reports which approach succeeded. Check the free 15-second preview on the repair page before deciding to unlock for $9/month or $29/year.

Corrupted MP4 versus incompatible MP4

Not every MP4 that refuses to play is corrupted. MP4 is a container that can hold many codecs, and a perfectly healthy file recorded in HEVC or with an unusual audio format may simply not be supported by an older player or TV. The tell is consistency: an incompatible file fails the same way everywhere that lacks the codec but plays fine in VLC, while a corrupted file fails even in VLC or plays only partly.

If VLC plays the file normally, the fix is conversion or a different player, not repair. If VLC shows errors, freezes at the same point, or refuses to open it, the file structure is damaged and repair is the right next step.