Repair an MP4 and enable fast start

What fast start means

An MP4 has an index, the moov atom, that tells a player where every frame is. Many cameras and editors write it at the end of the file. A player streaming the video from a website then has to download the whole file before it can start. Moving the index to the front, called fast start or web optimized, lets playback begin straight away.

How to repair an MP4

Drop the video above and press Repair MP4. The tool first tries MediaBunny, which copies every track into a new MP4 with the index at the front. If that can't read the file, FFmpeg regenerates missing timestamps, discards packets it can't decode, shifts timestamps that start below zero and copies the streams into a fresh MP4 with fast start. Only when the codecs can't be placed in MP4 at all does it re-encode to H.264 and AAC, and the result tells you which path it took.

What can and can't be fixed

This helps with files that play with no sound sync, show the wrong length, refuse to seek, start slowly online or were cut short by a crash or full memory card when the container is still readable. It can't rebuild a file whose index is missing entirely, such as a recording that stopped before the camera finished writing, and it can't restore data that was never saved.

Your file stays on this device

Convertur reads the file in this browser tab and builds the result there. The file is never sent to a server. Larger browser engines load only when this tool needs them.

Before you start

Input: MP4, MOV, M4V, MKV, and AVI. Output: repaired MP4. The settings appear before anything runs, so you can check them first. Your original file is never changed.

Video work uses a lot of memory and CPU. Try a short clip first and check that the sound and playback work. On a phone, split large batches into smaller ones.

Keep the original until you have opened and checked the result. The work happens in this browser tab, so the size you can handle depends on your device's memory, your browser and the file itself.

How to repair an MP4 and enable fast start

1. Drop the damaged or slow-starting video into the box above.
2. Press Repair MP4.
3. Read whether it was a lossless rebuild or a re-encode.
4. Download the repaired MP4 and test it in your player.