Why videos break - and what can actually be fixed
An MP4 or MOV file is a chain of "boxes": ftyp (the file type), moov (the index that tells players where every frame is) and mdat (the actual footage). Most "corrupt" videos have perfectly healthy footage - it is the structure around it that is damaged. The three common faults:
- moov at the end of the file. Cameras write the index last. If a player or browser needs it up front, the video appears broken or won't seek. Fixable: this tool moves the moov to the front and corrects every internal offset (the same repair as the classic "faststart" fix).
- Truncated file. An interrupted download or recording leaves a length header pointing past the end of the file, and strict players refuse the whole file. Fixable: the header is patched to the real size, so everything that exists plays.
- Missing moov. The camera died before writing the index at all. Not fixable in a browser - the index must be rebuilt frame by frame against a healthy reference clip from the same device, which desktop tools like untrunc do. This tool detects the case and tells you, instead of pretending.
The rescue re-encode
Browsers are remarkably tolerant decoders. If your damaged file still plays - even badly - the Rescue re-encode button plays it internally and re-records it with your device's hardware encoder, producing a brand-new, structurally clean file. It is the bluntest tool here, but it rescues files that strict editors and phones reject.
How to repair a video
- Drop the file above. The diagnosis runs instantly and lists every box, its size and position.
- Read the verdicts: green is healthy, yellow is fixable, red is not fixable locally.
- Press whichever repair button lights up, then download the repaired copy. Your original file is never modified.
Before you start: a 30-second checklist
First, work on a copy - this tool never modifies your original, but keeping an untouched copy of a damaged file is basic recovery hygiene, because a failed experiment elsewhere can make things worse. Second, try other players before repairing: VLC in particular plays files that Windows and phones reject, and if VLC plays it, the file may only need the faststart fix rather than a rescue re-encode. Third, be realistic about what is recoverable: a file whose video data was never written (a recording interrupted at the very start) contains nothing to repair - no tool can recover frames that were never saved. The diagnosis step here tells you which situation you are in before you commit to anything, which is exactly the information paid repair tools make you pay to find out.
FAQ
Does this Video Repair tool upload files to a server?
- No. The file structure is read and rewritten entirely in your browser. Nothing is transmitted.
What is the moov atom and why does it matter?
- The moov atom is the MP4 index - it tells players where every frame lives. If it sits at the end of the file, browsers and some players cannot start playback; if it is missing, the video is unplayable. This tool detects both cases and can move a misplaced moov to the front.
Can you fix a video with a missing moov atom?
- Not in a browser - the index must be rebuilt frame by frame using a healthy reference video from the same camera, which desktop tools like untrunc do. This tool tells you honestly when that is the case instead of pretending.
What does Rescue re-encode do?
- If your browser can still decode the video despite the damage, the tool plays it internally and re-records it with your device's hardware encoder, producing a brand-new clean file. It is the most robust fix for files with index damage or trailing garbage.
My download stopped midway - can the file be saved?
- Often, partially. If the index (moov) was stored at the front of the file, this tool can patch the truncated length header so players accept the file and play everything that was downloaded.