Repair an MP4 file online

Repair an MP4 file online in three steps

This page is the FixMP4 repair tool. It fixes corrupted video files in your browser: drop the broken MP4, MOV or M4A, add a reference clip if it is a video, and press Repair. The file is read on your device and never uploaded. When the repair finishes, a preview cut from the actual repaired file plays on the page. Only if it plays do we offer the full download.

Before you start: make a working copy

Repair is read-only, so FixMP4 never changes your original file, but the device that produced it might. If the broken video is still on a memory card or phone, stop recording on that device and copy the file to your computer first. New recordings can overwrite the frames you are trying to save. Work from the copy.

Step 1: choose the broken file

Click the first box or drag the file onto it. FixMP4 accepts MP4, MOV, M4A, M4V and 3GP. The file is read immediately (you will see the bytes counted) and, for files up to a few hundred megabytes, held in memory so that nothing that happens on disk afterwards can interfere. If the file is still downloading or syncing (from iCloud, Google Photos, AirDrop or a cloud drive), wait until it is complete. A partially downloaded file is a different problem from a corrupted one.

The tool then diagnoses the structure. Typical findings are:

• No index (moov atom) found: the recording was interrupted. This is the classic "moov atom not found" and the most repairable case.
• Media data truncated: the file stops in the middle of a frame; everything before the cut is kept.
• Header damaged: the first bytes were overwritten; the media data is located by scanning.
• Index survived but points past the end of the file: typical for fast-start files cut off during a copy; the surviving index is reused.

Step 2: add a reference file (for video)

A reference is a short, healthy recording from the same device with the same settings: same camera or phone, same resolution, same frame rate, same HDR/HEVC setting, same stabilization or lens mode. FixMP4 reads only its codec settings, the H.264 or HEVC parameter sets and the AAC configuration, which were lost together with the index of the broken file.

The easiest way to get one is to record ten seconds of anything right now with the same device and settings. For iPhone videos, check Settings > Camera > Formats (High Efficiency = HEVC, Most Compatible = H.264) and the resolution/frame rate you used. For GoPro and DJI, use the same video mode. A reference from a different resolution or quality setting usually produces scrambled or green pictures, and the preview will show you that before you pay.

Audio-only files such as iPhone Voice Memos usually repair without a reference: FixMP4 detects AAC frames at common sample rates by itself. If a memo plays at the wrong speed, add a reference memo recorded with the same settings.

Step 3: repair, watch the preview, download

Press Repair. The progress bar walks through the stages: reading the file, analyzing its structure, repairing (scanning every audio and video frame and building a new index), building the preview, and verifying that the preview actually plays in your browser. Most phone videos finish in seconds; multi-gigabyte files take longer because every byte is scanned. You can cancel at any point.

When it is done you see:

• the preview player with the first seconds of your repaired file,
• a repair summary: duration recovered, codec, resolution, frame rate, number of frames and audio tracks, repaired file size,
• a steps log showing what the engine did,
• and, next to it, the checkout and the Download full file button.

If the preview does not play, the repair did not work for this file and no download is offered. Try another reference recorded with exactly the same settings, or contact support with the device model.

How to fix a corrupted video file that won't play

Not every video that refuses to play is corrupted. Before repairing, rule out the quick wins:

1. Try VLC. It plays many codecs that the Windows Photos app or older QuickTime versions cannot. If VLC plays the file normally, you need a codec or converter, not a repair.
2. Check the size. A 1-minute 1080p recording should be roughly 100 MB or more. A file of a few KB, or 0 bytes, contains no video to repair. Use file recovery on the original card instead.
3. Check whether it was fully copied. Re-copy it from the source if you still can.

If VLC also fails, shows 0:00, or players report "moov atom not found", "invalid data found when processing input", error 0xc00d36c4 or "file is not supported", the index is almost certainly missing or damaged. That is what this tool repairs.

What this tool can and cannot repair

Can repair: interrupted recordings (dead battery, full card, crash, app killed), missing or deleted moov atoms, zeroed or overwritten headers, truncated files, fast-start files with a missing end, files with damaged blocks in the middle (the damaged seconds are skipped and audio stays in sync), long recordings and files over 4 GB (64-bit offsets are written automatically).

Codecs: H.264/AVC and HEVC/H.265 video (8- and 10-bit), AAC audio (mono, stereo, multiple tracks). Containers: MP4, MOV, M4A, M4V, 3GP.

Cannot repair (yet): ProRes, AV1, VP9 and MPEG-4 Part 2 video; PCM, ALAC, Opus and AC-3 audio; fragmented and DRM-protected MP4; MKV and AVI (different formats); files whose frames themselves were overwritten; 0-byte files.

Tips for the best result

• Record the reference with identical settings. This matters more than anything else.
• If you have several broken files from the same shoot, one reference works for all of them.
• For HEVC (H.265) videos, verify the preview in Safari or in Chrome/Edge on hardware with HEVC support. Some desktop browsers cannot decode HEVC at all, and the page will tell you if playback failed.
• Keep the original broken file until you are happy with the result.

Privacy

The tool is a static web page plus a repair engine written in TypeScript. Your file is opened with the browser's File API, processed in memory and saved with a normal download. No part of it is uploaded. The only data our server sees is whether you are signed in and subscribed.