Convert WebM, MKV or MOV to MP4
A standard MP4 that opens everywhere. Repackaged without quality loss when the codecs already fit, converted when they don’t. Nothing is uploaded.
Processed on your device. Your video is never uploaded. How we know
Drop a WebM, MKV or MOV to convert
Drop a video here, paste it with Ctrl+V, or choose one from your device.
ClipHeron runs in your browser. We don’t receive your videos, their names or anything in them. Ads on this page load separately and are not given your files. What happens to your video
How to convert a video to MP4 without uploading it
“Won’t play” and “won’t upload” are usually the same problem: a container or codec the other side doesn’t understand. MP4 with H.264 video and AAC audio is the one combination that opens on every phone, computer, TV and website. ClipHeron gets you there by the cheapest route: if the streams already fit, it just moves them into an MP4 in seconds; if not, it decodes and re-encodes only what it must.
The formats guide explains containers versus codecs with a compatibility table, and the iPhone HEVC guide covers the most common case of all.
- 1
Add the file
Drop or choose a WebM, MKV or MOV. The file card shows the container, the video and audio codecs, and the path ClipHeron will take.
- 2
Read the path
“No quality loss: just repackaging” means seconds. “Video kept as is, audio converted” means a short wait. “Video converted to H.264” means a real encode, with a time estimate.
- 3
Convert and download
Press Convert. The progress bar shows frames done, speed and time left. Download the MP4, or tick “Also compress” first to set a target size as part of the same job.
What this tool does
- WebM, MKV, MOV and M4V in; MP4 with H.264 video and AAC audio out, fast start enabled
- Smart path shown before you click: repackage only, audio converted only, or full conversion
- Repackaging is lossless and takes seconds, even for a 2 GB file
- iPhone HEVC footage converted to H.264 with its rotation kept, so portrait video stays upright
- HDR sources tone-mapped to standard color so the MP4 looks right everywhere
- MediaRecorder WebM files with no duration handled correctly, with the real length worked out
- Opus and Vorbis audio converted to AAC; AAC copied as is
- “Also compress” hands the file to the compressor with it preloaded
Three common files and the path each one takes
| Input | Video | Audio | Path | What happens |
|---|---|---|---|---|
| 2 GB MKV from a screen capture tool | H.264 | AAC | Repackage | Streams copied byte for byte into an MP4 with a fresh index. About as long as copying the file; every sample identical to the source. |
| Browser screen recording, WebM (MediaRecorder) | VP9 | Opus | Full | No duration in the file, so the clusters are scanned to find the real length. VP9 decoded and encoded to H.264 at the Best preset; Opus decoded and encoded to AAC. Timestamps preserved, so a 4-minute recording is 4 minutes long. |
| iPhone 4K .mov, 3:12, 1.42 GB | HEVC 10-bit HDR | AAC | Full video, audio copied | Decoded with the device’s hardware decoder where the browser exposes it, tone-mapped to standard color, encoded to H.264 at 2160p (1080p on phones), rotation written into the MP4 so it plays upright. AAC copied untouched. |
A MOV from a camera with H.264 video and uncompressed PCM audio takes the middle path: the video is copied and only the audio is converted to AAC at 192 kbps.
Containers versus codecs
The extension on a video file names the container: the box that holds the streams and the index that says where each frame is. MP4, MOV, WebM, MKV and AVI are containers. The codec is how the pixels inside are compressed: H.264, HEVC (H.265), VP8, VP9, AV1 for video; AAC, Opus, MP3, Vorbis, PCM for audio. A player needs to understand both. “Convert to MP4” therefore means two different things depending on what is inside: changing the box (cheap, lossless) or changing the compression too (slow, lossy by nature).
MP4 and MOV are close relatives (both are ISO base media files), which is why a MOV with H.264 and AAC can become an MP4 by rewriting a few bytes of the header. WebM is a restricted form of Matroska that only allows VP8, VP9, AV1, Opus and Vorbis, none of which belong in an everyday MP4, so a WebM almost always needs a real encode.
AVI: not supported yet, and why Xvid never will be
AVI is a 1992 container, and most AVI files you meet carry Xvid, DivX or other MPEG-4 Part 2 video, or MJPEG from an old camera. No browser can decode those codecs and ClipHeron doesn’t ship a decoder for them. The engine does not read AVI at all yet, so an AVI is refused with a message that says so and names the formats that work (MP4, MOV, WebM, MKV). A desktop converter such as HandBrake turns an AVI into an MP4 once, and you can bring the result here for everything else.
Screen recordings, iPhones and the other quirks
MediaRecorder WebM files, which every browser-based screen recorder produces, have no duration element and no seek index, which is why they show “0:00” in some players and refuse to scrub. ClipHeron scans the clusters to measure the real length and writes a proper index into the MP4.
iPhone footage stores its rotation as a matrix in the header rather than rotating the pixels. The converter carries that rotation into the MP4 (or bakes it in when it has to re-encode), so portrait videos don’t arrive sideways on Windows. Live Photo movies (3 seconds) work like any other MOV. Spatial video from recent Pro models is converted from the left eye only, with a note.
Subtitle tracks, chapters and extra audio tracks are listed on the file card as “not carried over”: the MP4 keeps the first video track and the default audio track.
Good to know
- Repackaging never changes quality. Full conversion re-encodes, so keep the original if you may need it again.
- Full conversion of HEVC or VP9 depends on your device: a laptop with hardware decoding does a 3-minute 4K clip in a few minutes; a phone works at 1080p output and takes longer.
- Protected (DRM) files, including iTunes-purchased .m4v, are refused.
- No “paste a link” box, on purpose: fetching a video from another site would make the no-upload promise false and invites abuse.
Related tools and guides
Questions people ask
Will converting lose quality?
Not on the repackaging path: the video and audio samples are copied exactly. When the video has to be re-encoded (HEVC, VP8, VP9, AV1 sources) there is some loss; ClipHeron uses its Best preset so it is hard to see, and the before/after comparison in the compressor lets you check.
Why does it say my file is “protected”?
The file has encrypted tracks (DRM), typically a purchase or rental. ClipHeron doesn’t open protected media and the terms don’t allow processing content you have no right to.
Why isn’t my AVI accepted?
The engine does not read AVI yet, and most AVI files carry Xvid, DivX or other MPEG-4 Part 2 codecs that no browser can decode anyway. Convert the AVI once with a desktop tool such as HandBrake, then use ClipHeron for anything else.
Is my video uploaded?
No. The converter runs in your browser and we never receive the file or its name. The how it works page shows how to verify it with your browser’s network log.
Where did the subtitles go?
MP4 carries text subtitles poorly and most players ignore them, so subtitle tracks are listed on the file card as not carried over. Burning subtitles into the picture is planned for a later release.
Can I convert several files at once?
One at a time in this release. A batch mode with a single zip download is planned.
What happens to your video
Processed on your device. Your video is never uploaded. ClipHeron runs in your browser. We don’t receive your videos, their names or anything in them. Ads on this page load separately and are not given your files.
Demuxing, decoding, encoding and muxing run in a background worker on your device. For the repackaging path nothing is decoded at all. The MP4 is assembled in the browser’s private temporary storage and handed to your download, then deleted. Nothing about the file is remembered.
The page does load advertising and analytics scripts, separate from the tool and described in our privacy policy. They are never given your files.