Convert WebM to MP4 in your browser
Browser screen recordings, Discord clips and WebM downloads to a standard MP4 with H.264 and AAC. The real length is worked out even when the file has none. Nothing is uploaded.
Processed on your device. Your video is never uploaded. How we know
Drop a WebM to convert it to MP4
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 WebM screen recording to MP4 without uploading it
WebM is the format browsers record in. Screen recorders built into Chrome, Edge and Firefox, meeting tools that record locally, Discord clips, Loom-style extensions and many “record your screen” web apps all produce it, because WebM is what the browser’s MediaRecorder writes. It plays fine in a browser and almost nowhere else: Windows Media Player, QuickTime, iPhones, most video editors and many upload forms expect MP4. Converting is unavoidable, and because WebM holds VP8 or VP9 video, which MP4 does not carry, converting means a real encode to H.264.
ClipHeron does that encode in your browser using the same VP9 decoder the browser already has, so a 1080p recording typically converts at three to five times real time on a laptop, and nothing is uploaded first. It also fixes the two things WebM recordings are notorious for: a missing duration (the file shows 0:00 and will not scrub) and odd timestamps from paused recordings. The formats guide explains why a WebM cannot simply be renamed, and the general converter takes MKV and MOV too.
- 1
Add the WebM
Drop or choose a .webm file. If it is a MediaRecorder file with no duration, the engine scans it and the file card shows the measured length with an “about” mark.
- 2
Check the card
The card names the video codec (VP8 or VP9), the audio codec (usually Opus) and the path: video converted to H.264, audio converted to AAC. The time estimate is based on your device.
- 3
Convert and download
Press Convert to MP4. The progress bar shows frames done, speed and time left. Download the MP4, or tick “Also compress” first to set a size target in the same job.
What this tool does
- Input locked to WebM, so the page does exactly one job; any other file is handed to the general converter in memory
- VP8 and VP9 decoded by your browser and encoded to H.264, usually at several times real time
- Opus and Vorbis audio converted to AAC inside the engine; no second tool needed
- MediaRecorder files with no duration and no seek index measured by scanning the clusters, so the MP4 has the right length and scrubs properly
- Timestamps preserved, so a 10-minute recording with pauses stays 10 minutes long and in sync
- Transparent (alpha) WebM flattened onto black with a note
- “Also compress” hands the file to the compressor with a target size in the same job
- Output is MP4 with H.264 and AAC, fast start enabled
A 10-minute 1080p browser screen recording
| Step | What the engine does | Why it matters |
|---|---|---|
| Index | Finds no Duration and no Cues in the file (normal for MediaRecorder), scans the clusters and measures 10:02. | Without this the MP4 would report 0:00 and some players would refuse to seek. |
| Decode VP9 | Hands each VP9 frame to the browser’s decoder, hardware where available. | Screen recordings have long runs of identical frames; the decoder is rarely the bottleneck. |
| Encode H.264 | Encodes at the Best preset for 1080p with a keyframe every 4 seconds, keeping the original timestamps. | Pauses and variable frame rate survive intact; audio stays in sync to the end. |
| Audio | Decodes Opus with the browser and encodes AAC 192 kbps in the engine. | MP4 players expect AAC; Opus in MP4 is still poorly supported. |
| Result | About 2 to 3 minutes on a recent laptop; an MP4 of 120 to 250 MB depending on how much changed on screen. | Opens in Windows Media Player, QuickTime, PowerPoint, Premiere and every upload form. |
A 30-second Discord clip takes seconds. A 2-hour meeting recording works too, but needs a computer rather than a phone: the whole file is held in memory in this version, and the drop zone states the limit for your device.
Why a WebM cannot just be renamed to .mp4
A container is a box, and a codec is the compression inside it. WebM is a restricted Matroska box that may only hold VP8, VP9 or AV1 video and Opus or Vorbis audio. MP4 players expect H.264 or HEVC video and AAC audio. Renaming the file changes the label on the box and nothing inside it, so a player that opens a renamed WebM still finds VP9 and gives up. Converting means decoding every frame and encoding it again as H.264, and decoding every audio packet and encoding it again as AAC. That is what this page does, with the browser’s own decoders doing the heavy half.
What is odd about browser recordings, and how it is handled
No duration. MediaRecorder writes the file as it goes and never comes back to fill in the length, so the header says “unknown”. The engine scans the clusters and measures it; the MP4 gets a correct duration and a proper index, which is why it scrubs where the WebM did not.
Gaps and variable frame rate. A screen recording emits frames when something changes, so a minute of a still slide may hold a handful of frames. The converter keeps each frame’s timestamp rather than inventing a fixed rate, so nothing speeds up or drifts out of sync with the audio.
Alpha. Some WebM files from design tools carry a transparent background. MP4 cannot, so the transparency is flattened onto black and the result card says so.
Odd sizes. Recordings of a browser tab can be 1366×768 or 1536×864 with odd margins. The output keeps the size; the resize tool turns it into a standard 1080p or 720p afterwards if a platform insists.
Which browsers can do this
The conversion needs the browser to decode VP8 or VP9 through WebCodecs: Chrome, Edge, Brave, Opera and Firefox on computers and Android do. Safari decodes VP9 only on devices with hardware support, so some WebM files on a Mac or iPhone are refused with a message that says so; opening the page in Chrome on the same device is the fix. There is no server fallback, on purpose: a fallback that uploaded your recording would make the privacy promise false.
Good to know
- This page accepts WebM files only; drop an MP4, MOV or MKV and it offers to open the general converter with the file already loaded, without a reload.
- A WebM with AV1 video converts where the browser decodes AV1 (most current browsers on computers); very old graphics drivers may refuse.
- Recordings over about 1 GB need a computer in this version; the drop zone states the limit for your device.
- Converting is lossy by nature; keep the WebM if you may need to edit again, and treat the MP4 as the sharing copy.
Related tools and guides
Questions people ask
Why does my WebM say 0:00 and refuse to scrub, and will the MP4 be the same?
Browser recorders never write the final duration into the file. The converter measures the real length by scanning the file and writes a proper index into the MP4, so the result shows the right length and scrubs normally.
How long does the conversion take?
Usually three to five times faster than real time for 1080p on a recent laptop, so a 10-minute recording takes 2 to 3 minutes. Phones and older machines are slower; the estimate on the file card is based on your device.
Will the audio stay in sync?
Yes. Every video and audio timestamp from the WebM is carried into the MP4 rather than re-timed to a fixed frame rate, which is the usual cause of drift in paused or variable-rate recordings.
Why is this WebM refused in Safari?
Safari can only decode VP9 on devices with hardware support, and ClipHeron has no server to fall back to. Open the page in Chrome, Edge or Firefox on the same computer and it will work.
Does the MP4 keep the transparency of my WebM?
No. MP4 with H.264 has no transparency, so a transparent background becomes black and the result card tells you. Keep the WebM for anything that needs the alpha channel.
Can I make the MP4 smaller at the same time?
Tick “Also compress” before you press Convert and the file is handed to the compressor, where you name a target size in megabytes and the conversion and compression happen in one encode.
What if the file is not really a WebM?
The tool reads the file’s own bytes, not its name. A .webm that is really an MKV or MP4 is reported as such, with a button that opens it in the general converter, file already loaded.
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.
Indexing, decoding, encoding and muxing run in a background worker on your device. 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.
Sources and review
Page reviewed by the ClipHeron team · How it works · Changelog · Report a problem