WebM to MP4 Converter - Convert WebM Videos to MP4 Online Free

By the team at VidHaul

A WebM to MP4 converter fixes a very common problem: a video plays perfectly in your browser but nowhere else. WebM is a video format made for the web. Recordings made in the browser and many videos saved from websites come as .webm files. Outside the browser, things go wrong: the Windows Photos app may refuse the file, your iPhone may not show it, an editor may not import it and a smart TV may skip it. Converting WebM to MP4 with H.264 video makes it play almost everywhere. This page lets you convert WebM to MP4 free and online, but without uploading: the conversion runs on your own device. Below we explain what is inside a WebM file, exactly what the converter does, why the MP4 is often bigger than the WebM, and what to do when you need the other direction.

How does the WebM to MP4 converter work?

The converter opens the WebM container with the open-source Mediabunny library and decodes it with WebCodecs, the video technology built into your browser. Then:

• Video (usually VP8 or VP9) is re-encoded to H.264 at Mediabunny’s medium (“balanced”) quality setting. The width, height, frame rate and length stay the same. The converter only copies video unchanged when it is already in the output codec (H.264 for MP4), and WebM recordings use VP8 or VP9, so MP4 output is always a re-encode.
• Sound (usually Opus or Vorbis) is re-encoded to AAC, the sound format MP4 players expect.
• Both are written into an MP4 file with the index at the start, so it can begin playing before it is fully loaded.

Worked example (computed). For a 1920×1080 recording, Mediabunny’s medium quality gives the H.264 encoder a target of about 3,227 kbps. A 5-minute 1080p screen recording therefore becomes about 3,227 × 300 ÷ 8 = 121 MB of video, plus sound. The same library would aim for about 1,937 kbps if the output were VP9, because it rates VP9 as needing about 60% of H.264’s bitrate. That difference is the main reason an MP4 can be larger than the WebM it came from. Real sizes vary with the content: a mostly still screen recording usually comes out well under the target.

What is a WebM file?

WebM is an open video format from the WebM Project. A .webm file is a container, a “box” based on Matroska, holding video compressed with VP8 or VP9 and sound compressed with Vorbis or Opus. Newer WebM files can also hold AV1 video. These formats are free to use, which is why browsers support them well and why browser-based recorders often save WebM.

Why won’t my WebM video play?

Most players, phones, TVs and editors were built around H.264 video and AAC sound in an MP4 file. They may open the WebM box but not understand the VP8, VP9 or Opus streams inside, or refuse the file completely. The fix is to change both the box and the codecs: MP4 with H.264 and AAC, which is exactly what this converter produces.

How to convert WebM to MP4 online for free

1. Choose your file. Click the box at the top of the page or drag the .webm file onto it. You see a thumbnail, the file size and the resolution.
2. Check the output. MP4 is selected by default. To pick MOV or MKV instead, open More options under the settings.
3. Press Start. A progress bar shows the percentage done. You can press Cancel at any time.
4. Download. When it finishes you see the size before and after. Click Download, or Convert another file to keep going.

Convert WebM to MP4 on any device

On Windows

Open this page in a current Chrome, Edge or Firefox. There is no converter download to install and no codec pack to find. Drop the file in, press Start and the MP4 plays in the standard Windows player.

On Mac

Use Safari, Chrome or Firefox. The MP4 opens in QuickTime Player and imports into Mac editors.

On Android

Open the page in Chrome, tap the box and choose the WebM from your files or downloads. Keep the screen on while it converts.

On iPhone

Open the page in Safari, tap the box and pick the WebM from the Files app. The MP4 is saved to Files, from where you can save it to Photos. Phones are slower than laptops and everything happens in the phone’s memory, so big recordings are better converted on a computer.

Will the MP4 be bigger than the WebM?

Often, yes, and that is expected. VP9 squeezes video more efficiently than H.264, so at a similar quality the H.264 file needs more bits. These are the targets the library gives the encoder at the medium quality the converter uses:

If you need the MP4 smaller for WhatsApp, email or Discord, run it through the video compressor afterwards, which can aim for a size in MB.

Converting screen and meeting recordings

Screen recordings often show emails, chats, dashboards or client work, so uploading them to an unknown converter is a real privacy risk. Here the file never leaves your device. The converter keeps the original resolution, so a 1080p recording stays 1080p and small text stays as sharp as the encoder allows. After converting, you can cut the start and end with the video cutter, crop away a toolbar or second monitor with the video cropper, or keep only the sound of a meeting with video to MP3.

Do you need FFmpeg or VLC to convert WebM to MP4?

No. Many guides show how to convert WebM to MP4 with FFmpeg on the command line or through VLC’s Convert menu. Both work well, but you have to install them, and FFmpeg means typing commands and choosing codec options. If you only want an MP4 that plays, this page is simpler: the same kind of re-encoding to H.264 and AAC happens in your browser, with no install. FFmpeg is still the better tool for batch jobs or special settings, which this page does not offer.

How to convert MP4 to WebM

This page accepts WebM files only. To go the other way, open the video cutter, choose your MP4, leave Start and End at 0 to keep the whole video, open More options and pick WebM. The video is re-encoded to VP9 with Opus sound. The video compressor can also output WebM if you want it smaller at the same time.

Converting large WebM files

There is no fixed size limit, because nothing is uploaded. The conversion runs in your device’s memory, so files over about 1.5 GB can be slow or fail on a phone, and the page warns you when a file is that large. A laptop or desktop handles long recordings better. Speed depends on your device and the video’s resolution and length, not on your internet connection.

How accurate is the WebM to MP4 converter?

• Resolution, frame rate and length are kept. The converter does not scale or trim.
• It is always a re-encode. VP8 and VP9 can’t go into a widely compatible MP4 as they are, so there is a small, usually invisible quality loss and it takes longer than a copy.
• The size is not fixed. The bitrates above are targets. Still screens often come out much smaller; fast motion can come out larger.
• Some files won’t open. If your browser can’t decode a codec in the file, for example AV1 on an older device, the page tells you instead of failing silently.
• Transparency is not kept. WebM can carry see-through video; H.264 MP4 cannot, so transparent areas are not preserved.

Why this tool is different

Typical online WebM converters upload your file, convert it on a server and send it back. This one runs the whole conversion in your browser using WebCodecs and the open-source Mediabunny library. Your recording is read from your device, converted in memory and saved back to your device, so there is no upload wait and no copy of your file on anyone else’s server. Once the page has fully loaded, the conversion needs no connection.

We also tell you what really happens to your file. The converter always re-encodes to H.264, the MP4 is often bigger than the WebM because H.264 is less efficient than VP9, and transparency is not kept. The numbers on this page come from the same formula the converter uses, so you can compare them with the before and after sizes the tool shows, which Copy summary lets you keep.

What we measure

When you choose a file, the tool reads its size, duration, width and height to show them and to check that it can be opened. The encoder’s bitrate target is computed by the Mediabunny library from the resolution. After converting, the tool shows the size before and after. It does not measure picture quality, does not look at what your recording shows and does not send any information anywhere.

Only convert videos you own or have the right to use.

Sources

• WebM Project: Frequently Asked Questions, on what WebM files contain.
• MDN: Media container formats (WebM and MP4).
• MDN: Web video codec guide (VP8, VP9, AV1 and H.264).
• Mediabunny: Converting media files, the library that runs the conversion.
• MDN: WebCodecs API.