MP4 to WebM Converter
Get a WebM copy of an MP4 with VP9 video and Opus audio, ready for an HTML5 video tag or any upload form that wants an open format. The encoder favors speed over compression, so don't count on the WebM being smaller than the MP4.
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Why Convert MP4 to WebM
MP4 files almost always carry H.264 or H.265 video with AAC audio, codecs covered by patent licensing pools. WebM is the open alternative backed by Google: a trimmed-down Matroska container limited to royalty-free codecs, VP8, VP9 or AV1 for video and Vorbis or Opus for audio. Some sites, open-source projects and HTML5 players prefer or require WebM, and many web pages offer it next to an MP4 so each browser can choose.
MiConvert re-encodes the whole video on its servers with FFmpeg. Video goes to VP9 through libvpx in constant-quality mode (CRF 32, the libvpx default when no bitrate is set), using the real-time deadline and the fastest speed setting with row-based multithreading. Audio is encoded to Opus at about 96 kbps for stereo and resampled to 48 kHz where needed.
The resolution and frame rate of the MP4 are kept. The picture is converted to standard 8-bit 4:2:0 color, so 10-bit or HDR sources lose their extra depth. One video track and one audio track are carried over; subtitle tracks and data tracks such as timecode are removed. The fast settings are the trade-off: VP9 can beat H.264 on size at equal quality when it is given time, but at maximum speed the WebM may come out about the same size as the MP4 or even larger. If a smaller file is the goal, MiConvert's Compress MP4 tool is the better starting point.
This page suits developers adding a WebM source to a video element, people uploading to services that ask for WebM, and anyone looking for a general video to WebM converter: besides MP4, the upload box takes MOV, MKV, AVI, FLV, WMV and 3GP files and converts them with the same settings.
How to Convert MP4 to WebM
- Make sure the MP4 plays all the way through on your computer. Interrupted downloads and split parts often lack the index an MP4 needs, and those can't be read.
- Trim it first if you only need part of the video, since the whole file is re-encoded and shorter input finishes sooner.
- Upload the file. Guests can send up to 50 MB, free accounts 100 MB, Plus 200 MB and Pro 1 GB, with up to 10 files per batch on the free tiers.
- Convert. Short clips take a few seconds; long or high-resolution videos can take half a minute or more.
- Download the .webm and compare its size and quality with the MP4 before replacing anything.
- On a website, list the WebM and the MP4 as two sources in the same video element so every browser finds one it can play.
What You Get in the WebM
- VP9 video encoded with libvpx at CRF 32
- Opus audio, about 96 kbps for stereo
- Original resolution and frame rate
- 8-bit 4:2:0 color for broad playback
- Subtitle and timecode tracks removed
- Also accepts MOV, MKV, AVI, FLV, WMV and 3GP uploads
Video Tutorial
Watch this step-by-step tutorial to learn how to convert MP4 files to WEBM format online for free using MiConvert.com. No software installation required.
MP4 to WebM FAQ
Will the WebM be smaller than my MP4?
Not reliably. VP9 is the more efficient codec, but the converter runs it at its fastest settings, and a well-compressed MP4 can match or beat the WebM on size. Compare both files and keep whichever suits you.
Which codecs does the WebM use: VP8, VP9 or AV1?
VP9 for video and Opus for audio. VP8 and AV1 are not offered on this page.
Why did my MP4 fail with 'Invalid data found when processing input'?
The MP4 is incomplete or damaged, which is the most common failure here. Files named like 'Part-1', downloads that stopped early and recordings from an app that crashed usually lack the index MP4 keeps at the start or end of the file. Download or export the complete video again and check that it plays from start to finish.
What does 'Output file does not contain any stream' mean?
FFmpeg found nothing it could put into a WebM: no usable video or audio track. This happens with MP4s that hold only data or subtitle tracks, or with a video codec FFmpeg can't decode. Open the file in a media player; if it shows no picture and plays no sound there either, the source is the problem.
Are subtitles kept?
No. Subtitle tracks are dropped from the WebM. For captions on a web page, keep them as a separate WebVTT file and attach it with a track element.
Does WebM play on iPhone and Safari?
Current Chrome, Firefox and Edge play VP9 WebM. Apple added WebM support to Safari later than other browsers, and older iPhones and Macs may not play it, so keep the MP4 as a fallback source.
Is there a file size limit?
Yes. A single file can be up to 50 MB without an account and 100 MB with a free one; Plus lifts that to 200 MB and Pro to 1 GB.
Do you keep a copy of my video?
Not a lasting one. The uploaded MP4 is deleted right after conversion, and the WebM is held only for the roughly four-hour download period before it is removed automatically.