Free online MP4 to WEBM converter

A website, CMS, or HTML5 player may require a .webm upload or use WebM as an efficient browser video source. The conversion also makes sense when VP9 or AV1 can reduce delivery bitrate compared with an H.264 MP4 at similar visual quality.

What MP4 is

MP4 is a multimedia container format based on MPEG-4 Part 14, usually identified by the .mp4 extension. A container can hold video, audio, subtitle tracks, chapters, and metadata; MP4 does not specify one mandatory video codec.

Most MP4 files use H.264/AVC video with AAC audio, while newer camera and phone files may use H.265/HEVC. Phones, cameras, screen recorders, video editors, streaming services, and social platforms commonly produce MP4 because H.264/AAC playback is widely supported by devices, browsers, editors, and media players.

What WebM is

WebM is an open container format intended for web media, normally using the .webm extension. Standards-compliant WebM commonly contains VP8, VP9, or AV1 video with Opus or Vorbis audio.

VP9 and AV1 can often use less bitrate than H.264 for comparable quality, but their encoding time is usually longer. Actual file size depends on resolution, frame rate, source material, encoder settings, and the source codec; an efficiently encoded HEVC MP4 may be as small as, or smaller than, a poorly configured WebM file.

Current Chrome, Firefox, Edge, Opera, and Android browsers support WebM, but codec support differs by browser and operating-system version. VP9 is generally the safer WebM choice; AV1 support is less consistent on older devices. H.264/AAC MP4 remains the safer single format for older browsers, smart TVs, legacy editing software, and older Apple systems.

Convert an MP4 file to WebM

To convert without installing software, upload the MP4 on 101convert.com, select WebM as the output format, start the conversion, and download the resulting .webm file. Test the downloaded file in the browser, player, or upload system that requires it before replacing the original.

For local conversion and codec control, FFmpeg can create a VP9/Opus WebM file:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 32 -b:v 0 -pix_fmt yuv420p -c:a libopus -b:a 128k output.webm

With VP9, -crf 32 selects constant-quality encoding and -b:v 0 prevents FFmpeg from imposing a target video bitrate. Lower CRF values increase quality and file size; values from about 24 to 36 are practical starting points. The yuv420p pixel format improves playback compatibility, particularly for standard dynamic-range sources.

For AV1 output, use an FFmpeg build that includes libaom-av1:

ffmpeg -i input.mp4 -c:v libaom-av1 -crf 32 -b:v 0 -cpu-used 6 -c:a libopus -b:a 128k output.webm

Higher -cpu-used values encode faster at the cost of compression efficiency. AV1 is usually slower than VP9, so use it only when the intended players support it or when lower delivery bitrate justifies the additional encoding time.

Current HandBrake releases can also create WebM files. Open the MP4, select Summary → Format → WebM, select VP9 or AV1 under Video, select Opus under Audio, set a destination ending in .webm, and choose Start Encode.

Quality and compatibility limits

Changing a typical H.264 or HEVC MP4 into VP8, VP9, or AV1 requires video re-encoding; changing AAC audio to Opus or Vorbis usually requires audio re-encoding as well. Re-encoding is lossy, so retain the original MP4 and avoid repeated conversions between lossy formats.

MP4 subtitle tracks, chapters, HDR metadata, and surround-audio layouts may not transfer as expected. Check captions, audio synchronization, color, and playback after conversion; HDR sources may require separate tone-mapping or color-management settings for standard dynamic-range WebM delivery.

When broad web compatibility is required, publish the WebM file alongside an H.264/AAC MP4 fallback rather than using WebM as the only source. Use constant-quality encoding for best visual results; use a bitrate target or two-pass encoding only when an exact output-size limit is required.