How do I convert a WebM to MP4?

Choose a file, set Output to MP4 and Codec to H.264, then press Convert once the browser check passes. Keep retains sound when the source has it; Opus or Vorbis audio is re-encoded as AAC at 128 kbps with at most two channels. The finished file has a Download link. Renaming .webm to .mp4 alone does not change the container or its codecs.

How do I convert MP4 to WebM, VP8 or VP9?

Open the MP4, set Output to WebM and choose VP9, VP8 or AV1 under Codec. Leave Size on Original to keep its dimensions, or choose a smaller available size. Set Audio to Remove audio for a silent WebM, or Keep to retain its sound as Opus when re-encoding is needed. Press Convert after the browser check passes.

Is this a free converter, and is my video uploaded?

It is free browser software with no account or installation required. The file is read and converted in this tab, and the browser downloads the result locally. Only the output settings are remembered in this browser, never the video itself. Start over at the top of the page forgets those settings.

Which files can it convert?

MP4, MOV, WebM, MKV and MPEG-TS videos up to 250 MB, plus animated GIFs with at most 500,000 frames and 67,108,864 pixels, the area of 8192 x 8192. The actual container is read from the file's bytes. AVI, MPG, FLV, WMV, still images and single-frame GIFs are refused with a reason; audio-only files belong in the Audio Converter. Only MP4 and WebM are written here, so this is not a 3GP or Ogg converter.

Can I convert on Android, iPhone, Mac, Windows or Linux?

Open the page in a browser on Android, iPhone or iPad with iOS, Mac or macOS, Windows, Linux or Ubuntu. Conversion depends on that browser's codecs and available memory. Before Convert is enabled, the page checks the source decoder and selected encoder at the requested size. A missing codec produces an explanation and available alternatives; this is a browser tool, with no native app, APK or desktop download.

Why does it say my browser cannot write this codec?

Browsers expose different encoders. Unsupported codec buttons are struck through, and the message suggests a supported codec, the other format or a smaller size. If MP4 audio needs re-encoding but AAC is unavailable, try WebM with Opus or Remove audio. A stream that already fits the chosen format may be copied without an encoder. The page never silently substitutes another codec.

Can it convert a 4K WebM to MP4 at high quality?

Keep Size on Original for the source dimensions, including a 3840 x 2160 video. A different codec is re-encoded at high quality, subject to your browser supporting that codec at that size. There is no promise of identical pixels, a smaller file or a particular speed. Choose a smaller size if the browser cannot encode the original dimensions.

Is WebM to MP4 or MP4 to WebM lossless?

A matching video codec at Original size can be copied into a compatible container, such as AV1 from WebM to MP4 when AV1 is selected. Sound may still need re-encoding. VP9 or VP8 to H.264, and H.264 to VP9 or VP8, re-encode the picture, so those conversions are not lossless. A stream copy also requires compatible timestamps and rotation.

Does a GIF keep its timing when it becomes a video?

Each frame keeps its own delay and disposal. For example, delays of 100, 200 and 300 ms make a 0.600 s video with frames beginning at 0.000, 0.100 and 0.300 s. Plays repeats the animation once, 2, 3 or 5 times inside the video, within the frame limit. Transparent parts use the white or black Behind colour; the output does not preserve alpha transparency.

Will the sound stay in sync?

The result compares the start of sound against picture with the original offset and reports the difference in milliseconds. For AAC output, the page measures the browser's encoder delay and writes an MP4 edit list to skip it. If that delay cannot be measured, the result says the start was not checked. A file's length and start offset are checks, rather than a guarantee about every player's playback.

Can I stop a conversion partway through?

Cancel stops the work and discards its output. The source remains open, so you can change settings and convert again without choosing it a second time.

Can I convert 2 GB files or use a WebM link?

No. Choose a local file no larger than 250 MB; there is no URL fetcher or unlimited-size mode. The output is assembled in this tab's memory, so the file bound also applies on a powerful computer.

Does the converter work offline?

The file processing is local, but the page and conversion library must load first. The library loads when a supported file is opened, so opening this page without a connection is not a guaranteed offline workflow. Keep a connection available while loading the page and opening the file.

Video Converter

Convert a local WebM to MP4, an MP4 to WebM, or an animated GIF to video. Choose the output format, codec, size and audio below; the page checks what your browser can handle before conversion and reads the finished file back. Your file stays in this tab.

Open a video or an animated GIF

· or drop one anywhere on the stage

Never uploadedNo accountUp to 250 MBReads MP4, MOV, WebM, MKV and GIF

Output

Codec

Size

Audio

Download

Ready. Choose a video or an animated GIF, or drop one on the stage.

Three-frame GIF, delays 100, 200 and 300 msGIF to MP4, played once
0.600 s long, frames start at 0.000, 0.100 and 0.300 s
WebM with VP9 video and Opus audioto MP4
H.264 + AAC, the AAC encoder's start-up delay measured and skipped, audio start checked to one frame, 33.3 ms at 30 fps
An AVI, an MPG, or a codec this browser cannot encodechecked before any work
refused with the reason; nothing is converted or uploaded

Output picks the file: an MP4 is written with H.264, HEVC or AV1 video, and a WebM with VP9, VP8 or AV1. A stream whose codec the new file already holds, kept at its own size, is copied without re-encoding, so an H.264 MOV becomes an MP4 with its picture untouched; anything else is decoded and encoded again by your browser at a high quality bitrate chosen for its size. Sound is copied where the format holds it (AAC or MP3 in an MP4, Opus or Vorbis in a WebM) and re-encoded at 128 kbps otherwise. Size brings the shorter side down to the number of pixels chosen and never enlarges. A GIF keeps each frame's own delay and its disposal, plays through as many times as Plays says, and gets the Behind colour where it is transparent, because a video has no transparency. When the work ends, the file's first bytes are checked against its extension, then it is read back: its codecs and size must match the plan or it is not offered, and its length and the start of its sound are measured against the source.

Accuracy. Re-encodes only with codecs your browser can decode and encode, copies a stream untouched only where the new format already holds it, and refuses a file or codec it cannot handle, AVI and MPG included, with the reason before any work starts, never sending it to a server. Animated GIF frame timing and disposal are kept, with delays of 0 and 10 ms played as 100 ms as browsers show them, and every finished file is read back: its first bytes must match its extension, and its length and the start of its sound are measured against the source, with the result saying whether each lands within one frame. Files up to 250 MB are converted on your device, and a re-encoded video is close to the original rather than identical to it.

The encoding is done by your browser, and the file handling by one open source library served from this site. Containers are read and written by Mediabunny 1.55.7 under the MPL-2.0 (licence text, the exact package, source), bundled unmodified. Video and sound are decoded and encoded by the WebCodecs codecs built into your browser, which is why the codecs on offer depend on it. GIFs are decoded by this page itself.

Common questions

How do I convert a WebM to MP4?: Choose a file, set Output to MP4 and Codec to H.264, then press Convert once the browser check passes. Keep retains sound when the source has it; Opus or Vorbis audio is re-encoded as AAC at 128 kbps with at most two channels. The finished file has a Download link. Renaming .webm to .mp4 alone does not change the container or its codecs.
How do I convert MP4 to WebM, VP8 or VP9?: Open the MP4, set Output to WebM and choose VP9, VP8 or AV1 under Codec. Leave Size on Original to keep its dimensions, or choose a smaller available size. Set Audio to Remove audio for a silent WebM, or Keep to retain its sound as Opus when re-encoding is needed. Press Convert after the browser check passes.
Is this a free converter, and is my video uploaded?: It is free browser software with no account or installation required. The file is read and converted in this tab, and the browser downloads the result locally. Only the output settings are remembered in this browser, never the video itself. Start over at the top of the page forgets those settings.
Which files can it convert?: MP4, MOV, WebM, MKV and MPEG-TS videos up to 250 MB, plus animated GIFs with at most 500,000 frames and 67,108,864 pixels, the area of 8192 x 8192. The actual container is read from the file's bytes. AVI, MPG, FLV, WMV, still images and single-frame GIFs are refused with a reason; audio-only files belong in the Audio Converter. Only MP4 and WebM are written here, so this is not a 3GP or Ogg converter.
Can I convert on Android, iPhone, Mac, Windows or Linux?: Open the page in a browser on Android, iPhone or iPad with iOS, Mac or macOS, Windows, Linux or Ubuntu. Conversion depends on that browser's codecs and available memory. Before Convert is enabled, the page checks the source decoder and selected encoder at the requested size. A missing codec produces an explanation and available alternatives; this is a browser tool, with no native app, APK or desktop download.
Why does it say my browser cannot write this codec?: Browsers expose different encoders. Unsupported codec buttons are struck through, and the message suggests a supported codec, the other format or a smaller size. If MP4 audio needs re-encoding but AAC is unavailable, try WebM with Opus or Remove audio. A stream that already fits the chosen format may be copied without an encoder. The page never silently substitutes another codec.
Can it convert a 4K WebM to MP4 at high quality?: Keep Size on Original for the source dimensions, including a 3840 x 2160 video. A different codec is re-encoded at high quality, subject to your browser supporting that codec at that size. There is no promise of identical pixels, a smaller file or a particular speed. Choose a smaller size if the browser cannot encode the original dimensions.
Is WebM to MP4 or MP4 to WebM lossless?: A matching video codec at Original size can be copied into a compatible container, such as AV1 from WebM to MP4 when AV1 is selected. Sound may still need re-encoding. VP9 or VP8 to H.264, and H.264 to VP9 or VP8, re-encode the picture, so those conversions are not lossless. A stream copy also requires compatible timestamps and rotation.
Does a GIF keep its timing when it becomes a video?: Each frame keeps its own delay and disposal. For example, delays of 100, 200 and 300 ms make a 0.600 s video with frames beginning at 0.000, 0.100 and 0.300 s. Plays repeats the animation once, 2, 3 or 5 times inside the video, within the frame limit. Transparent parts use the white or black Behind colour; the output does not preserve alpha transparency.
Will the sound stay in sync?: The result compares the start of sound against picture with the original offset and reports the difference in milliseconds. For AAC output, the page measures the browser's encoder delay and writes an MP4 edit list to skip it. If that delay cannot be measured, the result says the start was not checked. A file's length and start offset are checks, rather than a guarantee about every player's playback.
Can I stop a conversion partway through?: Cancel stops the work and discards its output. The source remains open, so you can change settings and convert again without choosing it a second time.
Can I convert 2 GB files or use a WebM link?: No. Choose a local file no larger than 250 MB; there is no URL fetcher or unlimited-size mode. The output is assembled in this tab's memory, so the file bound also applies on a powerful computer.
Does the converter work offline?: The file processing is local, but the page and conversion library must load first. The library loads when a supported file is opened, so opening this page without a connection is not a guaranteed offline workflow. Keep a connection available while loading the page and opening the file.

Re-encodes only with codecs your browser can decode and encode, copies a stream untouched only where the new format already holds it, and refuses a file or codec it cannot handle, AVI and MPG included, with the reason before any work starts, never sending it to a server. Animated GIF frame timing and disposal are kept, with delays of 0 and 10 ms played as 100 ms as browsers show them, and every finished file is read back: its first bytes must match its extension, and its length and the start of its sound are measured against the source, with the result saying whether each lands within one frame. Files up to 250 MB are converted on your device, and a re-encoded video is close to the original rather than identical to it.