Why does converting MP4 to WEBM take so long?

Because this is not a container swap, it is a full re-encode. The H.264 picture inside the MP4 is decoded and compressed again from scratch with VP9 or AV1. VP9, and AV1 in particular, use far more complex algorithms than H.264 and are noticeably heavier computationally. On long, high-resolution files it is normal for the process to take several minutes, and it cannot be rushed.

Does quality drop after the conversion?

Yes, some loss is unavoidable. H.264 is already a lossy codec, and when you decode that data and recompress it with VP9 you go through a second round of loss. With a good quality setting the loss stays at a level most viewers will not notice, but technically it is there and it cannot be undone. That is why it matters to convert from the best source you have and to keep the original MP4.

Will my file actually get smaller?

Usually yes, but it is not guaranteed. VP9 and AV1 can carry the same visual quality with less data than H.264, so with sensible settings you should expect a reduction. However, if the source MP4 was already compressed very aggressively, the gain will be limited. And if you pick an excessively high quality setting, the file can grow rather than shrink.

Can I upload my WEBM file to social media?

The large majority of platforms will not accept WEBM uploads, or will not process them reliably. WEBM's natural home is your own website; browsers play it without trouble. For videos heading to social media, a phone or a television you should stay with MP4. In other words, only do this conversion if your target is web publishing.

How to Convert an MP4 File to WEBM: A Guide for Website Video

When should you do this conversion?

Let us be clear up front: moving from MP4 to WEBM is not something everyone needs. MP4 is a format that opens everywhere, and in most cases the right move is to leave it alone.

There is one main scenario in which this conversion makes sense: when you are going to publish the video on your own website. Browsers play WEBM without trouble, and the VP9 or AV1 codecs inside WEBM can carry the same visual quality with less data than H.264. That means a faster-loading page, less bandwidth and video that starts sooner for viewers on a mobile connection.

If your video is going to social media, being sent to a phone or played on a television, do not do this conversion — WEBM most likely will not open there.

Three things to know before you start

This process is a re-encode. Some format conversions only change the container and leave the picture untouched. This is not one of them. The H.264 picture inside the MP4 will be decoded and compressed again from scratch with VP9 or AV1. That means some quality loss — unavoidably.

The process is slow. VP9 and especially AV1 encoding is computationally heavy. Even a short video takes a noticeable amount of time; on long, high-resolution recordings the process can run for minutes. This is not a performance problem, it is the nature of the codecs.

The better your source, the better the result. If you start from an MP4 that is already heavily compressed at a low bitrate, the WEBM version will be more degraded still. If you have a higher-quality original to hand, convert from that instead.

Step-by-step conversion

1. Work with the final version

Finish any cutting, joining or subtitle work before the conversion. Every extra conversion pass costs a little more quality; getting the order of operations right keeps the losses to a minimum.

2. Upload the file

Open our MP4 to WEBM converter and select or drag in your video.

3. Start the conversion and wait

Do not close the tab. Even if progress looks slow, work continues in the background. On long files you will need patience — this comes from the weight of the codec.

4. Download the output and test it

Your WEBM file will download once it is ready. Your original MP4 is unchanged, and you should not delete it.

Always check these after converting

Open it in a browser. The browser is WEBM's natural environment. Drag and drop the file into a browser and play it from start to finish.

Compare the visual quality. Open the original MP4 and the new WEBM side by side and compare them on a scene with movement. The places that stress quality hardest are fast motion, sharp edges and smooth colour gradients. If the difference is at a distracting level, try again with a higher quality setting.

Look at the file size. Your goal was to make the file smaller; did it actually shrink? If it did not, either the source was already heavily compressed or the quality setting was chosen too high. To work out which of the two it is, look at the bitrate of the original.

Verify audio sync. Check not only the beginning of the video but the middle and the end too. Sync drift usually grows as playback progresses.

Publishing on your site: serve both files

This is the most practical thing anyone working with WEBM should know. Moving to WEBM does not mean you have to give up MP4.

HTML's video tag lets you define more than one source file. The browser tries the list in order and plays the first file it can open. If you put WEBM first and MP4 second as a fallback, modern browsers pull the small WEBM while the exceptions that cannot open WEBM fall back to MP4. Nobody is left without video.

The price of this is hosting both files and running two separate encodes. For small sites with only a handful of videos it may not be worth the trouble; on video-heavy or high-traffic sites the gain is real.

Common problems

The file came out bigger than I expected. The quality setting may have been chosen too high. Or the source MP4 was already compressed very aggressively and there was no room left for VP9 to gain anything. In the second case this conversion is not doing you any good; staying with MP4 makes more sense.

The video does not play on some devices. This is WEBM's known limit. Phone galleries, smart televisions and most editing programs do not recognise WEBM. If you expect the file to work in those places, you have picked the wrong format.

Quality is visibly degraded. Your source was probably already compressed, and the second round of compression made the artefacts visible. If you have a higher-quality original, repeat the conversion from that.

The process is taking forever and never finishes. On long, high-resolution files this is normal. Shortening the video or lowering the resolution reduces the processing time noticeably.

In summary

Moving from MP4 to WEBM is meaningful for web publishing and mostly meaningless anywhere else. What you gain is a smaller file and a faster-loading page; what you pay is a long encoding time and a small but real quality loss. Before you make that trade, ask yourself one thing: is this video really only going to be watched in a browser? If the answer is yes, the conversion makes sense. If the answer is no, or if you are not sure, you already have an MP4 that works everywhere — leaving it alone is a valid decision too.