What is WebM, and what is MP4?
MP4 and WebM are both containers — boxes that hold a video track and one or more audio tracks. The box is not the video. What matters is which codecs each box is allowed to carry, and that is where these two part ways. Converting MP4 to WebM is almost never a rename; it is usually a full re-encode, and knowing why saves you a lot of confusion when the job takes four minutes instead of four seconds.
MP4
MP4 is the format your phone records in, your camera writes, and every editor exports by default. Inside, it nearly always holds H.264 video and AAC audio. That pairing has been the safe choice since roughly 2005, and it plays on everything — televisions, ancient tablets, the projector in a meeting room nobody has updated since. H.264 is also patent-encumbered, which is exactly the problem the web wanted to route around.
WebM
WebM is a trimmed-down profile of Matroska, built for the open web, with a deliberately short codec list: VP8, VP9, or AV1 for the picture, and Opus or Vorbis for the sound. Nothing else is allowed in. H.264 and AAC — the two things your MP4 almost certainly contains — are on neither list. So the picture genuinely has to be rebuilt as VP9 and the sound re-encoded as Opus. That short list is the point: every codec WebM accepts is royalty-free, which is why browsers ship decoders for all of them and why a WebM in a <video> tag plays without anyone paying a licensing body.
How MP4 to WebM conversion works
Since the picture is rebuilt rather than copied, the finished size cannot be read off the source file. It has to be predicted from the encoder settings, and the model every encoder uses is bits per pixel per frame.
VP9 bitrate = width × height × frames per second × 0.04 bits — then clamped to a floor of 200 kbps and a ceiling of 12 Mbps.
Work it through on a real file. Say promo.mp4 is 61 MB, runs 2:00, is 1920×1080 at 30 fps, and holds H.264 video with stereo AAC audio:
- 1920 × 1080 = 2,073,600 pixels per frame
- × 30 frames per second = 62,208,000 pixels per second
- × 0.04 bits = 2,488,320 bps, or about 2.49 Mbps of VP9 video
- + 128,000 bps of stereo Opus = 2,616,320 bps total
- × 120 seconds ÷ 8 = 39,244,800 bytes — 37.4 MB
61 MB in, 37.4 MB out: about 39% smaller, and that is the number the tool puts in front of you before you commit to anything. The 0.04 constant is not arbitrary. It puts 1080p30 at 2.49 Mbps and 720p30 at 1.11 Mbps, which is roughly where YouTube's own VP9 ladder sits — and YouTube re-encodes its entire library into VP9 precisely because it needs around 40% fewer bits than H.264 for a picture that looks the same.
Common MP4 to WebM conversions by resolution
Here is what the MP4 to WebM converter picks for the resolutions people actually feed it. The per-minute column assumes one stereo Opus track at 128 kbps alongside the video.
| Source video | VP9 bitrate | WebM per minute |
|---|---|---|
| 640×360 · 30 fps | 0.28 Mbps | 2.9 MB |
| 854×480 · 30 fps | 0.49 Mbps | 4.4 MB |
| 1280×720 · 30 fps | 1.11 Mbps | 8.8 MB |
| 1920×1080 · 30 fps | 2.49 Mbps | 18.7 MB |
| 1920×1080 · 60 fps | 4.98 Mbps | 36.5 MB |
| 2560×1440 · 30 fps | 4.42 Mbps | 32.6 MB |
| 3840×2160 · 30 fps | 9.95 Mbps | 72.1 MB |
| 3840×2160 · 60 fps | 12.00 Mbps (capped) | 86.7 MB |
Two patterns fall out of that table. Doubling the frame rate doubles the bitrate, which is why a 60 fps screen recording costs twice what the same footage at 30 fps would — worth remembering before you record. And 4K60 is the one row that hits the ceiling: the formula asks for 19.9 Mbps, the encoder is held to 12, and the result is a file that is smaller than the math predicts but softer in fast motion. If you are shipping 4K60 to the web, scale it down first and you will get a better-looking file than the cap will give you.
The second thing worth knowing is which parts of your file get rebuilt and which just move across. Not everything inside an MP4 is treated the same way.
| What's inside your MP4 | What happens | Why |
|---|---|---|
| H.264 or H.265 video | Re-encoded to VP9 | WebM has no slot for either |
| VP9, VP8, or AV1 video | Copied untouched | Already a legal WebM payload |
| MPEG-4 Part 2, MJPEG, MPEG-2 | Refused up front | No browser can decode them |
| AAC, MP3, Dolby Digital, FLAC, PCM | Re-encoded to Opus | Not on WebM's audio list |
| Opus or Vorbis audio | Copied untouched | Already legal |
| DTS, Apple Lossless | Left out, and the plan says so | No browser can decode them |
An MP4 holding VP9 already — some do — skips the re-encode entirely. The bytes are identical on the other side, so the tool reports the same size rather than inventing a saving it did not make.
When you'll need to convert MP4 to WebM
Putting video on a page you own. A VP9 file in a <video> tag downloads faster than the H.264 equivalent, and every browser that matters plays it. On a hero video or a background loop, 39% fewer bytes is the difference between a page that settles immediately and one that stutters on a phone connection.
Getting under an upload cap. Plenty of platforms accept WebM specifically because the files are small, and plenty of forms have a hard megabyte limit that your 61 MB clip misses by a little. Converting is the cheapest fix available.
Short looping clips and animated stickers. Several platforms take WebM for exactly this, since a few seconds of VP9 costs almost nothing.
Getting out of a licensing question. If you are shipping video inside a product and you would rather not think about H.264 patent pools at all, VP9 and Opus are royalty-free the whole way down.
Edge cases and gotchas
Safari can't do it. WebM encoding needs WebCodecs with a VP9 encoder behind it, and Safari ships neither reliably. It plays plenty of WebM files but won't make one. On an iPhone or iPad every browser is Safari underneath, so the limitation follows you across apps. Chrome, Edge, and Firefox all work, on desktop and on Android.
It takes about as long as the video is long. Every frame gets decoded and encoded again. A ten-minute clip is a ten-minute job, more on an older laptop. Keep the tab open; you can cancel at any point and nothing half-finished is left behind.
An odd dimension loses a pixel. VP9 stores colour at half resolution in both directions — the 4:2:0 arrangement nearly every consumer codec uses — so both dimensions must be even. A 1919-pixel-wide source becomes 1918. You will not see it, but the tool tells you rather than changing your video's dimensions quietly.
Surround gets mixed to stereo. WebM can carry multichannel Opus in principle, but browsers only encode it dependably in two channels. An honest downmix beats a silent centre channel.
Some files come out bigger. If a clip has already been squeezed hard — compressed down to a trickle by someone else — re-encoding at a sensible VP9 bitrate can add bytes. The estimate will say "about 12% larger" instead of pretending otherwise. Read it before you click.
The limits are real, and deliberately modest. Up to 2 GB in any browser, up to 4 GB in desktop Chrome and Edge, which can write the WebM straight to a file on your disk instead of holding it in memory. Length is capped at two hours. A full re-encode of a four-hour file inside a browser tab is a promise that can't be kept honestly — for that, a desktop encoder like HandBrake is the right answer, and saying so costs nothing.
Why this one runs in your browser
Most online video converters work the same way: you upload the file, it lands on someone's server, you wait in a queue, and then the interesting part arrives. A cap at 100 MB unless you upgrade. A watermark unless you upgrade. Two conversions a day, a countdown on a trial you didn't ask to start, a subscription that renews before you remember signing up. The video itself — which might be a client's unreleased cut, or your own face — sits on infrastructure you know nothing about while all of that happens.
None of that is a technical necessity. It's a business model wearing a technical costume. Your browser already ships a VP9 encoder; the only reason to send a 2 GB file across the internet and back is that somebody wants a relationship with you. This tool reads the MP4 off your disk in pieces, encodes it on your machine, and hands you the WebM. Open the network tab and watch — nothing is sent. That's also why the size limits here are your laptop's rather than a bandwidth budget's, and why there's no account, no queue, and no watermark to remove.
Microapp gives 10% of every dollar it earns to charity, off the top, audited quarterly. That's the whole business model on the video tools: they work, they're honest about what they can't do, and nothing about them is a funnel.
Related video and audio tools
Going the other direction, or between formats that hold the same codecs? MOV to MP4 and MKV to MP4 are rewraps rather than re-encodes — the same video bytes get a new container, and they finish in the time it takes to read the file, which is why they're so much faster than this one.
If you only want the sound, skip the video entirely: MP4 to MP3 pulls a shareable audio file out of a video, and MP4 to WAV gives you uncompressed audio for editing. GIF to MP4 handles the opposite problem — an animated GIF that's ten times the size it needs to be.
For still images, PNG to WebP applies the same idea to pictures, and the image compressor handles bulk. Planning a resize before you convert? The aspect ratio calculator gets the even dimensions right the first time.
Frequently asked questions
Is my video uploaded anywhere?
No. The page reads the MP4 off your disk in small pieces and builds the WebM on your device using the encoder your browser already ships. You can open the network tab and watch while it runs — nothing leaves the tab. That's also what makes big files practical, since there's no bandwidth bill to ration.
Will the WebM be smaller than the MP4?
Usually, and often by a lot — a 1080p30 clip typically lands around 39% smaller. VP9 is a generation newer than H.264 and needs roughly 40% fewer bits for the same-looking picture. The exception is a file that was already compressed hard, where a sensible VP9 bitrate can make it larger. The estimate on the plan is computed from your video's actual pixel count and frame rate, so check it before converting.
Why is this slower than converting MOV to MP4?
Because MOV to MP4 isn't really a conversion and this one is. MOV and MP4 are close cousins holding the same codecs, so moving between them just rewraps the existing bytes. WebM accepts only VP8, VP9, or AV1, and your MP4 almost certainly holds H.264 — so every frame has to be decoded and encoded again. Budget roughly the length of the video.
Why does my browser say it can't encode VP9?
That's Safari, including every browser on iPhone and iPad, which are all Safari underneath. It can play WebM but can't create one. Open the page in Chrome, Edge, or Firefox on a computer or an Android phone and it will work.
What happens to the audio?
It becomes Opus — the codec WebM is built around, and one of the best-sounding options under about 160 kbps. AAC, MP3, Dolby Digital, FLAC, and uncompressed PCM all re-encode to Opus at 128 kbps in stereo or 96 kbps in mono. Tracks that are already Opus or Vorbis are copied across untouched. DTS and Apple Lossless can't be decoded by any browser, so those are left out, and the plan names them before you convert.
Do I need WebM for Discord, or for a website?
For a website, yes — it's worth it. For chat platforms it depends: WebM plays fine in most of them, but animated-emoji and sticker slots have their own size and duration caps that no converter can change for you. If an upload is being rejected, check what the platform is actually complaining about first. The format often isn't the problem.
What's the biggest file I can convert?
Up to 2 GB in any browser, and up to 4 GB in desktop Chrome and Edge, which can write the WebM straight to a file you pick instead of holding it in memory. Length is capped at two hours, because a full re-encode of anything longer inside a browser tab isn't something to promise.