Video Compressor: Make Videos Smaller Online, No Upload

How the video compressor works

It uses your computer's own video encoder

Modern browsers can reach the video chip in your computer through WebCodecs. The compressor reads your file with an MP4 reader, decodes each frame, scales it if you asked for a smaller size, and encodes it again with H.264 (MP4) or VP9 (WebM) on the hardware encoder when there is one. That is usually several times faster than playing the video, and far faster than the older trick of running a copy of ffmpeg inside the page, which needs a 30 MB download and a single slow thread. The reading, encoding and file-writing code is a few small open-source libraries (mp4box, mp4-muxer, webm-muxer) loaded the first time you press Compress.

When WebCodecs is missing

Firefox and older Safari cannot encode video this way, and a few formats (WebM, MKV, AVI) cannot be read by the fast path. For those the page plays the video through a canvas and records it again with the browser's recorder. That works everywhere, but only in real time: a 10-minute video takes 10 minutes and the tab must stay open and visible. The page tells you which path each file takes before you start. A browser whose H.264 encoder cannot keep a constant quality uses a fixed bitrate for the picture size instead, so there the size cannot follow the picture.

Choose a quality: the size follows the picture

This works like HandBrake's Constant Quality (RF). Every frame is encoded with the same quantizer, the setting that decides how much detail is thrown away, so a slide or a talking head needs very little and a fast, busy scene more, instead of every video getting the same bitrate. Before compressing, the page encodes a few short stretches of the video (about ten seconds in all, which takes a couple of seconds) to measure how much detail it has, and shows the size to expect. Balanced is about HandBrake's RF 22, High keeps more fine detail, Small and Smallest trade detail for size. If the original is already compressed efficiently (a phone video or a download), the chosen quality could make it bigger, so the page lowers the quality just enough to keep it smaller and says so.

How it compares with HandBrake

On a 79-second 1080p talk-show video, HandBrake (x264, preset fast, RF 22) made a 9.3 MB file. Balanced here made 11.5 MB with the same measured quality (VMAF 96.2 against 96.0), in about 16 seconds on a 12-core Mac (a 2-second check, then the encode); High made 15.1 MB with a little more detail and Small 7.7 MB. Before this change the page used a fixed bitrate whatever the picture and made 39.5 MB from the same video. HandBrake's x264 is a slower and cleverer software encoder than the graphics chip, so at the same look our files are still bigger: about 10 to 30% for talking heads, screen recordings and slides, and up to about 70% for fast sport. When the smallest possible file matters more than speed and privacy in the browser, HandBrake is still the better tool.

Aim for a file size

Say 25 MB for an email or chat attachment. The page tests a few seconds of the video, then picks the quality that should just fit the size you set, with the sound included. Encoders do not hit a number exactly, so if the result is more than 3% over, the video is made a second time with a corrected setting; it usually lands a little under. If the video already looks its best well below that size, it stops at High quality and the file comes out smaller than you asked. If the size is too small for the length, the page warns you that the picture will look blocky.

Safe for your computer

The page reads your number of processor cores, the memory your browser reports, the free storage space and which encoders the browser supports, then picks sensible defaults (a weaker computer starts with a smaller picture) and warns before a job that is likely to fail, for example a 4K video on a small laptop or a result bigger than the free space. In Chrome, Edge and Brave you can save straight to disk, so even a huge result is written as it is made instead of being kept in memory. Browsers report memory and free space only roughly, so treat the warnings as guidance.

Which format?

MP4 with H.264 and AAC plays everywhere, including phones, so it is the default, and it is the one that keeps a constant quality. WebM with VP9 and Opus is fine for the web, but in our tests the browser's VP9 encoder was not smaller at the same look (it needed about 20% more), so WebM gets an average bitrate a little above the MP4 size. Compressing again and again loses quality each time, so keep your original.

Tips

The biggest savings come from a lower resolution (1080 to 720 often cuts the size by a third to a half) and a lower frame rate (60 to 30). Screen recordings and exports from editors such as Camtasia shrink enormously, often to a fifth; video that is already compressed by a phone shrinks less. A result that would be larger than the original is flagged.

Brave: “save straight to disk” is off

Brave turns off the File System Access API that lets a page write a file where you choose. To turn it on, open brave://flags/#file-system-access-api, set it to Enabled and restart Brave. Without it the result is kept in memory and downloaded at the end, which is fine unless the result is several GB.

Is it private?

Yes. Your video is read and compressed by your own browser. It is never sent to mes.fm or anywhere else, and closing the tab discards everything that was not saved. The only thing that downloads is the small open-source code libraries. The “Save straight to disk” option writes only where you tell the browser to.