Compress video

A video straight from a phone is large, and most places you send one to have a ceiling. This page writes the video again, smaller, so that it fits under the size you pick: 10, 20, 25, 50, 100, 200, 300 MB. Only sizes smaller than the file are offered, and the file that comes out is never larger than the one you chose.

The size decides the bitrate. A file’s size is its bitrate times its length, so the size you pick and the video’s length leave a number of bits per second, and the picture is encoded to exactly that in two passes: the first reads the whole video to see which parts need more bits, the second spends them there. x264, the H.264 encoder, lands close to its target that way; the page aims 3 % under the size to cover the difference, and in the rare case the file still comes out over, writes it again smaller before handing it over. Sound that is already AAC or MP3 and fits in a tenth of the budget is copied as it is; other sound is encoded as AAC at 128, 96 or 64 kbps, the largest that fits in that tenth, and never at less than 64. Two passes take longer than one; the estimate before the start counts both.

The bitrate decides the picture size. The same bits spread over a larger picture give each pixel less, and a busy scene then breaks into blocks. So the page also picks how large the picture is: the largest of 1080p, 720p, 480p and 360p at which each pixel still gets 0.05 bit per frame. A video of 50 or 60 frames a second is first played at half its rate — every other frame, each kept where it was — before any smaller picture: at the same size, a sharp picture at 30 frames a second looks better than a blurred smaller one at 60. Each size’s button says what it comes to before you choose, frame rate included when it changes, so a longer video shows a smaller picture at the same file size. When a size gives more than the picture needs — 0.1 bit per pixel per frame at the largest size — the rest is not spent and the file comes out smaller than you allowed, which the page says before it starts; no larger size is offered past that one, since it would write the same file. A 4K video comes out at 1080p at most.

Where the sizes come from. On 2026-09-24, Discord’s help centre gave 20 MB as the largest upload for an account without Nitro, Gmail’s help gave 25 MB for attachments from a personal account, Microsoft’s gave 25 MB for Outlook.com, and KakaoTalk’s help centre 300 MB for one video. Each is linked in the table above. Services change these figures — Discord says it is trying other limits with some users — so the page links them rather than promising them, and its own sizes are round numbers that include them.

First the page asks the browser itself. Browsers carry the video encoders they use for calls and screen recording — H.264, VP9, AAC, Opus, often on the graphics chip — and when this browser can read your file and write the result with them, that is what happens: nothing is downloaded, the time is measured here first, and the line under the badge says the browser’s encoder did the work. MP4, MOV, WebM and MKV files usually qualify. AVI, WMV and FLV do not, and neither does an MP3, so those go the way below.

The work is done by FFmpeg, the program most video software is built on, compiled to WebAssembly so that it runs inside this page. It is large — 16.8 MB — and it is not downloaded until you press the button that says so. Then it reads your file, encodes 2 seconds from the middle of it as a test, and tells you how long the whole file will take on this device, before anything starts. That figure is measured here rather than taken from a table: the same engine and settings, your processor, your video. It is still an estimate, and it is labelled as one.

It is slower than a program installed on a computer. Here FFmpeg runs on one processor core, because a browser only lets a page use more when the site is served with particular security headers, and it cannot use the graphics chip’s video encoder at all. A long high-resolution video can take many minutes, and longer on a phone. The estimate says so before you commit to it, the progress line shows the time left as it goes, and Cancel stops it at once. When the browser’s own encoder can do the job, it is far quicker, and nothing is downloaded.