Video compressor

1. 1

   Add the video

   Drop it in. The tool shows its frame size, length and file size.

2. 2

   Pick the limit

   Tap a size or type your own. The line below shows the bitrate and frame size it will use.

3. 3

   Compress and download

   Play the result to check it, then download the MP4.

A file's size is its bitrate times its length. The tool takes the limit, keeps a tenth back because encoders land near the bitrate rather than on it, sets aside 128 kbps for the sound, and spreads the rest over the video's length.

When that leaves too few bits for the frame, the picture turns to blocks. So the tool also steps the frame down, 1080p to 720p and so on, until the bitrate is enough for it. A smaller sharp picture looks better than a large blocky one.

Length is what matters most. Trimming a video to the part you need gives every second more bits, which shows more than any setting.

If the result comes out over the limit, pick the next size down and compress again. A video already under the limit doesn't need compressing.

FFmpeg's guide suggests two passes when you're aiming at a file size: the first studies the video, the second encodes it. Set -b:v to the video bitrate the tool shows:

• ffmpeg -y -i input.mp4 -c:v libx264 -b:v 2600k -pass 1 -an -f null /dev/null && ffmpeg -i input.mp4 -c:v libx264 -b:v 2600k -pass 2 -c:a aac -b:a 128k output.mp4

No. The video is decoded and the MP4 is encoded in your browser. Nothing is sent anywhere.

25 MB for personal Gmail, Google's limit for attachments. Other mail services set their own, often lower.

At a tight limit there aren't enough bits for the full frame, so the tool steps it down to keep the picture clean. A bigger limit keeps more of it.

Every frame is decoded and encoded again, so it depends on the video's length and your computer. Keep the tab open until it finishes.

Whatever your browser can decode: MP4, MOV, WebM and most MKV files. The result is always an H.264 MP4 with AAC sound.