Compress MP4 videos

MP4 is a container, not a codec

MP4 (MPEG-4 Part 14) is a file container: a box that holds a video track, one or more audio tracks, and sometimes subtitles and chapter marks. What decides the size and quality is the codec used for each track. Most MP4 files carry H.264 (also called AVC) video with AAC audio, the combination that plays on practically every phone, browser, TV and editing program. Newer MP4s may use H.265 (HEVC) or AV1, which compress better but are not supported everywhere.

That is why two MP4 files of the same length can be 20 MB and 2 GB. The container is the same; the resolution, frame rate and bitrate inside are not.

Bitrate decides the file size

A video's size is simple arithmetic: bitrate × duration. One minute at 8 megabits per second is 8 × 60 ÷ 8 = 60 MB. Phones and cameras record at generous bitrates because they have to encode in real time with little effort per frame; a slower, careful encode needs fewer bits for a picture that looks the same. For comparison, YouTube's recommended upload bitrates (opens in a new tab) for H.264 at 24–30 frames per second are 8 Mbps for 1080p, 5 Mbps for 720p and 2.5 Mbps for 480p.

Resolution and bitrate belong together. Squeezing a 1080p video into a very low bitrate gives a blurry, blocky picture; scaling it to 720p first leaves fewer than half the pixels to share the same bits and looks cleaner on a phone screen.

How to use the settings

The tool re-encodes the video in your browser with the hardware encoder your device exposes through WebCodecs and writes a new MP4 with H.264 video and AAC audio (Firefox, which has no AAC encoder, writes the sound as Opus). You can choose between two ways of controlling it:

• Target size. Enter the size you need, for example 24 MB for Gmail. The tool works out the bitrate from the duration: for a 2-minute clip, 24 MB × 8 ÷ 120 s is 1.6 Mbps in total, leaving about 1.5 Mbps for the picture after 128 kbps of audio. Hardware encoders do not hit a target exactly, so aim a little below the real limit.
• Quality preset. High quality, Balanced (the default) or Smallest file, when you have no hard limit.

Maximum resolution (1080p by default, applied to the shorter side, so a vertical phone video stays vertical), a frame rate cap (60, 30 or 24) and removing the audio track reduce the size further.

Limits of compressing video in a browser

Encoding speed depends on your device's hardware encoder: in our tests, a fast Windows PC in Chrome encoded a 60-second 1080p clip in about 20 seconds, while an older phone can take several times the clip's duration. Because a browser tab has limited memory, files above 4 GB on a computer or 1 GB on a phone are rejected with a message. Video needs WebCodecs, available in current Chrome, Edge and Opera, Firefox 130 and later and Safari 26 and later.

MP4 questions

Other video formats

iPhone recordings have their own page for MOV, and files with several audio or subtitle tracks are covered on the MKV page. All video tools are listed on the video compressor page, and how it works explains why nothing is uploaded.