Compress an MP4

MP4 is a box, not a size

MP4 is a container. It says how the streams are packed, not how heavily they are compressed. Inside a file called clip.mp4 there is a video stream — usually H.264, sometimes H.265 — and that stream's bitrate is what decides the file size. Two MP4s of the same 30-second clip can be 4 MB and 400 MB and both are perfectly valid MP4s.

So "compress MP4" always means one of two things: spend fewer bits per second, or have fewer pixels to spend them on.

Quality mode, and what CRF actually does

This page defaults to constant quality (CRF), which is the right default when you do not have a number to hit. Instead of forcing a bitrate, you tell the encoder how much loss is acceptable and it spends whatever each scene needs: a static talking head gets few bits, a handheld shot of leaves in wind gets many. The file ends up as big as this clip needs, which is why the size is an estimate until it finishes.

Each step of +6 on the CRF scale roughly halves the bitrate. That is why Small is so much smaller than High, and why going past 28 falls apart quickly.

If you need an exact size instead

Switch the tool to Target size and type a number. It reads the clip's duration, subtracts the audio, solves for the video bitrate that fits and encodes two-pass. That is the mode to use when a form, a mail server or a Discord channel has given you a hard limit.

Before you reach for H.265

H.265 genuinely produces a smaller file at the same visual quality — usually 25–30% smaller. In this browser tab it also encodes around four times slower, and a handful of destinations refuse it. Use it when you control the player and you care about size more than time; otherwise H.264 is the right answer.