Video Compressor Online: Reduce MP4 Size With Measurable Results
Re-encoding does not guarantee a smaller file. An 80 MB clip might become 32 MB after a lower-resolution encode, which is a 60 percent reduction. A clip that is already tightly compressed could grow instead. This tool tells you the actual input and output sizes rather than promising a fixed percentage. Choose a short H.264 MP4 above, select a quality setting, and wait for the browser to make a new MP4. Your video remains on your device. Nothing is uploaded to a server for processing.
Which MP4 files can this compressor handle?
The upload limit is 100 MB. The video must be no longer than three minutes and no larger than 1920 x 1080 pixels, including portrait footage with its dimensions reversed. Its video stream must be H.264 in an MP4 container. We inspect the stream inside the file rather than trusting the filename. MOV, WebM, and MP4 files with other video codecs belong in the video to MP4 converter instead. A short clip with a misleading .mp4 extension still has to pass the codec check.
These limits protect your tab, but they are not a promise that every phone can finish encoding a 100 MB clip. Loading the file into the ffmpeg.wasm virtual file system uses memory, and the encoder needs more memory for decoded frames and its output. A 1920 x 1080 decoded frame contains more than two million pixels before compression. Use a computer for longer or larger files. If the tab closes, there is no server job that keeps running in the background.
Quality settings, CRF, and the 720p limit
Smaller file uses an H.264 constant rate factor (CRF) of 34 and AAC audio at 96 kbps. Balanced uses CRF 28 and 128 kbps audio. Higher quality uses CRF 23 and 160 kbps audio. With this encoder, a larger CRF allows more visual detail to be discarded. No setting targets a specific file size or guarantees that text on screen will remain readable. The FFmpeg codec documentation describes the H.264 encoder controls and supported options. Different source footage responds differently to the same CRF value.
A 1920 x 1080 clip is scaled to 1280 x 720. That is about 0.92 million pixels instead of 2.07 million pixels per frame, roughly 56 percent fewer pixels to encode. That number is not a forecast of the file-size reduction because motion, audio, and the original bitrate matter too. Smaller videos are not enlarged. Portrait sources keep their shape within the same maximum dimensions. When an input exceeds 30 frames per second, the tool limits the output to 30 fps. Inputs at 30 fps or below keep their frame rate.
Why the result can grow or look different
Imagine a 720p H.264 clip that was already exported at a low video bitrate. Re-encoding at CRF 23 may produce a larger file even though it cannot restore lost image detail. Choosing Smaller file may reduce its size, but fine lines, captions, and grain may look worse. The tool re-encodes the first audio stream to AAC if one exists. An input with no audio stays silent. It ignores extra audio tracks, subtitles, chapters, and source metadata in the downloaded copy. Keep the original if you need any of those streams later.
The displayed size change is calculated as (1 - output bytes / input bytes) x 100. It uses the files' real byte counts after encoding, not an estimated bitrate. A result labeled 25 percent smaller means an 80 MB original became about 60 MB. If you see "not smaller," try the Smaller file setting or use the original. Repeated lossy re-encoding can only discard more information, so do not use a previous compressed export as the source for another pass if you still have the master.
When a different tool is the better choice
If your goal is an MP4 from WebM, AVI, MKV, or MOV, use the video converter. It performs format conversion with the same processing limits. If you have audio and an album cover but no video footage, the image and audio video makerbuilds a still-image MP4 from one or more songs. If the video soundtrack itself needs a different codec or bitrate, the audio converter handles audio files rather than video containers.
For rough uncompressed audio size estimates, use the sample rate calculator. The music time converterhelps you plan the length of a clip against bars or beats before you export. Play the entire finished MP4, especially transitions and the final seconds, before sharing it. Hardware and browser support differ, and a successful download is not the same as a quality check.
The encoder library downloads on first use. This site does not receive your media. Encoding a three-minute video can take longer than three minutes on a slow device. Cancel if you need to stop the job, and keep the browser tab open while it works. The MP4 preview and reported output size appear only after the new file has been made.
