How MP4 compression works
An MP4 file is a container. Inside it is a video stream (usually H.264 or HEVC), an audio stream (usually AAC) and some metadata. Almost all of the size comes from the video stream, and its size depends on the bitrate: how many bits the encoder spends on each second of video. Cameras and phones record at high bitrates to preserve detail for editing, which is why a short clip can be hundreds of megabytes. Re-encoding at a sensible bitrate, and optionally a lower resolution, is how you make it smaller.
This page defaults to Quality: High at up to 1080p. Quality mode uses H.264's constant rate factor (CRF): the encoder spends more bits on complex scenes and fewer on static ones, so the file looks consistent throughout. It is the best choice when you do not have a strict size limit.
Quality mode vs target size
- Quality (High): visually close to the original. Good for archiving, sharing with clients, or uploading to platforms that re-encode anyway.
- Quality (Medium): a noticeably smaller file that still looks good on phones and laptops. A sensible default for social media.
- Quality (Low): the smallest files, fine for previews, drafts and reference copies.
- Target size: enter a number of megabytes and the tool works out the bitrate. Use it for hard limits like email attachments or 10 MB uploads.
How to compress an MP4
- Drag the MP4 onto the tool, or tap “Choose video”. The file is read locally.
- Pick a quality level and a resolution. If you are unsure, start with High at 1080p and see how much you save.
- Press Compress video. The progress bar shows the encoding progress.
- Compare the original and compressed sizes, then download. If the saving is small, try Medium or a lower resolution.
How much smaller will my MP4 get?
It depends entirely on the source. Raw phone and camera footage often shrinks dramatically, especially when going from 4K to 1080p. Screen recordings with large static areas shrink a lot as well. Files that were already optimised for streaming — exports from editing software at web settings, or downloads from video sites — may only shrink a little, and occasionally a re-encode can even come out larger. The tool shows you the before and after size so you can decide whether the result is worth keeping.
Choosing a resolution
- Original: keep the resolution and only lower the bitrate.
- 1080p: full HD — sharp on laptops and TVs. The best target for most 4K phone footage.
- 720p: ideal for messaging apps and small embedded players.
- 480p: small files for long recordings, lectures and previews.
Troubleshooting
- The output is larger than the input: the source was already efficient. Choose Medium or Low, or a lower resolution.
- The audio is out of sync: some variable-frame-rate recordings can drift. Try compressing at the original resolution, or re-export from your editor first.
- It is slow: browser encoding uses one CPU core. Lower resolutions encode faster.
Have a different format? The tool also accepts MOV, WebM, MKV and AVI, and always outputs MP4. For the most conservative settings, read how to compress video without losing quality.