What this tool does

MP4 Fragment Cutter lets you extract a time-based segment from any MP4 video file and download it as a separate .mp4.

You can work with:

Remote URLs — paste a direct link to an .mp4 file (server must support Content-Length and HTTP Range requests with CORS)
Local files — pick a file from your disk; everything stays on your machine

How to use

Choose source — switch to Remote URL or Local file and provide the video.
Analyze — click Analyze. The tool reads only the MP4 metadata (moov atom), not the entire file. For large remote files this is typically under 1 MB transferred.
Select range — drag the handles on the timeline to set start and end. You can also type times directly into the Start and End fields in H:MM:SS, M:SS, or plain seconds format.
Set filename — enter the desired output name (.mp4 is added automatically).
Cut & Download — only the byte range covering your selection is downloaded.

GOP alignment (keyframe snapping)

MP4 video is stored in Groups of Pictures (GOPs). A GOP begins with a key frame (I-frame) and is followed by predicted frames that depend on it. You cannot start playback — or a valid clip — in the middle of a GOP.

When you set a start time, the tool automatically snaps it back to the nearest preceding keyframe. The orange line on the timeline marks the actual start; the Actual start note below the inputs shows the adjusted time.

The keyframe tick marks on the timeline help you place your start point exactly on a GOP boundary when precision matters.

Technical details

The tool reads the moov atom (MP4 metadata box) using HTTP Range requests — typically the first 64 KB of the file.
Samples (frames) are collected for the selected range.
A new, self-contained MP4 file is built in memory with correct ftyp, moov, and mdat boxes and accurate chunk offsets (stco/co64).
The result is packaged as a Blob URL and downloaded directly.

Limitations

CORS for remote URLs: The server must send Access-Control-Allow-Origin and accept Range requests. Local network streams and most CDN-hosted files work; some authentication-gated services may not.
Memory: The entire fragment is assembled in memory before the download starts. Fragments over a few hundred megabytes may be slow.
Codec agnostic: The tool copies samples as-is; it does not re-encode. The output codec is whatever the source uses (H.264, H.265, VP9, etc.).