Merge videos

How Merge Videos works

The file is read with a FileReader, rewritten in memory and offered as a download. This is why large PDFs are limited by your device's RAM rather than by an upload size limit. A 200 MB scan never crosses the network, so the slowest part is your own disk.

That is the whole architecture: static files, one script, no upload endpoint to leak or log. Close the tab and the work is gone.

Why some joins take seconds and others take minutes

An MP4 is a container holding two streams. When every clip uses the same codec, resolution and frame rate, joining is a matter of concatenating the compressed packets and writing a new index — no decoding at all. That is why a stream copy finishes before you switch tabs, and why the picture is bit-identical afterwards.

The moment one clip differs, the container cannot describe the result: two different H.264 profiles, a 1080p clip next to a 720p one, or 30 fps next to 25 fps. The only honest fix is to decode every frame and encode one consistent stream, which is slower and slightly lossy but produces a file that plays everywhere.

The order is the edit

Clips are joined in the order the file picker returns them, which is the order you selected them. There is no timeline to drag — if the order is wrong, clear the input and select again in the right sequence.

That constraint is deliberate. A drag-and-drop timeline is a video editor; this is a joiner, and the simpler model is what makes it fast and reliable.

What a browser can hold

Every clip is loaded into the tab's memory before the join starts. That is why the practical ceiling is around 220 MB in total: past that, the tab runs out of room and either stalls or crashes.

For a two-hour recording, a desktop tool is the right choice. For the ten phone clips that need to become one video for a WhatsApp status, this is faster than any app install.

Good to know

• Works offline once loaded, including on flights and in locked-down networks.
• No file size limit beyond your device's own memory.
• Free to use, with no account and no usage counter.
• Results are produced locally, so they are not logged or shared with anyone.

Questions