Merge videos

Video is often captured in pieces that need to be presented as a single file: multiple takes of an interview, separate clips from different cameras covering the same event, sequential chapters of a course that were recorded individually, or a highlight reel assembled from footage saved in separate files.

Merging before uploading also simplifies distribution. One file is easier to share, track, and archive than five. Platforms that charge per video or impose upload limits reward combined files.

Merging uses two different strategies depending on whether the input clips are compatible. The fast path uses FFmpeg's concat demuxer with -c copy, which reads each clip and writes the streams into the output container without decoding or re-encoding. This is near-instantaneous and produces no quality loss. A concat list file (containing one file path per line) is written to the in-browser filesystem and passed to FFmpeg as the input.

If the fast path fails (which happens when clips have different codecs, frame rates, or resolutions) the tool falls back to a filter_complex re-encode. Each input is scaled and padded to match the first clip's dimensions using scale=W:H:force_original_aspect_ratio=decrease,pad=W:H:(ow-iw)/2:(oh-ih)/2,setsar=1. Audio streams are resampled to 44100 Hz with aresample=44100. All scaled streams are then concatenated using the concat filter and re-encoded to libx264 with the ultrafast preset and AAC audio.

The fast path (stream copy) works when all clips have the same resolution, codec, frame rate, and audio sample rate. Clips recorded on the same device in the same settings will almost always take the fast path. Mixing clips from different cameras or different apps will usually trigger the re-encode fallback.

If the re-encode path runs, the output resolution matches the first clip you added. Reorder your clips so the one with the target resolution is first if you care about the output dimensions.

The 2 GB combined input limit exists due to browser memory constraints. For larger merges, use desktop FFmpeg with a concat list file.

Video editors handle merging natively in a timeline, but require creating a project, importing each clip, arranging them on a track, and exporting. For simple sequential merges with no editing, this is more setup than the task needs. Command-line FFmpeg with a concat list file is efficient but requires familiarity with the -f concat syntax and knowing when to use -c copy vs. re-encoding. This tool handles the codec compatibility check and fallback automatically.