Merge Videos

A video file stores its picture with a specific codec, size and set of encoder parameters, and its sound with a specific codec, sample rate and channel count. If every clip matches on all of those, the compressed data can simply be placed one after another with new time stamps. Nothing is decoded, so there is no quality loss and it is fast.

The tool checks each clip before you click anything. It compares the video codec, the picture size, the rotation, the encoder setup data stored in the file header, and the sound format. Clips recorded one after another on the same phone, or exported from the same editor with the same settings, usually pass. Clips from different phones, screen recordings mixed with camera clips, or a vertical clip next to a horizontal one don't, and the note under the list says which of these is the reason.

When the clips match you can still choose Re-encode to MP4, for example to get one MP4 from a set of WebM files or to change the size at the same time.

Every frame of a video has the same size, so clips with a different shape need a decision:

• Black bars keeps each clip whole and fills the rest of the frame with black. A phone clip in a 16:9 video gets bars left and right.
• Fill the frame scales the clip until there are no bars and cuts off what sticks out. Good when the difference is small, such as 4:3 into 16:9; harsh for a vertical clip in a wide frame, which loses most of its height.

The output size follows the first clip by default, so put the clip whose shape you want first. You can also pick the size of the largest clip, or 1080p or 720p in the first clip's orientation.

On our test machine (headless Chromium on a cloud server without a graphics card):

• Two identical 60 second 1080p H.264 clips of 61 MB each joined with the fast method in 0.7 seconds, giving a 122 MB, two minute MP4.
• Three short clips of different sizes and frame rates (two 1280×720 at 30 fps with sound, one 720×1280 at 25 fps without sound, 10 seconds in total) were re-encoded with the browser's own engine in 5.3 seconds.
• A vertical WebM followed by a horizontal MP4, 7 seconds in total, went through the FFmpeg fallback in 17.7 seconds.

The fast join takes about as long as reading the files. A re-encode takes roughly as long as the videos last or more, depending on your computer, and is faster with the browser's engine than with FFmpeg.

When at least one clip has sound, the merged video gets one sound track. A clip without sound gets silence of exactly its length, so the following clips stay in sync. Clips recorded at 44.1 kHz and 48 kHz, or in mono and stereo, are converted to 48 kHz stereo AAC. We checked this with a 1,000 Hz test tone recorded at 44.1 kHz mono: after merging it measured 1,000 Hz, so pitch and speed are kept.

• A clip shows no size or length. Your browser can't read that format (some AVI and older camera files). The merge still runs through FFmpeg, but slowly. Converting that clip with MOV to MP4 first can help.
• The merged file is huge. The fast join adds the clips' sizes together. Run the result through Compress Video to send it by email or chat.
• You need only part of a clip. Cut it first with Trim Video; fast mode keeps the quality.
• The tab reloads on a phone. Phones give a tab limited memory. Merge fewer or shorter clips at once, or use a computer.