What merging is good for
Joining clips end to end is the most common editing job there is, and it rarely justifies installing a full editor. Typical cases: a phone that split a long recording into several files, a set of short clips from one event that you want to share as a single video, a screen-recorded tutorial captured in parts, or an intro and outro that need to sit around a main recording. The tool does one thing: it plays the clips back to back, in the order you set, as one MP4.
What happens to your clips
All processing is done by FFmpeg compiled to WebAssembly, running in your browser tab. The files are read from your disk into the tab's memory and never uploaded. The engine is downloaded on first use and cached afterwards.
Clips from different sources almost never match. One might be 1080p at 60 fps from a phone, another 720p at 25 fps from a webcam, a third a WebM screen capture. A simple file join of mismatched streams produces a broken video, so every clip is normalised first:
- Frame size. The first clip decides the shape: landscape footage produces a 1280 by 720 video, vertical footage a 720 by 1280 one. Every clip is scaled to fit inside that frame without cropping and centred, so a vertical phone clip in a landscape merge gets black bars on the sides and a 4:3 webcam clip gets narrow bars left and right.
- Frame rate. Every clip is converted to a constant 30 fps, so 24, 25, 50 and 60 fps material plays at its normal speed side by side.
- Encoding. Video is encoded as H.264 at CRF 23, a widely used quality level that looks clean at 720p, using the fastest encoder preset to keep browser processing time reasonable. Audio is encoded as AAC at 128 kbps.
- Fast start. The index of the MP4 is moved to the beginning of the file, so the video starts playing on websites and in messengers before it has fully downloaded.
The result is a single merged.mp4 that plays on practically any phone, browser, TV or editing app. Unless the file is very large, a copy is also kept in this browser's local download history, so you can download it again if you close the tab.
Preparing clips for a clean merge
The merger works on whole files, so do the editing per clip before joining:
- Trim first. Remove false starts, dead air and the moment you reach for the stop button with the video trimmer. Ten seconds removed from each of six clips is a minute less to encode.
- Put the right clip first. Because the first clip sets the orientation, start with a vertical clip if you are making a Story or Reel, and with a landscape one for YouTube or a TV. Rotate sideways footage with the video rotator before merging.
- Silent clips are fine. Screen recordings and exported animations often have no audio track at all. Such clips get a stretch of silence of the same length, so the sound of the other clips stays in sync. If none of the clips has sound, the result has no audio track.
- Order with the arrows. Clips are listed in the order you add them. Use the up and down arrows to move a clip, and the cross to drop one. The list is numbered so you can confirm the sequence before you start.
Time, memory and file size
Processing is roughly real time on a typical laptop: three one-minute clips take around three minutes. Phones are slower and can run out of memory with large inputs.
Each input can be up to 500 MB, but all clips have to fit in browser memory at the same time, together with the output. If the merge fails partway on long footage, try fewer or shorter clips, close other heavy tabs, or switch to a desktop browser.
Output size depends on the content rather than on the inputs. At 720p and CRF 23, a static talking-head shot or screen recording may need only a few megabytes per minute, while busy handheld footage can take several times that. If the merged file is still too large for an email or messenger limit, run it through the video compressor afterwards.
When this tool is not the right fit
Because every clip is re-encoded to 720p, merging is not lossless. If your clips are already 4K and you want 4K out, or you need transitions, titles, background music or precise per-frame editing, a desktop editor is the right tool. For the everyday job of stitching a handful of clips into one shareable file, without installing anything or sending private footage to a server, this is the quicker route.