Merge videos
In your browser, no upload
MP4
several clips → one MP4 · as stored when they match
Video
videos → MP4Specification
9 rows| Item | Value | Notes |
|---|---|---|
| Input | Two or more videos | Any mix of MP4, MOV, WebM, AVI… |
| Output | MP4, H.264 + AAC | At the first clip’s size |
| Order | The order you pick | Files chosen together keep their list order |
| Same settings | Joined as stored | Same codec, size, sound and rotation: no engine, nothing re-encoded |
| Picture | Re-encoded as H.264 | x264, veryfast preset, CRF 23 |
| Sound | AAC 128 kbps | A clip with no sound plays silence |
| Time | Measured before it starts | A 2-second test encode on this device |
| Engine | This browser’s own encoder, or FFmpeg (16.8 MB) | FFmpeg is downloaded only when the browser cannot do the job, and only when you press the button that says so |
| Processed in | This browser | Not sent to a server |
What happens
7What happens, and why
Put two or more videos into one file, in the order you choose them. The clips can come from different phones and cameras and be in different formats; the result is one MP4 with H.264 picture and AAC sound.
Clips recorded with the same settings — one phone, one camera mode — are joined as they are stored. The page checks this from the files as soon as they are picked: the same codec and configuration, picture size, frame timing, sound format and rotation as the first clip. Then the picture and sound carry straight on from one clip into the next, nothing is re-encoded, the engine is not downloaded and it takes seconds; the badge says Not re-encoded. Each clip’s sound is placed where its picture starts, so they stay in step however many clips there are.
Clips that do not match cannot simply be laid end to end, so everything is re-encoded to one shape. The first clip decides it: the size — later clips are scaled to fit inside it, with black bars where their shape differs — and the frame rate, up to 60 frames per second. The sound becomes 48 kHz stereo AAC at 128 kbps, and a clip that has no sound plays silence for its length, so the clips after it stay in step with their pictures.
To cut a clip down before joining it, use the Trim page first: it does not re-encode, and clips trimmed from the same camera’s videos still join here as they are stored.
First the page asks the browser itself. Browsers carry the video encoders they use for calls and screen recording — H.264, VP9, AAC, Opus, often on the graphics chip — and when this browser can read your file and write the result with them, that is what happens: nothing is downloaded, the time is measured here first, and the line under the badge says the browser’s encoder did the work. MP4, MOV, WebM and MKV files usually qualify. AVI, WMV and FLV do not, and neither does an MP3, so those go the way below.
The work is done by FFmpeg, the program most video software is built on, compiled to WebAssembly so that it runs inside this page. It is large — 16.8 MB — and it is not downloaded until you press the button that says so. Then it reads your file, encodes 2 seconds from the middle of it as a test, and tells you how long the whole file will take on this device, before anything starts. That figure is measured here rather than taken from a table: the same engine and settings, your processor, your video. It is still an estimate, and it is labelled as one.
It is slower than a program installed on a computer. Here FFmpeg runs on one processor core, because a browser only lets a page use more when the site is served with particular security headers, and it cannot use the graphics chip’s video encoder at all. A long high-resolution video can take many minutes, and longer on a phone. The estimate says so before you commit to it, the progress line shows the time left as it goes, and Cancel stops it at once. When the browser’s own encoder can do the job, it is far quicker, and nothing is downloaded.
FAQ
6When is nothing re-encoded
When every clip was recorded with the same settings as the first — the same codec and configuration, picture size, sound format and rotation — as clips from one phone in one camera mode are. If one differs, the page says which, and the clips are re-encoded. Where two clips meet, the sound can be off from the picture by up to one sound frame, about a fiftieth of a second, as in any join that does not re-encode; it does not add up from clip to clip.
Can I change the order
The order is the order you pick the files in. To change it, press Other videos and pick them again in the new order.
What happens to a vertical clip next to horizontal ones
It is scaled to fit inside the first clip’s frame, keeping its shape, with black bars on the sides. Put the vertical clip first and the whole video is vertical instead.
Why does it take so long
Every frame is decoded and encoded again, on one core of your processor, inside the browser. Installed programs use every core and usually the graphics chip’s encoder; a web page gets neither by default. The page measures how long it will be on your device before you start, so the wait is never a surprise.
What if my phone runs out of memory
The page stops and says so, rather than handing back half a file. The video you drop is read from disk in pieces, never copied whole into memory, but the new file is built in memory until it is finished, so a long, high-resolution video can be too much for a phone. A shorter clip, a smaller size, or a computer will get through.
Is the video uploaded
No. The engine is downloaded to your browser, and your file is read and written there. There is no upload endpoint on this site, so there is nowhere for the video to go.