How to merge several videos or several MP3s into one file

How to merge several videos into one: the short answer

Upload two or more files, put them in the right order with the list arrows and click the merge button. Videos come out as a single horizontal HD MP4 (1280×720), standardized so they join without a jolt; audio files come out as a single 320 kbps MP3 with the volume leveled across tracks. Everything happens in the browser, with no video editor or audio program.

What merging is for

Merging means placing the files one after another to form a single one. It's the difference between sending a handful of loose files the other person has to open one by one and handing over one thing they just press play on and watch or hear all the way through. The most common cases:

• Build one video from parts recorded separately: a party, a trip, a tutorial in segments.
• Stitch short horizontal clips into one to send or post.
• Join the chapters of an audiobook or the episodes of a podcast into a single track.
• Gather voice recordings or songs recorded at different volumes into a sequence that sounds even.

Merge several videos, step by step

1. Open the Join videos (MP4) tool and upload at least two videos (MP4, MOV, WebM, MKV, AVI or M4V, even mixed): drag them onto the "Drag your files" area or click "Select files." With only one, the button stays disabled and reads "Upload at least 2 videos."
2. Set the order in the list with the ↑ and ↓ arrows: the first in the list plays first. No need to re-upload anything.
3. Click "Join N videos and download." Each video is standardized before joining, so it may take a moment; on first use the video engine also warms up.
4. Download the result: a single file named atlasdocs-video.mp4, at 1280×720.

What video merging does (and what changes in the result)

• Standardizes everything to horizontal HD 1280×720 at 30 frames per second. That's what makes it possible to join videos from different cameras, phones and formats.
• A vertical phone video (Stories, Reels, TikTok) gets black bars on the sides; a 4K video is reduced to 720p. If your clips are vertical, the result isn't ideal for posting.
• A video with no sound gets a silent track, so the join doesn't fail or skip.
• Quality is good (H.264, the phone standard), but there is recompression: it's not a bit-for-bit copy.
• There are no transitions, cuts or volume adjustments between videos.

How to merge several MP3s into one, with leveled volume

For audio the path is the same: the Merge MP3 files tool asks for at least two files (with only one, the button reads "Upload at least 2 files"), the order is the list order, and the "Merge N files and download" button produces a single audio-unido.mp3 at 320 kbps. The detail that matters: when you join tracks recorded at different volumes, one comes out quiet and the other blares. Here the tool measures each file's volume in your browser and adjusts them all to match the loudest, with a limiter that keeps the sound from distorting. A file that is already close to the ceiling is raised less, precisely so it doesn't distort. The result sounds even from start to finish. If the audio is still inside a video, pull it out first: how to extract the audio from a video and save it as MP3.

Common problems and limits

• The button won't activate: you uploaded only one file. Merging needs at least two, for video and for audio alike.
• Wrong order: move the items with the list arrows before clicking merge; the list order is the final order.
• It's slow: each video is re-encoded before joining, using your own device's processor. Long videos take minutes; keep the tab open.
• Black bars: they're expected when the videos aren't 16:9. It isn't a defect in the file.
• Memory: the upload limit is 2 GB per file, but on a phone the browser's memory can run out first with several large videos. If it freezes, use a computer.

Both the video merge and the MP3 merge happen in your own browser: the files are never sent to any server. The only download is the conversion engine, about 9 MB, once, on first use. No app, no account.

Instead of sending five loose videos or ten loose audio files, you hand over a single file, in the right order, with even sound and easy to play from start to finish. To lay music under the merged video, see how to make a photo slideshow video with music; and if a single video only needs a format change, how to convert a video to MP4.