Merge video and audio

Merge video and audio

Processing uses remux: the original tracks are copied into a new container without re-encoding, so there is no upload wait and no loss of quality.

How to use it

1. Select or drag in the video file with no sound.
2. Select or drag in the matching audio file.
3. Wait while the tool reads the format and duration of both files.
4. Press Merge files, then download the merged video.

When you need this tool

High-quality downloads (for example 1080p and above from YouTube) often come as separate video and audio streams. After downloading both files, use this tool to merge them back into one normal, playable video.

Supported file formats

The video slot accepts .mp4 or .webm; the audio slot accepts .m4a, .mp4, .webm or .opus. The tool reads the file’s real format instead of trusting its file extension, so a renamed file is still detected correctly.

What it takes to merge successfully

The video and audio must be the same container family (both mp4 or both webm) — mixing mp4 with webm triggers an error and you will need to pick a matching file. If the two files’ durations differ by more than 2 seconds, the tool warns you in case you picked the wrong audio, but still lets you continue after confirming.

Notes for mobile use

If the combined size of both files is over 150 MB and you are on a phone, the tool asks for confirmation before merging, since large files can freeze a mobile browser or close the tab on their own. For large files, using a computer is recommended.

Frequently asked questions

Are the files uploaded to a server?

No. The entire merge happens right in your browser — the files never leave your device.

Does merging reduce video quality?

No. The tool remuxes: it copies the original tracks into a new container without decoding or re-encoding them.

Why does it say the files can’t be merged?

This is usually because the video and audio are in different container families (one is mp4, the other webm). Pick files from the same family instead.

What format is the result saved in?

The downloaded file keeps the same container as the original video: mp4 if the video was mp4, webm if it was webm.

Related tools

If you are downloading a high-quality YouTube video and need to merge in audio, go back to the video download page to get the video and audio files first, then use this tool.