sync.video

You filmed an interview on a camera and recorded the voice on a separate microphone, and now the two files have to be lined up. Or you have a single video whose sound arrives late, thanks to a Bluetooth microphone or an export setting. sync.video is a free browser tool that handles both cases.

For the first, you drop the video and the recording in together, and it lines them up by matching the shape of their sound. The site says that's accurate to well under a millisecond on clean audio, and it works with two to eight cameras and recorders at once.

For the second, Detect automatically looks for sharp moments, like a clap or a slate closing, in both the picture and the sound, and suggests a delay with a confidence figure. When it can't tell, you step to the frame where the hands meet and mark it as the clap. Playback slows to a quarter speed around that moment, and you can nudge the sound earlier or later in 10 ms or 1 ms steps.

There's also a difference between a fixed offset and drift, where the gap grows as the video plays because two clocks ran at slightly different speeds. A drift fix stretches the sound between two sync points.

When only the timing changes, sync.video copies the compressed picture and sound into a new file with the sound's timestamps shifted. Nothing is decoded, so there's no quality loss and a long video is written in seconds. Drift fixes and mixes re-encode the sound, but the picture is still copied. It opens MP4, MOV, MKV and WebM files, along with WAV, MP3 and M4A.

The team behind sync.video says the usual way to fix lip-sync or line up a separate microphone is scrubbing waveforms by hand in an editor, or paying for a desktop tool such as PluralEyes. They built this one to find the offset from the shared sound and export in a browser tab, with no upload.

They're also plain about its limits. Automatic detection works best on clean sound with a clear clap or hit, and noisy or very quiet recordings may need the manual clap frame. For multicam work it lines the files up and hands over an FCPXML or Premiere XML file. It isn't a multicam editor.

Other tools sit on the same site. There's a beat-sync maker that cuts clips and photos on the beats of a song, a BPM finder and tap tempo counter, and a subtitle timing fixer for SRT and VTT files. You'll also find an audio delay test, a clapperboard for your phone, and a sync player that plays one video in step across several devices.

It's free, with no account or watermark. The privacy page, updated November 7, 2026, says files are read, analysed and written inside your browser tab and never uploaded.

The sync player is the one tool that talks to a server. Each device opens its own copy of the video, and a relay the site runs on Cloudflare introduces the devices and passes on play, pause and clock messages. The relay sees the room code, any device name you give and the connection details browsers exchange, including IP addresses. It keeps only the room code and its timing, deleted when the room closes or after 12 hours at most.

Google Analytics sets cookies and records which tools were used, never the files themselves, with advertising signals denied. It can be turned off from any page. If you've got an interview with a separate recorder or a clip with a nagging delay, drop it in and see what the automatic detection suggests.