No upload, 100% local, no account

Merge video files

Drop your video files, drag to reorder them, and download the merged MP4. Uses fast stream copy when possible, re-encodes when needed. Nothing is uploaded.

How Merge video files works

Join multiple video files into a single clip using ffmpeg compiled to WebAssembly, entirely inside your browser. Drop your clips, arrange them in order, then choose between fast copy mode or re-encode mode. Copy mode stitches streams via the concat demuxer without touching codec data, so it is near-instant and lossless, but all clips must share the same codec, resolution, and frame rate. Re-encode mode runs the concat filter and produces a uniform H.264/AAC stream, handling mismatched sources at the cost of extra processing time.

Memory is the main constraint: very large files held in your browser's address space can slow down or crash the tab. Merging four or five shorter clips at once is more reliable than loading dozens of gigabytes in one pass. The tool has no file count limit, but your device's available RAM sets the practical ceiling.

Written by Bastien Sulyan

How to use Merge video files, step by step

  1. Drop or select all the clips you want to combine.
  2. Drag the clips into the order you want them to appear in the output.
  3. Pick Copy mode if all clips share the same codec and resolution, or Re-encode mode for mixed sources.
  4. Click Merge and wait for ffmpeg to process on your device.
  5. Download the joined video file when it is ready.

Common use cases

  • Assembling multiple interview segments recorded separately into one continuous video before uploading to YouTube.
  • Combining daily GoPro clips from a trip into a single highlight reel, all clips shot at the same 1080p60 settings so copy mode works.
  • Joining screen-recording chunks that were split automatically by recording software to get around a file size limit.
  • Merging an intro bumper, a main presentation, and an outro clip into one file for a webinar recording.

Frequently asked questions

Do my video files leave the device when being merged?

No. ffmpeg.wasm runs the concat demuxer (Copy mode) or the concat filter (Re-encode mode) inside your browser tab. Every file is read into local memory, joined there, and the combined video is saved to your device. Nothing is sent to Sunasty or any external server. Open DevTools and check the Network tab while the merge runs to confirm.

Why do I get a jerky join or audio sync issues in copy mode?

Copy mode does not re-encode, so any mismatch in frame rate, codec profile, or audio sample rate between clips can cause sync drift or playback stuttering. Switch to Re-encode mode, which normalises all streams into a single H.264/AAC timeline and eliminates those inconsistencies.

Can I merge clips of different resolutions?

In Copy mode, clips must have matching resolution and codec or the output may be corrupted. Re-encode mode scales and pads all clips to a common resolution automatically, making it the right choice when sources differ in size.

Is there a limit on how many clips or how large the files can be?

There is no enforced limit, but the browser needs to hold all input files in RAM simultaneously. If a merge of many large files makes the tab unresponsive, reduce the batch size. Merging in two passes (first set then second set, then merge the two results) is a practical workaround.

What format does the merged file use?

Copy mode preserves the original container (MP4 when sources are MP4, for example). Re-encode mode always outputs MP4 with H.264 video and AAC audio, which plays on every modern device without plugins.

Does the tool work offline after the first visit?

Yes. The ffmpeg.wasm engine is sizeable and downloads on first use, but once the page and engine have been fetched the merge runs entirely on your device. You can disconnect from the network and it will keep working.

Can I merge video clips on my phone?

Yes, for a couple of short clips. Merging holds every input file in the browser tab's memory at once, and a phone gives a tab far less RAM than a laptop, so merging many long clips is more likely to make a mobile tab unresponsive. Merge two or three clips at a time on a phone rather than a whole day's footage.

Related tools

Keep everything local, explore complementary tools.

All video tools