How to Convert OBS MKV to MP4 (Free, Fast)

How to Convert OBS MKV to MP4 (Free, Fast)

You have a folder of .mkv files and an editor that will not open them. That is the whole problem, and it takes about ten seconds to solve once you know that OBS ships the converter you need.

The important thing to understand before you start: you are not converting these files. You are remuxing them. Nothing about the video is re-processed, nothing is re-compressed, and the output is bit-for-bit identical in quality to the input. That is why a 12 GB recording finishes in a couple of seconds instead of forty minutes.

This guide covers the single-file method, the batch method for a whole folder, what to do when the remux fails, and the two problems that still bite people after the file is technically an MP4.

If you want OBS to stop producing MKV files in the first place, that is a settings change covered in OBS Recording Saves as MKV: How to Change to MP4. This page is about the files you already have.

Remux vs Convert: Why One Takes Seconds and the Other Takes an Hour

A video file has two parts: the compressed streams inside it, and the container wrapping them.

Your OBS recording contains an H.264, HEVC, or AV1 video stream and an AAC or PCM audio stream. MKV and MP4 are just two different wrappers around those same streams.

Re-encoding decodes every frame back to raw pixels and compresses them again from scratch. It is slow, it burns your CPU or GPU for the entire duration of the file, and because compression is lossy, the output is measurably worse than the input.

Remuxing unwraps the streams and re-wraps them. The video data is copied byte for byte. There is no decode stage, no encode stage, and no quality change. The only limit on speed is how fast your drive can read and write.

For MKV to MP4, remuxing is always the correct operation. If a tool tells you a 20-minute recording needs 25 minutes to convert, that tool is re-encoding and you should not use it.

Method 1: Convert a Single File in OBS

1. Open OBS Studio and click File in the top menu bar.
2. Select Remux Recordings.
3. In the window that opens, click the ... button in the OBS Recording column and pick your .mkv file.
4. Click Remux.

The Target File column fills in automatically with the same filename and an .mp4 extension, written to the same folder. Your original MKV is never touched or deleted, so there is no risk in trying this.

A status message confirms the result. On a normal SSD you should expect roughly one second per gigabyte.

Method 2: Convert a Whole Folder at Once

Most people find this after remuxing forty files one at a time. The Remux window is a multi-row table, not a single-file dialog.

1. Open File > Remux Recordings.
2. Select every .mkv file in your recordings folder and drag them all into the Remux window together. Each one lands on its own row.
3. Check that every row shows a target path ending in .mp4.
4. Click Remux once. OBS processes the entire queue in sequence.

You can also click the ... button and multi-select with Shift or Ctrl held down. Either way, one click handles the whole batch, and OBS clears completed rows as it goes so you can see progress.

When Remux Fails: The Four Real Causes

The Remux button reporting a failure is informative, not random. There are four common causes and each has a different fix.

1. The target file already exists. OBS will not silently overwrite. Rename the existing .mp4, or edit the target path in the second column, then remux again.

2. The source file is truncated. If OBS crashed or the machine lost power during that recording, the MKV may be missing its closing structures. Remux cannot rebuild a broken file, it can only rewrap an intact one. Work through OBS File Corrupted After Crash: Recovery Guide first, then remux the repaired file.

3. The audio codec cannot live in MP4. If you set your recording audio to PCM in Settings > Output > Recording, that works fine inside MKV but is not a normal MP4 audio format. Remux either fails or produces a file some players refuse. Record AAC if MP4 is your destination.

4. The file is still locked. A file that is open in a player, mid-upload to cloud storage, or sitting on a network share with restricted write permissions will not remux. Close the player, wait for the sync, or copy the file to a local drive first.

Method 3: The FFmpeg Fallback

If OBS is not installed on the machine holding the files, or you want this scriptable, FFmpeg does the same job in one line.

Single file:

ffmpeg -i input.mkv -c copy output.mp4

The -c copy flag is what makes this a remux. Without it, FFmpeg re-encodes and you lose both time and quality.

Every MKV in the current folder, on macOS or Linux:

for f in *.mkv; do ffmpeg -i "$f" -c copy "${f%.mkv}.mp4"; done

Every MKV in the current folder, in Windows Command Prompt:

for %f in (*.mkv) do ffmpeg -i "%f" -c copy "%~nf.mp4"

Run these from inside the folder containing the recordings. FFmpeg writes each MP4 alongside its source and leaves the originals in place.

After Conversion: Two Things That Still Break

The container is now MP4, but two problems survive the change.

Your extra audio tracks may not follow. OBS can record up to six separate audio tracks, which is how people keep microphone and desktop audio separable in the edit. MKV holds all six comfortably. MP4 technically supports multiple tracks, but a lot of editors and nearly every player will only ever expose the first one. If you rely on split audio, keep the MKV as your master and treat the MP4 as a delivery copy.

Your editor may still refuse the file. If Premiere Pro or Resolve rejects an MP4 you just produced, the container was never the problem. The codec inside it was. Recording with HEVC or AV1 produces files that many editors will not decode natively regardless of the wrapper. That case genuinely needs a re-encode:

ffmpeg -i input.mp4 -c:v libx264 -crf 18 -c:a aac output.mp4

This one is slow and does cost a little quality, which is exactly why the right long-term answer is to record H.264 in the first place.

Quick Reference

If You Would Rather Skip the Step Entirely

Remuxing is a chore you inherited from a broadcast tool. OBS defaults to MKV because it was built for people who cannot afford to lose a stream to a crash, and that safety decision is what leaves you with files your editor rejects.

Cubix Capture writes a finished, editor-ready file when you press stop, on Windows and Mac, with no container choice to make and no conversion step to remember. There is a free Starter plan and no account required to try it.

If you are staying on OBS, turn on Settings > Advanced > Recording > Automatically remux to mp4 and this problem stops recurring.

Keep Reading

• OBS Recording Saves as MKV: How to Change to MP4 changes the default so new recordings arrive as MP4.
• OBS MKV vs MP4: Which Should You Use? covers the container decision itself, including fragmented MP4.
• OBS File Corrupted After Crash: Recovery Guide is the right first stop when a file will not remux.
• OBS Recording File Too Large: How to Reduce Size if the MP4 you just made is inconveniently big.