Convert OBS Studio recording to MP4
OBS Studio's default output is MKV because MKV survives a crash without corrupting; MP4 doesn't. Wave converts your MKV, FLV, or WebM output to MP4 in the browser - remuxing when it can, re-encoding only when the source codec doesn't fit MP4.
What actually changes
OBS Studio defaults to MKV for a specific reason: recording to MP4 requires the container index (the moov atom) to be finalised when the file is closed. If OBS crashes mid-recording, the moov never gets written and the MP4 becomes essentially unrecoverable - the frames on disk are unreadable without the index that describes them. MKV writes its index incrementally, so a crashed MKV can still be played and salvaged. The trade-off is that MKV needs conversion for playback anywhere outside VLC and MKV-aware players, which is where Wave comes in. Full options for keeping crash safety while getting MP4 output are in OBS recording settings for MP4.
What OBS puts inside the MKV is by default MP4-compatible: H.264 video from the encoder you selected (NVENC on NVIDIA hardware, QuickSync on Intel, VideoToolbox on Apple Silicon, or x264 software fallback), with AAC audio at your chosen bitrate. On this default configuration, Wave copies both streams into an MP4 container without re-encoding a single frame - a "remux" (see Remux vs re-encode). It's the same operation as OBS's own Tools → Remux Recordings feature, running in the browser without needing OBS installed on the machine where you're handling the file.
WebM output from OBS is a different story. When you selected WebM in Settings → Output, OBS records VP9 video with Opus audio (or AV1 with newer OBS versions and hardware support). MP4 rejects all three (the codec-set mismatch is why - see Codecs vs containers), so Wave must actually decode and re-encode via WebCodecs. A 30-minute 1080p60 WebM recording typically re-encodes in a few minutes. FLV output is more forgiving - most OBS FLVs contain H.264 + AAC and copy through cleanly.
The other OBS output worth mentioning is fragmented MP4 (fMP4), added in OBS 30. It's crash-safe like MKV because the file is written as a series of self-contained fragments, each with its own tiny moov, rather than one big moov at the end. Wave reads fragmented MP4 fine and remuxes it to standard MP4 for editors and pipelines that don't recognise fMP4. This is a good middle ground for streamers who want MP4 out of the box without giving up crash safety.
Three steps.
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Step 1
In OBS, click Stop Recording. Locate the .mkv (or .flv, .webm, .mp4) file in your recording folder.
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Step 2
Drop the file onto Wave. Wave inspects the streams and picks Remux or Re-encode automatically.
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Step 3
Convert and save the MP4. MKV-with-H.264 remuxes in seconds; WebM re-encodes in a few minutes on hardware-accelerated H.264.
Answers, specifically.
OBS's own Tools → Remux Recordings does the same thing - why use Wave?
Same operation, different place. Remuxing in OBS requires OBS installed on the machine that has the recording. Wave runs in the browser on any machine you have the file on. Handy when a streamer sends you a raw MKV and you don't want to install OBS just to remux it.
Should I record directly to MP4 in OBS to skip this step?
Only if you're confident OBS won't crash and your workflow needs immediate MP4. A stream crash mid-recording turns MP4 into unrecoverable garbage; MKV survives. If you want crash safety and MP4 out of the box, choose fragmented MP4 in OBS 30+.
Streamlabs OBS - same file format?
Yes. Streamlabs Desktop is an OBS fork with the same encoding pipeline. Recording defaults are usually configured identically, and the MKV/FLV/MP4 outputs are byte-compatible with what OBS Studio produces.
Multiple audio tracks from OBS?
OBS supports multi-track output for separate desktop, browser, microphone, and game audio - useful for editing later. Wave takes the default-flagged track. Multi-track passthrough is on the roadmap; for now, a specific alternate track requires the command line: `ffmpeg -i input.mkv -map 0:v -map 0:a:1 -c copy output.mp4`.
Replay Buffer output?
The Replay Buffer saves in the same format as your configured recording output. Same conversion path applies - MKV replays remux in seconds; WebM replays re-encode.
My recording is 4 hours and 20 GB. Will Wave handle it?
Wave caps at 8 GB per file. Above that, ffmpeg from the command line is the practical path: `ffmpeg -i input.mkv -c copy output.mp4`. Alternatively, split the recording in OBS by hitting Stop and Start Recording periodically - each chunk becomes its own file.
Variable frame rate (VFR) from OBS - does the MP4 play right?
Wave preserves the source timestamps. Modern MP4 players handle VFR fine. Older editors sometimes prefer CFR (constant frame rate) input; if the MP4 shows audio drift in an old editor, re-render from OBS with the Enforce streaming service encoder settings option turned off and Common FPS Values set to a fixed rate.