How to Convert MKV to MP4 Online in 3 Steps
- Upload the MKV in the converter above (drag and drop or click to browse, up to 100 MB).
- Click Convert. The server re-encodes to H.264 (CRF 23) with AAC 192 kbps audio and adds faststart.
- Download the MP4. It plays in browsers, on iPhone and Android, and in every video editor.
The online route always re-encodes. If your MKV already holds H.264 or HEVC with AAC audio, a remux is faster and lossless (see the tools below).
Can You Really Convert MKV to MP4 Without Quality Loss?
Yes — but only under specific conditions. The key concept is remuxing: repackaging the video and audio streams from one container (MKV) into another (MP4) without touching the compressed data. Since the actual video and audio bits remain identical, the result is zero quality loss.
Think of it like moving books from a cardboard box to a plastic bin. The books (your video and audio data) are untouched — only the container changes. This process is nearly instant because no encoding or decoding occurs. If containers and codecs still blur together, read codec vs container: what the difference is.
However, remuxing only works when the codecs inside your MKV are compatible with the MP4 container. When they're not, re-encoding is required — and that's where quality considerations come in.
When Remuxing Works (Zero Quality Loss)
Remuxing is possible when your MKV contains codecs that MP4 supports natively:
| MKV Contents | Remux to MP4? | Quality Loss |
|---|---|---|
| H.264 video + AAC audio | Yes — direct remux | Zero |
| H.265 video + AAC audio | Yes — direct remux | Zero |
| H.264 video + MP3 audio | Yes — direct remux | Zero |
| H.264 video + DTS audio | Partial — audio must be re-encoded | Audio only (video untouched) |
| H.264 video + FLAC audio | Partial — audio must be re-encoded | Audio only (video untouched) |
| VP9 video + Opus audio | No — full re-encode needed | Minimal with CRF 23 |
The FFmpeg command for a lossless remux is remarkably simple:
ffmpeg -i input.mkv -c copy -movflags +faststart output.mp4
The -c copy flag tells FFmpeg to copy all streams without re-encoding. The -movflags +faststart moves the MP4's metadata to the beginning of the file, enabling instant web playback.
When Re-encoding Is Needed
Re-encoding is required when your MKV contains codecs that MP4 doesn't support or supports poorly:
Incompatible Audio Codecs
The most common scenario. Many MKV files from Blu-ray rips contain DTS, DTS-HD Master Audio, Dolby TrueHD, or FLAC audio — codecs that MP4 players rarely support. These must be re-encoded to AAC:
ffmpeg -i input.mkv -c:v copy -c:a aac -b:a 192k -movflags +faststart output.mp4
Note: -c:v copy keeps the video stream untouched (zero video quality loss). Only the audio is re-encoded. AAC at 192 kbps is transparent quality for the vast majority of listeners.
Incompatible Video Codecs
If your MKV contains VP9, VP8, Theora, or other codecs that MP4 doesn't handle well, the video must be re-encoded to H.264 or H.265:
ffmpeg -i input.mkv -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 192k -movflags +faststart output.mp4
Subtitle Handling
MKV supports advanced subtitle formats (SSA/ASS, PGS) that MP4 cannot contain. During conversion:
- Text subtitles (SRT): Can be kept as a sidecar .srt file or embedded as MP4 mov_text
- Styled subtitles (ASS/SSA): Converted to plain mov_text (styling lost), burned into the video, or stripped
- Bitmap subtitles (PGS): Must be burned in or stripped (cannot be converted to text)
What CRF 23 Means for Quality
When re-encoding is necessary, quality depends on the CRF (Constant Rate Factor) value. CRF is a single number that controls output quality for H.264 and H.265 encoders:
- CRF 0: Mathematically lossless (enormous files, 10–50x larger)
- CRF 18: Visually transparent — virtually no perceptible quality loss
- CRF 23: Default. Excellent quality, a small loss that is hard to see for most content
- CRF 28: Good quality, smaller files, some loss visible in demanding scenes
- CRF 33+: Noticeable degradation, mainly for previews or low-priority content
CRF 23 is the sweet spot for MKV-to-MP4 conversion: the loss is small and hard to see at normal viewing distance. Use CRF 18–20 if you plan to edit or archive the result.
File size after re-encoding: From older codecs such as Xvid or MPEG-2, H.264 at CRF 23 often gives a smaller file. From VP9 or HEVC, which are more efficient than H.264, the MP4 is often larger.
How to Check Your MKV's Codecs Before Converting
Knowing what's inside your MKV helps you predict whether remuxing or re-encoding will occur:
Using FFprobe (Command Line)
ffprobe -v error -show_entries stream=codec_type,codec_name -of csv=p=0 input.mkv
Example output:
video,h264— H.264 video (remuxable)audio,dts— DTS audio (needs re-encoding to AAC)subtitle,ass— ASS subtitles (will be stripped or burned in)
Using VLC (GUI)
Open the MKV in VLC, go to Tools > Codec Information (or press Ctrl+J). The "Codec" field for each stream tells you exactly what's inside.
Using MediaInfo (GUI)
MediaInfo provides the most detailed view: codec, bitrate, channels, resolution, frame rate, and more. Available free for Windows, Mac, and Linux.
How Our Converter Handles MKV to MP4
Our online converter does not remux. Whatever the MKV contains, it re-encodes:
- Video: H.264 (libx264), CRF 23, medium preset, yuv420p, at the original resolution and frame rate
- Audio: AAC at 192 kbps from one audio track (FFmpeg picks the track with the most channels)
- Limits: files up to 100 MB; very long or 4K files can hit the 5-minute processing limit, so remux those locally with FFmpeg
The output always includes -movflags +faststart for instant web playback. For a lossless result, use the FFmpeg remux command above instead.
What you may lose: Multiple audio tracks (only one is kept), advanced subtitles (ASS styling and PGS images do not survive), embedded fonts, and nested chapter markers may be simplified. If these features matter, keep the original MKV as your archive copy.
Convert MKV to MP4 with VLC, HandBrake, OBS or FFmpeg
VLC (Windows, Mac, Linux)
- Open Media → Convert / Save (Ctrl+R), click Add, choose the MKV and click Convert / Save.
- Pick the profile Video – H.264 + MP3 (MP4). To remux instead, edit the profile and tick Keep original video track and Keep original audio track.
- Set a destination file ending in .mp4 and click Start.
HandBrake
Open the MKV, choose a preset such as Fast 1080p30, set the format to MP4 on the Summary tab and click Start Encode. HandBrake always re-encodes; its Subtitles tab can burn one subtitle track into the picture.
OBS Studio recordings
OBS can record to MKV because an MKV survives a crash. File → Remux Recordings turns those files into MP4 without re-encoding.
FFmpeg, a whole folder at once
Windows Command Prompt: for %i in (*.mkv) do ffmpeg -i "%i" -c copy -sn "%~ni.mp4". macOS or Linux: for f in *.mkv; do ffmpeg -i "$f" -c copy -sn "${f%.mkv}.mp4"; done. The -sn flag skips subtitle tracks that MP4 cannot hold, which would otherwise stop the remux.