How to Convert MKV to MP4 Without Losing Quality (Free)

To convert MKV to MP4 without losing quality, copy the video and audio streams into a new MP4 container instead of re-encoding them. This is called a remux, it takes seconds, and it works for most MKV files because they already hold H.264 video. You can do it in your browser with the free WoowTools MKV to MP4 converter, with OBS, with VLC or with one FFmpeg command.

This guide explains what is inside an MKV file, which method to use, and how to fix the two problems people hit most: no sound after converting and a file that will not play on a phone or TV.

Why convert MKV to MP4 at all?

MKV (Matroska) is an open container that can hold almost any video codec, several audio tracks, subtitles and chapters. That flexibility is great for archiving and for recording, which is why OBS Studio records to MKV by default: if OBS crashes, an MKV file is still readable, while an unfinished MP4 is usually lost.

The trouble starts when you want to use the file:

• iPhones, many smart TVs and some Android galleries will not play MKV.
• Instagram, TikTok and WhatsApp expect MP4.
• Premiere Pro, iMovie and many phone editors import MP4 more reliably than MKV.

MP4 is the format almost everything accepts, so converting is about compatibility, not quality.

Remux or re-encode: the one thing to understand

A video file is a container (MKV, MP4, MOV) holding streams that were compressed with codecs (H.264, H.265, AAC, Opus). Converting MKV to MP4 can mean two very different jobs:

Job	What happens	Quality	Speed
Remux	Streams are copied as they are into an MP4 container	Identical to the original	Seconds, even for an hour of video
Re-encode	The video is decoded and compressed again with a new codec	Slightly lower, depends on settings	Roughly real time or slower

If your MKV holds H.264 video and AAC audio, a remux is all you need and the result is bit for bit the same picture. Re-encoding is only needed when the codec inside is one that MP4 players handle badly.

How to convert MKV to MP4 in your browser (free)

The WoowTools converter checks the codecs first and then picks the fastest safe path, so you never re-encode by accident.

1. Open the MKV to MP4 converter.
2. Drop your MKV file on the page or click to choose it.
3. Read the plan in the panel. It tells you whether the file will be copied (fast), whether only the audio will be converted, or whether a full re-encode is needed.
4. If a re-encode is needed, choose High quality or Balanced.
5. Click Convert to MP4, preview the result and download it.

The file is read and written on your own device with the browser's WebCodecs engine. Nothing is uploaded, so a 2 GB recording does not have to travel to a server first.

What happens to each kind of MKV file?

Here is what the converter does with the combinations you are most likely to have:

Inside your MKV	Result in the MP4	Typical time
H.264 video + AAC audio (OBS default)	Remuxed, identical quality	Seconds
H.264 + AC-3, FLAC, Opus or Vorbis audio	Video copied, audio converted to AAC	Seconds to a minute
H.265 (HEVC), VP9 or AV1 video	Re-encoded to H.264 + AAC	About real time or faster
DTS or TrueHD audio	Browsers cannot decode it: the video is saved without sound and the tool tells you	Seconds

Not sure what is inside your file? The converter shows the codecs as soon as you drop it in, before anything is converted.

How to convert OBS MKV recordings to MP4

If you stream or record with OBS, keep recording to MKV. It protects you from losing a recording when the PC or OBS crashes. Then convert afterwards, either in OBS itself (File, Remux Recordings) or by dropping the file into the browser converter.

To make sure the conversion is always an instant remux, set OBS to:

• Video encoder: x264 or NVENC H.264 (not HEVC or AV1).
• Audio encoder: AAC.

With those settings you get MP4 files in seconds with no loss. If you use several audio tracks in OBS (for example game sound and microphone on separate tracks), note that the MP4 from the browser tool keeps the main video and the main audio track only.

Other free ways to convert MKV to MP4

You do not need paid software for this. Each of these is free:

VLC (Windows, Mac, Linux)

Open Media, then Convert / Save, add the MKV, choose the profile "Video H.264 + MP3 (MP4)" and start. VLC re-encodes by default, which is slower and loses a little quality, so it is best when you already have VLC and the file is short.

FFmpeg (command line)

For a lossless remux, FFmpeg's stream copy is the classic method:

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

The -c copy option copies the streams without re-encoding, as described in the FFmpeg documentation. It fails if the MKV holds a subtitle or audio format that MP4 cannot store, and then you need extra options, which is where most people give up.

HandBrake

HandBrake always re-encodes. Use it when you also want to shrink the file or change the resolution, not for a simple container change.

Method	Lossless remux	Install needed	Best for
WoowTools in the browser	Yes, automatic	No	Quick one off files, phones, school or work PCs
OBS Remux Recordings	Yes	Already have OBS	Your own OBS recordings
FFmpeg	Yes, with -c copy	Yes	Batches and scripts
VLC	No, re-encodes	Yes	Short clips if VLC is already installed
HandBrake	No, re-encodes	Yes	Shrinking or resizing at the same time

Common problems after converting

The MP4 has no sound. The MKV audio is most likely DTS or TrueHD, which is common in movie rips. Browsers cannot decode those formats. Use FFmpeg or HandBrake to convert the audio to AAC instead.

Subtitles disappeared. MP4 handles subtitles poorly, so the converter does not carry them over. Keep the original MKV if you need the subtitle tracks, or burn the subtitles in with a desktop editor.

The MP4 still will not play on my TV. Very old TVs only play H.264 up to 1080p. A 4K or 10 bit file may need a re-encode to 1080p H.264.

The file is too big to send. A remux never makes a file smaller. Run the MP4 through the video compressor afterwards, or cut it first with the video trimmer.

Tips for the best result

• Convert the original MKV, not a file that has already been converted, so you avoid a second round of compression.
• If you only need the sound from a recording, use MP4 to MP3 after converting.
• For MOV files from an iPhone, use the separate MOV to MP4 converter.

Convert your MKV now

For most files, converting MKV to MP4 is a lossless copy that takes a few seconds. Drop your file into the free MKV to MP4 converter, check the plan it shows, and download a video that plays everywhere.

Frequently asked questions

Does converting MKV to MP4 lose quality?

Not if the MKV holds H.264 video. The streams are copied into the MP4 container (a remux), so the picture is identical. Only H.265, VP9 or AV1 videos need a re-encode, which costs a little quality.

How do I convert OBS MKV recordings to MP4?

Use File, Remux Recordings in OBS, or drop the MKV into a browser converter that remuxes. With OBS set to H.264 video and AAC audio, the conversion takes seconds and is lossless.

Why does my converted MP4 have no sound?

The MKV audio is probably DTS or TrueHD, which browsers cannot decode. Convert the audio to AAC with FFmpeg or HandBrake instead.

Is it safe to convert MKV to MP4 online?

With WoowTools the file never leaves your computer: it is read and written in your browser. Many other online converters upload the whole video to a server first.

Can I keep subtitles when converting MKV to MP4?

The browser converter keeps the main video and audio only. Keep the original MKV for its subtitle tracks, or burn the subtitles into the video with a desktop editor.