How to convert MKV to MP4 without losing any quality

Do not convert it. Remux it. MKV and MP4 are containers, not codecs, so if the video inside your file is already H.264 or HEVC and the audio is AAC, AC-3 or MP3, an MP4 can hold exactly the same compressed bytes. Nothing is decoded, nothing is re-compressed, and the picture is bit-for-bit identical to what you started with.

A remux takes seconds on a two-hour film, because the work is rewriting an index rather than rebuilding every frame. Running the same file through HandBrake or a paid desktop converter takes an hour and throws quality away by definition. The MKV to MP4 converter on this site reads your file in the browser, tells you which of the two paths applies, and copies the streams when it can.

Containers and codecs are two different things

Almost every bad answer to this question comes from treating "MKV" as a video format. It is not. A video file has two layers, and confusing them is what leads people to re-encode footage that never needed touching.

The container — MKV, MP4, MOV, AVI — is the filing system: where each frame begins, which tracks exist, how sound and picture line up in time. It holds no picture information of its own. The codec — H.264, HEVC, VP9, AV1, AAC, AC-3 — is the compression, and it is the only layer that can lose quality.

So when someone tells you MKV has better quality than MP4, or the reverse, they are describing files they happen to have met rather than the formats. A 4K master and a blurry phone clip can both be MKV. Swapping a wrapper costs nothing.

What "lossless" actually means here

There are two things a converter can do to your file, and only one deserves the word lossless.

A stream copy, or remux, lifts the already-compressed frames out of the MKV and writes those same bytes into an MP4 structure. No decoder ever runs on the picture. The result is not "very high quality" — it is identical quality, because the video data was never altered.

A transcode decodes every frame back to raw pixels and compresses them again with new settings. That is generational loss at any quality setting, including the highest. It can be small enough to be invisible on a static talking head; it still shows on gradients, fast motion, film grain and small text, and it shows more once a platform re-compresses your upload a second time.

That last row is why so much advice on this topic is wrong. HandBrake is a good encoder and not a remuxer at all. Pointing it at an MKV that only needs rewrapping turns a five-second job into an hour and costs quality you cannot get back. The wider comparison is in the guide on using a browser-based HandBrake alternative.

How to remux an MKV to MP4 in your browser

The converter here runs FFmpeg compiled to WebAssembly inside your own tab. Your file is never uploaded, because there is no server to upload it to — which matters when the footage belongs to a client, or simply is not yours to hand to a third party.

1. Drop the MKV onto the MKV to MP4 page. It reads the header locally and prints the container, video codec and profile, resolution, frame rate, duration, audio codec and channel layout, file size and bitrate.
2. Read the line that says what it is about to do. If the streams will be copied, that is the lossless path — press Convert.
3. If it says the video will be re-encoded, read the two sections below first.
4. Save the MP4 alongside the original, and keep the MKV until the new file has played where you needed it.

Two limits are worth knowing in advance. Files above 2 GB are refused outright — a 32-bit WebAssembly heap cannot address them — and above roughly 300 MB the page warns you that the job wants around three times the file size in free memory. Feature-length rips often exceed 2 GB; for those, use a desktop remuxer such as FFmpeg or MKVToolNix, which do the same stream copy with no ceiling.

Check that it really was a copy

You do not have to take the claim on trust. Compare the two file sizes: a genuine remux lands within a fraction of a percent of the original, because only the index changed, while a file that came out half the size was re-encoded. You can also feed the MP4 back into the tool, or the codec checker, and confirm the codec, profile, resolution and frame rate read back the same.

When a copy is not possible

MP4 is stricter than MKV. Matroska carries almost any codec ever shipped; MP4 has a far shorter list of things ordinary players accept from it. A stream copy is available when your MKV holds:

• H.264 video — copies, and plays essentially everywhere.
• HEVC (H.265) video — copies. It plays fine on modern Apple hardware and recent Windows, and fails on older Windows installs; if that is your destination, see HEVC video that will not play on Windows 11.
• AAC, MP3 or AC-3 audio — copies.

A copy is not available when the MKV holds VP9 or AV1 video, or Opus, Vorbis, FLAC or PCM audio. Those are legal in Matroska and not usefully playable out of an MP4, so the tool re-encodes them and says so before it starts. VP9 and AV1 are what files pulled off the web tend to contain; a screen recorder or disc rip almost always gives you H.264.

Subtitles, extra audio tracks and chapters

This is the part most "lossless" guides skip, and it is where people lose something real without noticing.

A ripped MKV often carries several audio tracks, forced and full subtitle tracks, and chapter marks. This converter keeps the first video stream and the first audio stream and drops the rest. It tells you how many subtitle and extra audio tracks it found before you convert, so the loss is visible rather than silent. The phone app drops them too.

Even a full desktop remux cannot carry everything. Blu-ray rips use PGS subtitles, which are bitmap images, and MP4 has no place for them — its own subtitle format is timed text. They can be extracted to a separate file, or burned into the picture by a re-encode, but they cannot ride along. If subtitles are the reason you keep the MKV, playing it in a player that handles Matroska often beats converting at all. Where MKV really fails is elsewhere: Premiere Pro refuses the container entirely, and televisions are fussy, which the guide on MKV on a smart TV covers.

One more edge case. AC-3 audio copied straight into an MP4 is legal but not universally handled — some players decode it, some give you silence. If the picture is fine and the sound is missing, convert again with the audio option set to re-encode. The video still copies, so nothing is lost visually. The video plays but there is no sound guide walks through the rest of the causes.

If you must re-encode, lose as little as possible

Sometimes there is no copy path: the codec is VP9, or the target device only accepts H.264 below a resolution your file exceeds. The goal then shifts from zero loss to invisible loss.

• Use constant quality, not a bitrate. The presets here are CRF 20, 23 and 28 for High, Medium and Low. CRF holds picture quality steady and lets the size land where the content needs it. A fixed bitrate does the opposite: it starves complex scenes and wastes space on simple ones.
• Change nothing you do not have to. Leave resolution and frame rate on original; downscaling and frame-rate conversion each add damage of their own.
• Encode once. Every pass compounds. If the file is going into an edit and out again, re-encode at the end, not the start.
• Keep the source. The MKV is your only copy of the original quality.

The same reasoning applies between MOV and MP4, where a copy is nearly always available because both are ISO base-media containers — see converting MOV to MP4 without losing quality.

On a phone, or with a file over 2 GB

The browser tool has a hard ceiling; the Video Converter app for iPhone and Android has none, and it uses the device's hardware encoder rather than software in a tab, so long and 4K footage goes far faster there.

Be clear about the trade: the app always re-encodes. It has no stream-copy mode, so MKV to MP4 there is a transcode at 1, 2.5 or 5 Mb/s, not a lossless rewrap. For bit-for-bit preservation, use the browser tool under 2 GB and a desktop remuxer above it. For simply getting a big file onto your phone in playable form, the app is the practical choice — free and unlimited on Android, while on iPhone the first conversion is free and further ones need the paid upgrade.