Add Subtitles to an MKV File

An MKV is a bundle

Matroska, the format behind .mkv files, is a container built to hold almost anything: a video stream, several audio tracks in different languages, any number of subtitle tracks, chapter markers, even font files attached for styled subtitles. That flexibility is why film rips, OBS recordings and archive copies so often end up as MKV, and also why it's awkward to hand on, since phones, social platforms and many editing apps won't take it. MKV versus MP4 compares the two containers.

For burning subtitles, three parts of the file matter: the video codec, the main audio track and any subtitle tracks. The editor treats each one differently.

The subtitle tracks inside your MKV

The editor reads the video and audio from an MKV and ignores the rest. Subtitle tracks don't appear in the line list, can't be picked for burning and aren't carried into the download. That goes for every kind: text tracks stored as SRT or ASS, and image-based ones such as PGS from Blu-ray or VobSub from DVD.

So start by finding out whether the words you want already exist in the file. MKVToolNix GUI lists every track when you add the file, VLC shows them under Tools, then Codec Information, and ffprobe film.mkv prints them in a terminal.

Getting a text track out as an SRT

MKVToolNix

mkvextract film.mkv tracks 2:film.srt, where 2 is the track ID that mkvmerge --identify film.mkv reports.

FFmpeg

ffmpeg -i film.mkv -map 0:s:0 film.srt writes the first subtitle track; raise the last number for the second, third and so on.

ASS or SSA tracks

The editor imports only SRT and VTT. The same FFmpeg command converts an ASS track to SRT, keeping text and timing and dropping styling, positions and effects.

PGS or VobSub tracks

These are pictures of text. They need optical character recognition before they can become an SRT; Subtitle Edit, a free Windows tool, does it, and its output needs a proofread.

With the SRT ready, load the MKV, switch AI off if you won't need it, and use Import SRT / VTT. Tags left over from styled tracks, such as italics or position codes, are stripped on import; SRT to MP4 lists what the import keeps.

Burning subtitles into an MKV, start to finish

1. Check whether the MKV already holds the subtitles you want, and extract them to an SRT if it does.
2. Open the editor in Chrome or Edge and choose the .mkv. Leave AI subtitles on only if the speech needs transcribing.
3. Confirm the bar above the preview shows the frame size. A message about decoding means the codec is the problem, covered below.
4. Import the SRT, or review the AI draft, and play a few points across the video to make sure the timing matches this particular file.
5. Choose a look in Style. For long-form viewing, Classic or Cinema reads comfortably.
6. Click Render video, read the summary, especially the audio line, then render and download the MP4.

Codecs that work, codecs that don't

H.264

Renders in every supported browser.

HEVC, including 10-bit

Common in film and series files. Renders where the browser and graphics hardware can decode HEVC; otherwise the editor reports that it can't decode HEVC video.

VP9 and AV1

Rendered by current Chrome, Edge and Firefox.

MPEG-2, Xvid, DivX and other older codecs

Browsers don't decode them. Convert to H.264 with HandBrake first.

The preview works differently from the render: it plays the file through the browser's own video player. Chrome and Edge play most MKVs with common codecs. Safari's player isn't built for MKV, so the preview may stay blank there even when the file itself can be read, which is why this page suggests Chrome or Edge for MKV work.

What happens to the audio

MKVs often carry sound that delivery files rarely use: AC-3, E-AC-3 or DTS surround, FLAC, Opus or Vorbis. MP4 can store all of these, so the editor copies the main audio track across unchanged and the summary says "copied without re-encoding". A 5.1 mix stays intact, but the MP4 then plays sound only where that format is supported, and surround codecs such as DTS are often silent in browsers, on phones and on social platforms. For files heading there, convert the sound afterwards: ffmpeg -i film-subtitled.mp4 -c:v copy -c:a aac -ac 2 film-stereo.mp4 makes stereo AAC and copies the picture.

When the main track is in a form MP4 can't store, the editor converts it to AAC at 192 kbps if the browser can decode it. If it can't, the summary says this browser can't process that audio and rendering is blocked, so you never get a silent file by surprise. Extra audio tracks, such as a second language, come along into the MP4 too, but the summary describes only the main one and many phones play only the first track. If one particular language matters, remove the others in MKVToolNix before loading the file.

What stays behind

Going from MKV to MP4 leaves some extras behind: chapters, attached fonts, cover art and every original subtitle track. The subtitles you burn in can't be switched off afterwards. Anamorphic video, common in DVD rips that store a wide picture in a narrower frame, is written out with square pixels at its displayed shape, so it looks the same but reports a different frame size.

Keep the original MKV. And if what you really wanted was a switchable track rather than burned-in text, skip the render: fix the lines in the editor, press Export SRT, and add the file back into the MKV with MKVToolNix, which copies the video and audio untouched. Embedded versus sidecar subtitles weighs the options, hardcoding subtitles covers the burned-in side, and MKV to text is the route when you only need a transcript.

Three MKV jobs

About the subtitle editor on this page

Opens

MP4, MOV, M4V, WebM, MKV videos your browser can decode

Subtitles from

AI (from the speech, optionally translated into one of 21 languages), an SRT or VTT file, or typing them in

You download

An MP4 with the subtitles drawn into the picture at the original resolution and frame rate, plus SRT and VTT files

Cost

Editing, styling, rendering and SRT/VTT export are free, with no account and no watermark. AI subtitles cost 1 credit per minute (5-credit minimum per video); signing in with Google gives 100 free credits and accounts get 10 free credits a day

Privacy

The video never leaves your device and is rendered in your browser. Only for AI subtitles is the audio sent, over HTTPS, and it is deleted within 30 minutes of delivery

Rendering needs

Chrome or Edge 94+, Safari 16.4+, or Firefox 130+ on a computer. Editing and SRT/VTT export work in any current browser