Embed Soft Subtitles

About this tool

Embed Soft Subtitles adds your .srt or .vtt subtitle file as a separate, selectable subtitle stream inside an MP4 video using FFmpeg's mov_text codec. Unlike burned-in hardsubs, soft subtitles can be toggled on or off by the viewer in any modern player (QuickTime, VLC, iOS Video, Android, Plex, smart TVs). The video and audio streams are copied losslessly — only the subtitle stream is added. You can set the language code (3-letter ISO like 'eng', 'spa', 'fra'), give the track a human-readable title ('English', 'Español'), and optionally mark it as the default track that auto-displays. Everything runs in your browser via ffmpeg.wasm — your files never upload.

Why use it

• 100% private — neither video nor subtitle ever leaves your device

• Lossless video and audio stream-copy — zero quality reduction

• Toggle-able captions that viewers can show or hide

• Multi-language friendly — the track has language metadata for player UIs

• Near-instant processing thanks to stream-copy

• Works offline after the first ffmpeg.wasm cache

How to use

1. Drop your MP4 or MOV video into the first upload area (mov_text needs MP4-family containers).
2. Drop your .srt or .vtt subtitle file into the second upload area.
3. Set the 3-letter ISO language code — eng, spa, fra, deu, jpn, etc.
4. Set a human-readable track title — 'English', 'Español', etc.
5. Tick 'Mark as default subtitle track' if you want it to auto-display.
6. Click 'Embed Soft Subtitles' and wait briefly for ffmpeg.wasm to mux the stream.
7. Download the MP4 and test in VLC or QuickTime — the subtitle track should be selectable.

When it helps

• Distributing tutorials where viewers may want to disable captions
• Single-language releases where viewers should be able to switch captions on or off
• Plex / Jellyfin / Kodi media server libraries that surface soft subtitles natively
• Apple TV, Roku, smart TVs that show subtitle track menus
• Archive masters where keeping subtitles editable is more useful than burning them
• Producing client deliverables with a clearly labelled, selectable caption track

Examples

Tips

• Run it once per file with one subtitle track — on an MP4 that already has a subtitle stream, the language, title and default flag are applied to the first (existing) track and the new track is left untagged.
• Use 3-letter ISO 639-2 codes ('eng', 'spa', 'fra') so player UIs label tracks correctly.
• Untick 'Mark as default subtitle track' if you want captions to stay off until the viewer turns them on.
• If a player ignores your soft subs, switch to Burn Subtitles for permanent hardsub rendering.
• VLC is the most reliable player for verifying soft sub embedding worked correctly.
• After the first run ffmpeg.wasm is cached so subsequent embeds start instantly even offline.

Frequently Asked Questions

Glossary

mov_text

MP4-native subtitle codec that stores plain-text captions inside the MP4 container as a selectable stream.

Softsubs

Subtitles stored as a separate text track inside the file — toggle-able by the viewer in supported players.

Hardsubs

Subtitles burned into video pixels — permanent and visible in every player; opposite of softsubs.

ISO 639-2

Three-letter language code standard (eng, spa, fra, deu, jpn) used in subtitle track metadata.

Disposition

FFmpeg flag (-disposition:s:0 default) that marks a stream as default for auto-selection.

ffmpeg.wasm

WebAssembly port of FFmpeg that runs locally in your browser tab.

SharedArrayBuffer

JS memory primitive needed for multi-threaded WebAssembly; gated by COOP/COEP.