How to Add Subtitles to a Video Locally (Free & Private)
A step-by-step guide to adding accurate AI subtitles to any video — without uploading a single frame to the cloud. Powered by a local Whisper-based auto subtitle generator that runs entirely in your browser.
Why generate subtitles locally?
Most online subtitle tools upload your footage to their servers to run transcription. That's fine for cat videos, but it's a privacy problem for interviews, client work, medical content, legal recordings, or anything under NDA. A local AI caption generator keeps every byte on your machine.
- No uploads. Your video never leaves your device.
- No accounts required to try it.
- Works offline once the page has loaded.
- No per-minute fees or file-size caps from a cloud API.
Cloud subtitle tools vs. local Whisper AI
Popular cloud tools (Rev, Descript, Kapwing, Veed, etc.) transcribe on their own servers. MediaVault runs OpenAI's Whisper model directly in your browser via WebAssembly, so the audio is transcribed on your CPU — not theirs.
| Feature | Cloud tools | MediaVault (local) |
|---|---|---|
| Video leaves your device | Yes | No |
| Works offline | No | Yes |
| File-size limits | Usually | Limited only by your RAM |
| Pay per minute | Common | No |
Step-by-step: add subtitles to a video locally
1. Open the Subtitler tool
Head to MediaVault and open the Subtitler. Everything loads client-side — the tab becomes your subtitle studio.
2. Drop in your video
Drag any MP4, MOV, MKV, or WebM file onto the drop zone. The file is read straight from disk into memory. Nothing is uploaded.
3. Run the AI caption generator
Pick your language (or leave auto-detect) and start transcription. MediaVault extracts the audio track with FFmpeg-WASM, then feeds it to the local Whisper model. You'll see captions stream in with accurate word-level timestamps.
4. Edit, translate, and style
Fix any misheard words, split or merge cues, adjust timings, and restyle fonts, colors, and positioning. You can also translate the output track into another language while keeping the timings.
5. Export or burn in
Export as .srt or .vtt for YouTube, Vimeo, Premiere, DaVinci Resolve, and Final Cut — or burn the subtitles directly into a new MP4 so they always show, everywhere.
Tips for great AI subtitles
- Use a clean audio source when possible — noisy footage costs accuracy in every tool, cloud or local.
- For long videos, split into chapters and transcribe each — easier to review, same quality.
- Always re-read AI output for names, acronyms, and technical terms.
- Keep lines under ~42 characters for readability.
FAQ
Is MediaVault really 100% local?
Yes. Transcription, editing, and rendering all happen in your browser tab. Your video never touches a server.
Which languages does the auto subtitle generator support?
Whisper supports dozens of languages including English, Spanish, French, German, Portuguese, Italian, Dutch, Japanese, Korean, and Chinese.
Can I get an SRT file?
Yes — export SRT, VTT, or burn subtitles directly into the video.
Try the private AI caption generator
Add subtitles to your next video without sending it to the cloud.
Open MediaVault Subtitler