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How to add subtitles to an MP4 for free

You have an MP4 and you want words on screen. You can type every line by hand in a desktop subtitle editor, or let speech recognition write a first draft and spend your time fixing it instead. This guide shows the second approach using AutoSub, a free-to-start web app that runs in your browser.

Open AutoSub free30 free minutes when you sign up. Works best in Chrome or Edge.

Two ways to add subtitles to an MP4

Before you start, decide what kind of subtitles you need. It changes which export button you press at the end.

AutoSub produces both from the same edited transcript, so you can export an SRT for YouTube and a captioned MP4 for Instagram without redoing any work.

Step by step

1. Open AutoSub. Go to autosub.isroot.in in Chrome or Edge. There is nothing to download, though you can install it as an app from the address bar if you'll use it often.
2. Pick the spoken language. Choose the language people speak in the recording, or leave it on auto-detect. Picking it explicitly helps with short clips and heavy accents.
3. Drop in your file. Drag your MP4 onto the page (or click to browse). AutoSub decodes it in the browser and uploads only compressed, speech-only audio for transcription. Sign in when asked; new accounts get 30 free minutes.
4. Review and edit. The subtitles open in the editor. Click any line to jump the video to that moment, fix words, adjust start and end times, split or merge cues, and use find & replace for names the model misheard. Undo and redo cover every change.
5. Burn the captions in or export a subtitle file. To make the subtitles part of the picture, open the burn-in panel, choose a caption style, preview it on your video and export a new MP4. The video is rendered on your device. If you only need a separate subtitle file, export SRT or VTT instead.

Tips for subtitles people can actually read

• Keep lines short. Two lines per cue is the usual maximum. If a cue looks crowded, split it at a natural pause.
• Fix names first. Speech recognition is good at common words and weaker at brand names and people's names. Find & replace fixes every occurrence at once.
• Check timing at cuts. A caption that lingers into the next shot looks sloppy. Nudge end times so text leaves with the speaker.
• Preview before exporting. Scrub through the video in the burn-in panel to check that the style is legible against bright and dark scenes.

Why use AutoSub for this

Desktop tools such as video editors and dedicated subtitle editors are powerful, but adding captions there usually means either typing them yourself or exporting audio to a separate transcription service. Many online caption tools, on the other hand, ask you to upload the full video file, which is slow for long recordings and not ideal for unreleased or private footage.

AutoSub sits in between:

• No installation; it runs in Chrome or Edge and can be installed as an app.
• Your video never leaves your device. Burn-in rendering happens locally.
• Transcription uses Whisper large-v3-turbo and supports more than 40 languages.
• A real editor for text and timing, not just a text box.
• Exports to SRT, VTT, TXT and ASS, plus burned-in MP4 with eight caption styles.

After the free minutes, Pro is $9.99 a month for 1,000 minutes, or a one-time 300-minute pack is $7 and never expires.

Frequently asked questions

Is it really free to add subtitles to an MP4?

Every new account gets 30 free minutes of transcription, which covers several short videos. Editing, styling and exporting don't use minutes. Free burned-in exports carry a small AutoSub watermark in the corner; any purchase removes it.

Should I burn the subtitles in or use a separate SRT file?

Burn them in when the platform autoplays muted or doesn't accept subtitle files, which is typical for Shorts, Reels and TikTok. Use an SRT or VTT file when the player lets viewers switch captions on and off, such as YouTube, Vimeo or a video course platform.

Does my MP4 get uploaded to a server?

No. The video is decoded and, if you burn captions in, re-encoded on your own device. Only compressed audio of the speech is uploaded for transcription, and it is deleted within 24 hours.

I already have an SRT file. Can I just burn it into my MP4?

Yes. Importing an existing SRT or VTT file is free and doesn't need an account. Load your video, import the subtitle file, adjust anything you like in the editor, then export the MP4 with your chosen style.

Which browsers work?

Chrome and Edge on desktop work best, because AutoSub relies on the WebCodecs API to decode and encode video locally. Other browsers may not support every feature yet.