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Updated 2026-06-28

How to Add Subtitles to a Video Offline on iPhone

By the iPhone Captions editorial team · Updated 2026-06-28

Most caption apps and every browser tool send your video to a server to transcribe it, so they simply stop working with no signal. To caption offline you need an app that does the speech-to-text on the iPhone itself.

Can I add subtitles to a video without internet on iPhone?

Yes, but only with an app that transcribes on-device. Web tools and most popular caption apps upload your clip to the cloud to transcribe it, so they fail offline. Subly runs the transcription on the iPhone itself, so it captions and exports in airplane mode with nothing leaving your phone.

Why most caption tools need internet

Auto-captioning is speech-to-text, and many apps run that step on a remote server — they upload your audio, transcribe it in the cloud, and send the text back. That means no connection, no captions. It also means your footage leaves your phone, which matters if it's private or unreleased.

How on-device captioning works

Newer apps use an on-device speech model (Apple Silicon is fast enough to run one locally). The transcription happens on your iPhone, so it works with no signal and nothing is uploaded. Subly uses this approach with an on-device Whisper model across 90+ languages.

  1. Turn on Airplane Mode (optional test). If you want to prove it's offline, enable Airplane Mode first — a truly on-device app keeps working.
  2. Import the video. Open Subly and select your clip from Photos.
  3. Transcribe on-device. Generate captions; the speech-to-text runs locally, no upload.
  4. Edit and export. Fix wording, style it, and save the captioned video — all without a connection.
Quick test for any app: turn on Airplane Mode and try to caption a clip. If it spins forever or errors, it's a cloud tool.

Frequently asked questions

Does CapCut work offline for captions?
No. CapCut's auto-captions need an internet connection because the transcription runs in the cloud. You can edit offline, but generating captions won't work without signal.
Is offline captioning more private?
Yes. On-device captioning never uploads your video or audio, so nothing is stored or processed on someone else's server — useful for sensitive, personal or pre-release footage.
Is the transcription quality worse offline?
Not necessarily. Modern on-device speech models are very capable; Subly uses a Whisper model locally. As with any auto-caption, expect to fix a few words — clear audio gives the best result.

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Written for iPhone users. App features and iOS steps can change between versions — check the latest before you rely on them. How we test & our sources →