Auto-Subtitle a Video with AI, in French

You shot a decent video, it's clear, and it still goes unnoticed because nobody turns the sound on. Subtitling by hand easily takes 30 to 45 minutes of work for 60 seconds of video: writing, splitting, timing every line to the speech. With a transcription AI, the same video comes out subtitled in 2 to 3 minutes, and you only have two or three words left to fix.

This guide shows you two ways to do it in French: with the built-in features of your phone and social platforms, or with a specialized third-party tool. You'll also see the settings that truly matter (text size, position, number of words per line) and the typical mistakes automatic French produces: badly split liaisons, homophones, proper nouns. No technical skills required: if you can drop a file into a window, you can do this. For the text part, starting with proofreading the transcript, you can get help from the Skilzy chat before going back to the subtitling tool of your choice.

How an AI subtitles a video, concretely

A subtitling AI does three things: it listens to the audio track, it writes the text, then it syncs each sentence to the exact moment it's spoken.

The first step is called automatic speech transcription. The model analyzes the sound and produces text. The best known in the field is Whisper, OpenAI's open source model, trained on 680,000 hours of multilingual audio, a large share of it French. It's Whisper, or a derivative, running under the hood of most of the tools you'll come across.

The second step is timestamping: the AI assigns each word a precise moment (00:00:04,120, for example). That's what keeps subtitles from lagging half a second behind, the single most annoying flaw for the eye.

The third step is styling: font, size, color, word-by-word animation. At the end, you get one of two formats.

• A separate subtitle file (.srt or .vtt): a plain text file you upload alongside your video on YouTube, LinkedIn, or a website. Viewers can turn subtitles on or off, and search engines read their content.
• Burned-in subtitles (also called „burn-in"): the text is baked into the image. You can't turn it off, but it shows up everywhere, including on TikTok, Instagram, and Shorts, where playback often starts muted.

For vertical formats, burning in is almost always the right call. For a long video on YouTube or a website, keep the .srt file: it also feeds the page's search rankings.

Method 1: the built-in feature on your phone and social platforms

It's free, it's already installed, and for a story or a Reel published within the minute, it's often the best effort-to-result ratio.

No reason to hide this option: in plenty of cases, it gets the job done.

On TikTok. In the editor, open the right-hand menu, then „Captions." The app transcribes automatically in French and lets you correct each line before publishing. You can also generate translations. Styling stays limited to a handful of presets.

On Instagram. Two routes: the „Captions" sticker you add on top of the video in the Reels editor, or the automatic captions option in your account settings, which turns them on by default. Editing is possible but less comfortable than on TikTok.

On YouTube. After you upload the video, YouTube generates automatic French subtitles in YouTube Studio, under the Subtitles tab. You can edit them line by line or upload your own .srt file, which is preferable for a video you expect to have a long life. These subtitles get indexed — a real lever for a faceless YouTube channel powered by AI.

On your phone directly. On iPhone, Apple's Clips app displays live subtitles as you speak and burns them into the video. On Android, several manufacturers offer transcription built into the gallery editor, with availability varying by model and system version.

The limits of this route are always the same: styling is locked, you don't get a file you can reuse elsewhere, and transcription happens inside the app you're publishing from. If you're distributing the same video on four platforms, you redo the operation four times.

Method 2: specialized third-party tools

They're worth it when you want a highly polished look, batch processing, or a clean subtitle file to reuse.

Here are the families of tools you'll run into, in no particular order of preference.

Pricing for these services changes often: check the official page before committing, especially on annual plans.

A word on Whisper installed on your own machine: it's the cheapest solution to run and the most respectful of your files, since nothing leaves for a server. In exchange, you have to go through the terminal and wait longer if your computer has no dedicated graphics card. It's a good option for a high volume of sensitive content, not for subtitling a Reel between two meetings.

The settings that separate subtitles people read from subtitles people ignore

A subtitle is readable when it fits on two lines max, stays on screen long enough to be read, and doesn't hide behind the app's interface.

Four settings do most of the work.

Reading pace. Professional subtitling standards hover around 17 characters per second and 42 characters per line for a horizontal video. Beyond that, the eye gives up. AI tools often split too finely or too coarsely: take a look at the longest lines before exporting.

Number of words on screen. In a vertical format, 3 to 5 words per screen with the spoken word highlighted works better than a full sentence. On a long 16:9 video, do the opposite: complete sentences on two lines, easier on the eye.

The safe zone. TikTok, Instagram, and YouTube Shorts layer buttons, a handle, and a caption over the edges. Keep your subtitles between 20% and 70% of the image height, leaning toward the lower middle. A subtitle hidden behind the „Subscribe" button is useless.

Contrast. White text with a black outline, or text on a semi-opaque band. Avoid bright colors on a light background: it looks nice on your calibrated screen and is unreadable in full sun on a phone at 30% brightness.

These rules apply to any short-form content, including when you're producing Reels and TikToks with AI in series.

Typical mistakes in automatic French, and how to fix them fast

French trips models up on four specific points: homophones, liaisons, proper nouns, and numbers.

Proofreading 60 seconds of subtitles takes about two minutes. It's the best investment in the whole chain, because one glaring mistake displayed large in the middle of the screen costs more in credibility than sloppy editing.

The checklist:

• Grammatical homophones: a/à, ces/ses/c'est/s'est, et/est, ou/où, la/là. Models lean on context and get it wrong mostly at the start of a sentence.
• Badly split liaisons: „les enfants ont attendu" can come out as „les enfants nont attendu." Watch for words starting with n' or t' for no reason.
• Proper nouns, brands, and acronyms: your company name rarely lands on the first try. Many tools accept a custom vocabulary list: fill it in once and it serves all your videos.
• Numbers and units: „vingt-neuf euros quatre-vingt-dix" is correct but unreadable as a subtitle. Rewrite it as €29.90. Same logic for percentages and dates.
• Punctuation and capitals: some models return a block with no periods or commas. Add at least the periods — they guide how lines get split.
• Anglicisms: „engagement," „feed," and „prompt" often get transcribed phonetically. Fix the spelling your audience expects.

One time-saving trick: proofread the text before styling it. Fixing a word in a text editor takes three seconds; fixing it once the animation is applied takes thirty.

What to remember

If you publish occasionally, the subtitles built into TikTok, Instagram, or YouTube Studio are enough and cost nothing. If you produce several videos a week, go through a specialized tool where transcription, styling, and export live in the same place: what you mainly gain back is time on proofreading and multi-platform republishing. In every case, keep the rule of two minutes of proofreading per minute of video, place your subtitles outside interface zones, and stick to two lines. The rest is comfort.

The simplest way to form your own opinion: subtitle a video you've already published, using two different methods, and compare the time spent.