Subtitle converters that keep your files on your device

The short answer

A subtitle converter that works in your browser can convert a file without sending it anywhere, but a page that says so is only a claim. You can check any converter yourself in about two minutes with your browser's developer tools: convert a file and look for a request that sends it out.

This guide gives you criteria for judging a converter's privacy and a step-by-step way to test one. It does not rank tools. Free Subtitle Converter is one of the tools you could test, and you are welcome to test it the same way.

Why it matters

A subtitle file contains every line of dialogue in a video. For a home video that may not matter, but for an unreleased film, a client project under a non-disclosure agreement, a recorded medical or legal conversation, or an internal company training, the subtitles are as sensitive as the video itself.

When a converter uploads your file, it passes through and may be stored on a server you know nothing about. Some services say they delete files after a while, some use them to improve their products, and some say nothing at all. A converter that never receives the file cannot store, share or leak it.

Criteria for judging any converter

Use these criteria on any site, including this one. None of them proves privacy on its own, but together they give you a good picture.

Check any converter with the Network tab

Every desktop browser can show the requests a page makes. These steps use Chrome, Edge or Firefox; Safari has the same panel once you turn on its developer features in the settings.

• Open the converter page and let it finish loading.
• Open the developer tools with F12, or Ctrl+Shift+I on Windows and Linux, or Cmd+Option+I on a Mac, and choose the Network tab.
• Turn on Preserve log, so requests are kept if the page reloads, and clear the list.
• Choose a small test subtitle file, convert it and download the result.
• Look at the requests that appeared after you chose the file. Sort by the Method column, or filter by Fetch/XHR, and look for POST or PUT requests.
• Click any such request and open its Payload or Request tab. If you can see your caption text, or the request size is close to your file's size, the file was sent.
• Check how the download happens. A download from a blob: address was created inside the page. A download from a web address on a server usually means the file was converted there.

What you will see on a typical page

Expect some requests even on a converter that never uploads anything. Pages load fonts, images and scripts, and many free sites, including this one, use analytics to count visits, and may show ads. Those requests should describe the visit, not your file. On this site, analytics count page views and, at most, anonymous tool use such as which format was chosen; they never include file names, file contents or caption text.

The question to answer is narrow: after you chose the file, did any request carry its contents? If none did, the conversion happened on your device.

The offline test

A second check is simpler and works on any computer. Load the converter page, then cut the connection: switch off Wi-Fi, or choose Offline in the throttling menu of the Network tab. Now convert a file. If the conversion and download still work, the page did not need a server to convert it.

This test has limits. A page could store a file and send it once you are back online, which is rare, and a page that fails offline might simply load part of its code late. Use it together with the Network tab rather than instead of it. On this site, the tool has everything it needs once the page has loaded, so it should keep working when you go offline.

Reading privacy claims

Words such as secure, safe and private do not say where a file goes. A file sent over an encrypted connection to a server is still uploaded. Look for direct statements instead: processed in your browser, never uploaded, or uploaded to our server and deleted after a stated time. If a tool page and its privacy policy disagree, trust the stricter reading and test it.

Be careful with sites that say they run in the browser but also offer features that clearly need a server, such as machine translation, transcription or extracting subtitles from a video. Those features may upload even if simple conversion does not. Test the specific tool you plan to use.

When a desktop program is the safer choice

If you are not allowed to use any website for a project, or you cannot test the tool yourself, a desktop subtitle editor such as Subtitle Edit keeps files on your computer by design. Command-line tools such as FFmpeg can also convert common text subtitle formats. Some desktop programs have optional online features, such as translation, so check those before using them on sensitive files.

How this site handles your file

Free Subtitle Converter reads files with your browser's file reading features, converts them with code that runs in the page, and saves the result through a download created inside the page. Files are not uploaded, stored or logged. The site's own automated browser tests check that working with a file sends no upload-style requests, only the analytics that count visits. You do not have to take that on trust: the Network tab method above works on this site too.

Common questions

Is a converter that runs in the browser always private? Only if nothing is sent while you use it. Running in the browser makes it possible to keep files local; the Network tab shows whether a site actually does.

Can I check a converter on my phone? Phone browsers rarely have developer tools, but the offline test works: load the page, switch on airplane mode and try converting a file.

Are free converters safe for confidential work? Some are and some are not. Test the tool, read its privacy policy, and follow your organization's rules, which may forbid websites altogether.