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CONVERTERS

Subtitle Converter & Synchronizer

Paste or open a subtitle file, convert it to SRT, WebVTT, ASS/SSA, or SBV, shift the timing to fix a subtitle that's early or late, and preview it against your own video. Runs locally in the browser.

Tool statusReady to use in your browser
AvailableIn Testing
prontouso://converters/subtitle-sync
LocalSRT · VTT · ASS · SBV

Source subtitle

The format is detected automatically.

Options
SOURCE SUBTITLE
Format—
Cues0
EncodingUTF-8
Convert to

You can also pick the same format just to fix the timing or clean up the text.

Is the subtitle out of sync?

Describe the problem in plain terms — you don't need to remember whether the offset should be positive or negative.

Text formatting
Advanced options
● Local file · no upload · no account

Result

See what will change before you download.

RESULT
CC

Open a subtitle to begin

Once the format is detected, ProntoUso shows the conversion, the timing adjustment, and any styling that might be lost.

How it works

  1. Enter your input

    Fill in the values, paste your text, or upload the file this tool works with.

  2. See results instantly

    Most tools update live as you type; a few use a single button. Either way, the result appears right on this page.

  3. Use your results

    Copy, download, or share what the tool produces — you're always in control of the output.

100% Private & Secure — Runs locally in your browser. This tool does not upload your input.

This tool is still being fine-tuned, so you may run into the odd error. Spotted one? Let us know so we can fix it.

What is Subtitle Converter & Synchronizer?

ProntoUso's Subtitle Converter & Synchronizer detects a subtitle file's format automatically, converts it to SRT, WebVTT, ASS/SSA, or SBV, and shifts every timestamp forward or backward to fix subtitles that are out of sync — no software install and no upload.

ENTENDA A FERRAMENTA

How to convert and sync subtitles (SRT, VTT, ASS, SBV)

Convert between SRT, WebVTT, ASS/SSA, and SBV, fix subtitles that run early or late, and preview the result against your own video — entirely in the browser.

Converting between subtitle formats

Paste or open a subtitle file and the tool detects SRT, WebVTT, ASS/SSA, or SBV from its structure, not its file extension. Pick a destination format and the timestamps are rewritten to that format's own syntax: SRT uses a comma before milliseconds, WebVTT uses a period and adds the WEBVTT header, ASS/SSA uses centisecond precision, and SBV uses a comma-separated time pair with no cue index.

Fixing subtitles that are out of sync

Choose "Appears early → delay" or "Appears late → advance" instead of guessing whether an offset should be positive or negative, enter how far off it is, and every timestamp in the file shifts by that same amount. Quick presets cover the most common drifts (100 ms to 5 s); a custom value works in milliseconds or seconds.

This applies one constant offset to the whole file. It fixes subtitles that are consistently early or late throughout, not ones that drift gradually or only a few lines out of place.

What happens to styling when converting from ASS/SSA

SRT, WebVTT, and SBV don't support everything ASS/SSA can express. Basic italic, bold, and underline carry over when the destination format allows it. Positioning, karaoke timing, colors, and other override tags don't have an equivalent in the simpler formats and are dropped — the tool flags how many cues were affected instead of silently discarding them.

Choosing "Plain text only" strips all styling and formatting tags from the output regardless of source or destination format, useful when you just need the words.

Checking sync against your own video

Open a local video file to preview the converted, synced subtitle over it as it plays. The video is read with the browser's File API and never leaves your device — this is a sync check, not a video editor or muxing tool.

How to use

  • Paste your subtitle text or open an .srt, .vtt, .ass, .ssa, or .sbv file.
  • Pick the destination format, or keep the source format to only fix timing or clean up text.
  • If the subtitle is early or late, choose the direction and how far off it is.
  • Review the before/after timestamp preview and any warnings about dropped styling or negative timestamps.
  • Copy the result or download the converted file.

Runs entirely in your browser

The subtitle file is read locally and processed as text — nothing is uploaded. The optional video used to preview sync is also read locally with the File API and never sent anywhere; it exists only to play back next to the converted captions.

Frequently Asked Questions

Which subtitle formats are supported?

SRT, WebVTT (VTT), ASS/SSA, and SBV, in any direction. The input format is detected automatically from its structure when you paste or open a file.

How do I fix a subtitle that's out of sync?

Choose "Appears early → delay" if the text shows up before people speak, or "Appears late → advance" if it shows up after. Enter the delay you observed — for example 1200 ms — and every timestamp in the file shifts by that amount.

What's the difference between "Preserve when possible" and "Plain text only"?

"Preserve when possible" keeps italic, bold, underline, and other formatting the destination format supports. "Plain text only" strips every style tag, color, and positioning command, leaving just the spoken text — useful for a clean transcript or a format that doesn't support styling anyway.

Does converting from ASS/SSA lose anything?

Positioning, karaoke timing, colors, and most override tags don't exist in SRT, WebVTT, or SBV, so they can't carry over — only basic italic, bold, and underline do. The tool counts how many cues had styling that couldn't be preserved and shows it as a warning before you download.

What happens to cues that end up before 00:00:00 after a timing adjustment?

By default, a cue that starts before zero is clamped to 00:00:00, and a cue that ends before zero (entirely pushed off the start) is dropped, since a subtitle can't have a negative timestamp. Both are counted so you know exactly how many cues were clipped or removed. This safety clamp can be turned off in advanced options if you'd rather keep the raw shifted values.

Can I check the sync against my own video file?

Yes. Open a local video and the converted, synced subtitle overlays it as you play, so you can confirm the timing looks right before downloading anything.

Is my subtitle or video file uploaded anywhere?

No. Parsing, timing adjustments, format conversion, and the video preview all happen in your browser using the File API. Nothing is sent to a server.

Does the tool handle files that aren't UTF-8?

It tries UTF-8 first and falls back to Windows-1252 (common in older SRT files) if the file isn't valid UTF-8, so accented characters from older subtitle files still display correctly.

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