Subtitle Converter (SRT ↔ VTT), Explained

After reading this you will understand exactly what separates a SubRip file from a WebVTT file, how to convert between them without breaking timing, and which small formatting details cause players to reject subtitles.

What the two formats are

SubRip (.srt) and WebVTT (.vtt) are both plain text subtitle formats. Each stores a list of cues. A cue has a start time, an end time, and one or more lines of text that show during that window. Open either file in a text editor and you can read it directly.

Here is the same cue in both formats. SRT first:

2
00:01:02,500 --> 00:01:05,000
Keep your hands inside the vehicle.

Now WebVTT:

00:01:02.500 --> 00:01:05.000
Keep your hands inside the vehicle.

Three differences do all the work. SRT uses a comma before the milliseconds (02,500); WebVTT uses a dot (02.500). SRT numbers each cue on its own line; WebVTT makes that number optional. And a WebVTT file must begin with the line WEBVTT. Get those three right and the conversion is essentially a text substitution.

When to convert, and when not to

Convert to WebVTT when you serve video on the web. The HTML <track> element only accepts WebVTT, so a browser playing an <video> tag will ignore a raw .srt file. Convert to SRT when a desktop player, an encoder, or an editing tool expects SubRip, which remains the most widely accepted format for local playback and for muxing into containers like MKV.

Do not convert if the target already accepts what you have. VLC, mpv, and most set-top players read both. Converting a file that already works only risks introducing a formatting error. Convert when a specific tool refuses the file, not out of habit.

The conversion rules in full

There is no arithmetic here, only a fixed set of transformations applied to each line. Going from SRT to VTT:

1. Add the header line WEBVTT followed by one blank line at the top.
2. On every timestamp line, replace the comma before the milliseconds with a dot. Do this for both the start and the end time.
3. Keep the --> arrow and the text exactly as they are.
4. Cue numbers may stay or be removed; both are valid WebVTT.

Going from VTT to SRT reverses the first two steps and adds one more:

1. Remove the WEBVTT header and any styling blocks (STYLE, NOTE, REGION).
2. Replace the dot before the milliseconds with a comma on every timestamp line.
3. Number the cues sequentially starting at 1, because SRT requires an index line and some players fail without it.
4. Strip any WebVTT cue settings after the end timestamp, since SRT does not understand them.

Why the blank line matters

Both formats separate one cue from the next with a single blank line. That blank line is not decoration. It is the delimiter the parser uses to decide where one cue ends. If two cues run together with no blank line between them, a strict parser reads the second cue's index and timestamp as if they were part of the first cue's text.

Count the newlines in a well-formed cue block. A cue in SRT is: index line, timestamp line, one or more text lines, then exactly one empty line. If a file was edited by hand and someone deleted a blank line, or added a second one, the cue count can shift. The converter normalizes this: it collapses runs of blank lines to a single separator and guarantees one blank line between cues.

Reading a timestamp precisely

A timestamp is hours, minutes, seconds, and milliseconds. To sanity-check that a conversion preserved timing, convert a timestamp to a single number of milliseconds and compare before and after.

Here h is hours, m is minutes, s is seconds, and ms is the millisecond field. For the demo's second cue start, 00:01:02,500, that is 0 + 62000 + 500 = 62500 milliseconds. The WebVTT version 00:01:02.500 gives the same 62500. If the two numbers match, the punctuation swap changed nothing about when the cue appears.

Common mistakes

The three failures below account for most rejected files.

Missing WEBVTT header

A .vtt file without the WEBVTT first line is not valid WebVTT. Browsers silently show no captions. Renaming an .srt to .vtt does not add the header, so the rename alone never works.

Wrong separator

A comma in a VTT timestamp (00:01:02,500) or a dot in an SRT timestamp (00:01:02.500) causes strict parsers to skip the cue. The text before and after may still show, so the symptom is missing captions in the middle of a file.

Encoding and byte order marks

A UTF-8 byte order mark placed before WEBVTT pushes the header out of first position for some parsers. Save as UTF-8 without a BOM. Accented characters that appear as garbage usually mean the file was saved as a legacy code page rather than UTF-8.

Related tools

Conversion changes the format, not the timing. If the subtitles are correctly formatted but appear early or late against the video, that is a timing problem, not a format problem. Use the Subtitle Shifter to move every cue earlier or later by a fixed offset, then convert if you also need to change format.

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