Free Subtitle Checker
Check an existing SRT or WebVTT file for structure, timing, and readability problems — with every issue pinned to its line.
Free, no signup. It never changes your file and doesn't transcribe video.
Subtitles
Ctrl+Enter also runs the check.
How to check an SRT or VTT subtitle file
- Paste subtitles or upload an SRT/VTT file, then choose Check subtitles.
- Review the cue and line numbers for malformed timing, overlaps and readability warnings. Adjust the 42-character and 20-character-per-second guidance to your needs.
- Copy or download the report. Correct the original subtitles in your editor; the checker leaves them unchanged.
This checks structure, timing and readability. It cannot tell whether the words match the speaker. Overlaps can be intentional.
Input is sent to Cubix's server for processing. The input limit is 2 MiB. Use sample data when trying the tool.
Opens editor Home. Upload your video and add or import captions there; your subtitle file is not transferred automatically.
Line-Accurate
Every error and warning points to the exact source line, so you can jump straight to the fix.
Timing & Overlaps
Catches malformed timestamps, end-before-start cues, zero durations, and overlapping cues.
Readability
Flags long lines and fast reading speeds using thresholds you can tune to your style guide.
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Frequently Asked Questions
What does the subtitle checker look for?
It checks structure (missing timing lines, missing blank lines, cue numbers, WEBVTT header), timing (malformed timestamps, end before start, zero duration, overlaps), and readability (characters per line and characters per second).
What are the default readability limits?
42 characters per line and 20 characters per second. These are common guidance defaults, not hard rules — you can change both to match your style guide.
Does it change my subtitle file?
No. The checker only reads the file and reports what it finds. Use the Subtitle Time Shifter or SRT ↔ VTT Converter if you need to change it.
How are characters counted?
Formatting markup such as <i>, <b>, <font>, WebVTT tags and {\an8} overrides is ignored, and accented letters and emoji sequences count as one character each.
Is the subtitle checker free?
Yes. Check SRT and VTT files online for free with no signup and no software to install.
Related subtitle tools
Convert, shift, add, and remove subtitles.